Escherichia coli O157:H7 is a leading cause of foodborne illness. Based on a 1999 estimate, 73,000 cases of
infection and 61 deaths occur in the United States each year. In the ten CDC Foodborne Diseases Active
Surveillance Network (FoodNet) sites (which represent 15% of the US population), there was a 29% decline
in E. coli O157:H7 infection since 1996-98 (see FoodNet Reports).
http://www.cdc.gov/ncidod/dbmd/diseaseinfo/escherichiacoli_g.htm








Food-related Illness and Death in the United States-Synopses

PS Mead, L Slutsker, V Dietz, LF McCaig, JS Bresee … - … On WWW at http:== www. cdc. gov= ncidod= eid= vol5no5=
mead …, 1999


http://www.cdc.gov/ncidod/eid/vol5no5/mead.htm#19











Synopses

Food-Related Illness and Death in the United States
Paul S. Mead, Laurence Slutsker, Vance Dietz, Linda F. McCaig, Joseph S. Bresee, Craig Shapiro, Patricia M. Griffin,
and Robert V. Tauxe
Centers for Disease Control and Prevention, Atlanta, Georgia, USA


--------------------------------------------------------------------------------

    To better quantify the impact of foodborne diseases on health in the United States, we compiled and
    analyzed information from multiple surveillance systems and other sources. We estimate that
    foodborne diseases cause approximately 76 million illnesses, 325,000 hospitalizations, and 5,000
    deaths in the United States each year. Known pathogens account for an estimated 14 million
    illnesses, 60,000 hospitalizations, and 1,800 deaths. Three pathogens, Salmonella, Listeria, and
    Toxoplasma, are responsible for 1,500 deaths each year, more than 75% of those caused by known
    pathogens, while unknown agents account for the remaining 62 million illnesses, 265,000
    hospitalizations, and 3,200 deaths. Overall, foodborne diseases appear to cause more illnesses but
    fewer deaths than previously estimated.

More than 200 known diseases are transmitted through food (1). The causes of foodborne illness include viruses,
bacteria, parasites, toxins, metals, and prions, and the symptoms of foodborne illness range from mild gastroenteritis to
life-threatening neurologic, hepatic, and renal syndromes. In the United States, foodborne diseases have been
estimated to cause 6 million to 81 million illnesses and up to 9,000 deaths each year (2-5). However, ongoing changes
in the food supply, the identification of new foodborne diseases, and the availability of new surveillance data have made
these figures obsolete. New, more accurate estimates are needed to guide prevention efforts and assess the
effectiveness of food safety regulations.

Surveillance of foodborne illness is complicated by several factors. The first is underreporting. Although foodborne
illnesses can be severe or even fatal, milder cases are often not detected through routine surveillance. Second, many
pathogens transmitted through food are also spread through water or from person to person, thus obscuring the role of
foodborne transmission. Finally, some proportion of foodborne illness is caused by pathogens or agents that have not
yet been identified and thus cannot be diagnosed. The importance of this final factor cannot be overstated. Many of the
pathogens of greatest concern today (e.g., Campylobacter jejuni, Escherichia coli O157:H7, Listeria monocytogenes,
Cyclospora cayetanensis) were not recognized as causes of foodborne illness just 20 years ago.

In this article, we report new estimates of illnesses, hospitalizations, and deaths due to foodborne diseases in the United
States. To ensure their validity, these estimates have been derived by using data from multiple sources, including the
newly established Foodborne Diseases Active Surveillance Network (FoodNet). The figures presented include estimates
for specific known pathogens, as well as overall estimates for all causes of foodborne illness, known, unknown,
infectious, and noninfectious.

Data Sources

Data sources for this analysis include the Foodborne Diseases Active Surveillance Network (FoodNet) (6), the National
Notifiable Disease Surveillance System (7), the Public Health Laboratory Information System (8), the Gulf Coast States
Vibrio Surveillance System (9), the Foodborne Disease Outbreak Surveillance System (10), the National Ambulatory
Medical Care Survey (11), the National Hospital Ambulatory Medical Care Survey (12-14), the National Hospital
Discharge Survey (15), the National Vital Statistics System (16), and selected published studies.

Established in 1996, FoodNet is a collaborative effort by the Centers for Disease Control and Prevention, the U.S.
Department of Agriculture, the U.S. Food and Drug Administration, and selected state health departments. FoodNet
conducts active surveillance for seven bacterial and two parasitic foodborne diseases within a defined population of
20.5 million Americans (6). Additional surveys conducted within the FoodNet catchment area provide information on the
frequency of diarrhea in the general population, the proportion of ill persons seeking care, and the frequency of stool
culturing by physicians and laboratories for selected foodborne pathogens.

The National Notifiable Disease Surveillance System (7) and the Public Health Laboratory Information System (8) collect
passive national surveillance data for a wide range of diseases reported by physicians and laboratories. The Gulf Coast
States Vibrio Surveillance System collects reports of Vibrio infections from selected states (9), and the Foodborne
Disease Outbreak Surveillance System receives data from all states on recognized foodborne illness outbreaks (defined
as two or more cases of a similar illness resulting from ingestion of a common food) (10).

As components of the National Health Care Survey, the National Ambulatory Medical Care Survey and the National
Hospital Ambulatory Medical Care Survey measure health care use in various clinical settings, including physician
offices and hospital emergency and outpatient departments (11-14). These surveys collect information on patient
characteristics, patient symptoms or reasons for visit, provider diagnosis, and whether the patient was hospitalized. Up
to three symptoms are recorded using a standard classification (17), and up to three provider diagnoses are recorded
according to the International Classification of Diseases, 9th Revision, Clinical Modifications (ICD-9-CM) [18] (Table 1).


The National Hospital Discharge Survey, another component of the National
Health Care Survey, is a representative annual sample of discharge records
from approximately 475 nonfederal short-stay hospitals (15). The information
collected includes up to seven principal discharge diagnoses classified by
ICD-9-CM codes (18). Because these data include information on condition
at discharge, they can be used as a source of information on in-hospital
deaths. Additional information on food-related deaths was obtained from the
National Vital Statistics System, which collects death certificate data on causes
of death classified by 3- or 4-digit ICD-9 codes (16).

In addition to information from these formal surveillance systems, we used
data from two published population-based studies. The Tecumseh study was
conducted from 1965 through 1971 in 850 households in Tecumseh,
Michigan, with an emphasis on households with young children (19).
Households were telephoned weekly to identify incident cases of self-defined
diarrhea, vomiting, nausea, or stomach upset. The Cleveland study was
conducted among a selected group of 86 families followed from 1948 through
1957 (20). A family member recorded occurrences of gastrointestinal
illnesses and associated symptoms on a monthly tally sheet. Both studies also
collected information on extraintestinal illnesses (e.g., respiratory illness).
Other studies with similar designs were not included in our analysis, either
because they were relatively small or because they did not provide
information on the desired endpoints.

The Study

Food-Related Illness and Death from Known Pathogens

Total Cases
To estimate the total number of foodborne illnesses caused by known
pathogens, we determined the number of reported cases for each pathogen,
adjusted the figures to account for underreporting, and estimated the
proportion of illnesses specifically attributable to foodborne transmission.
Although data from various periods were used, adjustments for changes in
population size had minimal effect on the final estimates and were therefore
omitted.

Cases may be reported in association with documented foodborne outbreaks,
through passive surveillance systems (e.g., the National Notifiable Disease
Surveillance System, the Public Health Laboratory Information System), or
through active surveillance systems (e.g., FoodNet).
Sporadic illness
caused by some pathogens (e.g., Bacillus cereus,
Clostridium
perfringens, Staphylococcus aureus) is not reportable through
passive or active systems; hence, the only cases reported are those related to outbreaks.
For these
pathogens, we have assumed that if diagnosed sporadic cases were reported, the total number would be 10 times the
number of outbreak-related cases. This multiplier is based on experience with pathogens for which data are available on
both sporadic and outbreak-associated cases (e.g., reported cases of Salmonella or Shigella, Table 2). For all
pathogens, the number of outbreak-related cases was calculated as the average annual number of such cases reported
to CDC from 1983 to 1992, the most recent years for which published outbreak data are available. For pathogens also
under passive surveillance, we used the average number of cases reported to CDC from 1992 through 1997, and for
pathogens under active surveillance through FoodNet, we used the average rate observed for the surveillance
population from 1996 to 1997 and applied this to the total 1997 U.S. population (with some modification for E. coli O157:
H7; Appendix).



Table 2. Reported and estimated
a  illnesses, frequency of foodborne transmission, and hospitalization and case-fatality
rates for known foodborne pathogens, United States
-------------------------------------------------------------------------------------------------------------------------------------------------
                                                                       Reported Cases
                                    Estimated                by Surveillance Type
                                     total
cases          

%   Foodborne transmission  Hospital-
ization
rate
Case-
fatality
rate

by Surveillance Type


--------------------------------------------------------------------------------

Disease or Agent
Active
Passive
Outbreak


--------------------------------------------------------------------------------

Bacterial
          
Bacillus cereus
27,360
720
72
100
0.006
0.0000

Botulism, foodborne
58
29
100
0.800
0.0769

Brucella spp.
1,554
111
50
0.550
0.0500

Campylobacter spp
2,453,926
64,577
37,496
146
80
0.102
0.0010

Clostridium perfringens
248,520
6,540
654
100
0.003
0.0005

Escherichia coli O157:H7
73,480
3,674
2,725
500
85
0.295
0.0083

E. coli, non-O157 STEC
36,740
1,837
85
0.295
0.0083

E. coli, enterotoxigenic
79,420
2,090
209
70
0.005
0.0001

E. coli, other diarrheogenic
79,420
2,090
30
0.005
0.0001

Listeria monocytogenes
2,518
1,259
373
99
0.922
0.2000

Salmonella Typhib
824
412
80
0.750
0.0040

Salmonella, nontyphoidal
1,412,498
37,171
37,842
3,640
95
0.221
0.0078

Shigella spp.
448,240
22,412
17,324
1,476
20
0.139
0.0016

Staphylococcus food
185,060
4,870
487
100
0.180
0.0002

poisoning
          
Streptococcus, foodborne
50,920
1,340
134
100
0.133
0.0000

Vibrio cholerae, toxigenic
54
27
90
0.340
0.0060

V. vulnificus
94
47
50
0.910
0.3900

Vibrio, other
7,880
393
112
65
0.126
0.0250

Yersinia enterocolitica
96,368
2,536
90
0.242
0.0005

Subtotal
5,204,934
        
     
Parasitic
          
Cryptosporidium parvum
300,000
6,630
2,788
10
0.150
0.005

Cyclospora cayetanensis
16,264
428
98
90
0.020
0.0005

Giardia lamblia
2,000,000
107,000
22,907
10
n/a
n/a

Toxoplasma gondii
225,000
15,000
50
n/a
n/a

Trichinella spiralis
52
26
100
0.081
0.003

Subtotal
2,541,316
        
     
Viral
          
Norwalk-like viruses
23,000,000
  40
n/a
n/a

Rotavirus
3,900,000
  1
n/a
n/a

Astrovirus
3,900,000
  1
n/a
n/a

Hepatitis A
83,391
27,797
5
0.130
0.0030

Subtotal
30,883,391
        
Grand Total
38,629,641
        

--------------------------------------------------------------------------------

aNumbers in italics are estimates; others are measured.

b>70% of cases acquired abroad.



Irrespective of the surveillance system, many cases of foodborne illness are not reported because the ill person does
not seek medical care, the health-care provider does not obtain a specimen for diagnosis, the laboratory does not
perform the necessary diagnostic test, or the illness or laboratory findings are not communicated to public health
officials. Therefore, to calculate the total number of illnesses caused by each pathogen, it is necessary to account for
underreporting, i.e., the difference between the number of reported cases and the number of cases that actually occur
in the community. For Salmonella, a pathogen that typically causes nonbloody diarrhea, the degree of underreporting
has been estimated at ~38 fold (Voetsch, manuscript in preparation) (21). For E. coli O157:H7, a pathogen that typically
causes bloody diarrhea, the degree of underreporting has been estimated at ~20 fold (22). Because similar information
is not available for most other pathogens, we used a factor of 38 for pathogens that cause primarily nonbloody diarrhea
(e.g., Salmonella, Campylobacter) and 20 for pathogens that cause bloody diarrhea (e.g., E. coli O157:H7, Shigella).
For pathogens that typically cause severe illness (i.e., Clostridium botulinum, Listeria monocytogenes), we arbitrarily
used a far lower multiplier of 2, on the assumption that most cases come to medical attention. Details of the calculations
for each specific pathogen and rationale are provided in the Appendix. Where information from both active and passive
reporting was available, we used the figure from active surveillance when estimating the total number of cases.

Having estimated the number of cases caused by each pathogen, the final step was to estimate for each the percentage
of illness attributable to foodborne transmission. The total number of cases was then multiplied by this percentage to
derive the total number of illnesses attributable to foodborne transmission. The rationale for each estimate is presented
in the Appendix; although precise percentages are generally difficult to justify, in most instances there is ample support
for the approximate value used.

Results are presented in Tables 2 and 3. Known pathogens account for an estimated 38.6 million illnesses each year,
including 5.2 million (13%) due to bacteria, 2.5 million (7%) due to parasites, and 30.9 million (80%) due to viruses
(Table 2). Overall, foodborne transmission accounts for 13.8 million of the 38.6 million illnesses (Table 3). Excluding
illness caused by Listeria, Toxoplasma, and hepatitis A virus (three pathogens that typically cause nongastrointestinal
illness), 38.3 million cases of acute gastroenteritis are caused by known pathogens, and 13.6 million (36%) of these are
attributable to foodborne transmission. Among all illnesses attributable to foodborne transmission, 30% are caused by
bacteria, 3% by parasites, and 67% by viruses.

Hospitalizations
To estimate the number of hospitalizations due to foodborne transmission, we calculated for each pathogen the
expected number of hospitalizations among reported cases by multiplying the number of reported cases by pathogen-
specific hospitalization rates from FoodNet data (23, 24), reported outbreaks (10, 25), or other published studies
(Appendix). Not all illnesses resulting in hospitalization are diagnosed or reported. Health-care providers may not order
the necessary diagnostic tests, patients may have already taken antibiotics that interfere with diagnostic testing, or the
condition leading to hospitalization may be a sequela that develops well after resolution of the actual infection (e.g.,
Campylobacter-associated Guillain-Barré syndrome). Therefore, to account for underreporting, we doubled the number
of hospitalizations among reported cases to derive for each pathogen an estimate of the total number of
hospitalizations. Finally, we multiplied this figure by the proportion of infections attributable to foodborne transmission.
Because of gaps in the available data, this approach could not be used for some parasitic and viral diseases (Appendix).

Overall, the pathogens listed in Table 2 cause an estimated 181,177 hospitalizations each year, of which 60,854 are
attributable to foodborne transmission (Table 3). Excluding hospitalizations for infection with Listeria, Toxoplasma, and
hepatitis A virus, 163,015 hospitalizations for acute gastroenteritis are caused by known pathogens, of which 55,512
(34%) are attributable to foodborne transmission. Overall, bacterial pathogens account for 60% of hospitalizations
attributable to foodborne transmission, parasites for 5%, and viruses for 34%.

Deaths
Like illnesses and hospitalizations, deaths are also underreported. Precise information on food-related deaths is
especially difficult to obtain because pathogen-specific surveillance systems rarely collect information on illness
outcome, and outcome-specific surveillance systems (e.g., death certificates) grossly underreport many pathogen-
specific conditions. To estimate the number of deaths due to bacterial pathogens, we used the same approach
described for hospitalizations: first calculating the number of deaths among reported cases, then doubling this figure to
account for unreported deaths, and finally multiplying by the percentage of infections attributable to foodborne
transmission. As with hospitalization, this approach could not be used for some parasitic and viral diseases.

Overall, the specified pathogens cause an estimated 2,718 deaths each year, of which 1,809 are attributable to
foodborne transmission (Table 3). Excluding death due to Listeria, Toxoplasma, and hepatitis A virus, the number of
deaths due to pathogens that cause acute gastroenteritis is 1,381, of which 931 (67%) are attributable to foodborne
transmission. Bacteria account for 72% of deaths associated with foodborne transmission, parasites for 21%, and
viruses for 7%. Five pathogens account for over 90% of estimated food-related deaths: Salmonella (31%), Listeria
(28%), Toxoplasma (21%), Norwalk-like viruses (7%), Campylobacter (5%), and E. coli O157:H7 (3%).

Table 3. Estimated illnesses, hospitalizations, and deaths caused by known foodborne pathogens, United States


--------------------------------------------------------------------------------

Illnesses  Hospitalizations
Deaths


--------------------------------------------------------------------------------

--------------------------------------------------------------------------------

--------------------------------------------------------------------------------

Food-
borne
% of total
foodborne
Food-
borne
% of total foodborne
Food-
borne
% of total
foodborne

Disease or agent
Total
Total
Total


--------------------------------------------------------------------------------

Bacterial
              
Bacillus cereus
27,360
27,360
0.2
8
8
0.0
0
0
0.0

Botulism, foodborne
58
58
0.0
46
46
0.1
4
4
0.2

Brucella spp.
1,554
777
0.0
122
61
0.1
11
6
0.3

Campylobacter spp.
2,453,926
1,963,141
14.2
13,174
10,539
17.3
124
99
5.5

Clostridium perfringens
248,520
248,520
1.8
41
41
0.1
7
7
0.4

Escherichia coli O157:H7
73,480
62,458
0.5
2,168
1,843
3.0
61
52
2.9

E. coli, non-O157 STEC
36,740
31,229
0.2
1,084
921
1.5
30
26
1.4

E. coli, enterotoxigenic
79,420
55,594
0.4
21
15
0.0
0
0
0.0

E. coli, other diarrheogenic
79,420
23,826
0.2
21
6
0.0
0
0
0.0

Listeria monocytogenes
2,518
2,493
0.0
2,322
2,298
3.8
504
499
27.6

Salmonella typhi
824
659
0.0
618
494
0.8
3
3
0.1

Salmonella, nontyphoidal
1,412,498
1,341,873
9.7
16,430
15,608
25.6
582
553
30.6

Shigella spp.
448,240
89,648
0.6
6,231
1,246
2.0
70
14
0.8

Staphylococcus food poisoning
185,060 185,060
1.3
1,753
1,753
2.9
2
2
0.1

Streptococcus, foodborne
50,920
50,920
0.4
358
358
0.6
0
0
0.0

Vibrio cholerae, toxigenic
54
49
0.0
18
17
0.0
0
0
0.0

V. vulnificus
94
47
0.0
86
43
0.1
37
18
1.0

Vibrio, other
7,880
5,122
0.0
99
65
0.1
20
13
0.7

Yersinia enterocolitica
96,368
86,731
0.6
1,228
1,105
1.8
3
2
0.1

Subtotal
5,204,934
4,175,565
30.2
45,826
36,466
59.9
1,458
1,297
71.7

       
Parasitic
              
Cryptosporidium parvum
300,000
30,000
0.2
1,989
199
0.3
66
7
0.4

Cyclospora cayetanensis
16,264
14,638
0.1
17
15
0.0
0
0
0.0

Giardia lamblia
2,000,000
200,000
1.4
5,000
500
0.8
10
1
0.1

Toxoplasma gondii
225,000
112,500
0.8
5,000
2,500
4.1
750
375
20.7

Trichinella spiralis
52
52
0.0
4
4
0.0
0
0
0.0

Subtotal
2,541,316
357,190
2.6
12,010
3,219
5.3
827
383
21.2

        
Viral
              
Norwalk-like viruses
23,000,000
9,200,000
66.6
50,000
20,000
32.9
310
124
6.9

Rotavirus
3,900,000
39,000
0.3
50,000
500
0.8
30
0
0.0

Astrovirus
3,900,000
39,000
0.3
12,500
125
0.2
10
0
0.0

Hepatitis A
83,391
4,170
0.0
10,841
90
0.9
83
4
0.2

Subtotal
30,833,391
9,282,170
67.2
123,341
21,167
34.8
433
129
7.1

Grand Total
38,629,641
13,814,924
100.0
181,177
60,854
100.0
2,718
1,809
100.0


--------------------------------------------------------------------------------


Food-Related Illness and Death from Unknown Pathogens
Some proportion of gastrointestinal illness is caused by foodborne agents not yet identified. This conclusion is
supported by well-documented foodborne outbreaks of distinctive illness for which the causative agent remains unknown
(e.g., Brainerd diarrhea) (26), by the large percentage of foodborne outbreaks reported to CDC for which no pathogen
is identified (25), and by the large number of new foodborne pathogens identified in recent years.

To estimate food-related illness and death from unknown pathogens, we used symptom-based data to estimate the total
number of acute gastrointestinal illnesses and then subtracted from this total the number of cases accounted for by
known pathogens; this difference represents the illness due to acute gastroenteritis of unknown etiology. To determine
how much of this illness was due to foodborne transmission, we used the percentages of foodborne transmission as
determined above for acute gastroenteritis caused by known pathogens.

Total Cases
To determine the rate of acute gastroenteritis in the general population, we used data on the frequency of diarrhea from
the 1996 to 1997 FoodNet population survey. This survey did not collect data on the rate of vomiting among persons
without diarrhea, however, so we relied on the Tecumseh and Cleveland studies for information on the frequency of this
symptom. Because young children were overrepresented in the Tecumseh and Cleveland studies relative to the current
U.S. population, rates of illness for these studies were age-adjusted. For the Tecumseh data, we used the reported age-
and symptom-specific rates. For the Cleveland study, we used the method described by Garthright (27) to derive an
overall age-adjusted rate of gastrointestinal illness; we then multiplied this rate by the relative frequency of symptoms to
derive age-adjusted rates for specific symptoms.

In the 1996-97 FoodNet population survey, the overall rate of diarrhea was 1.4 episodes per person per year, and the
rate of diarrheal illness, defined as diarrhea ( 3 loose stools per 24-hour period) lasting >1 day or interfering with normal
activities, was 0.75 episodes per person per year (H. Herikstad, manuscript in preparation). We used the lower 0.75 rate
for our analysis. To this we added the average age-adjusted rate of vomiting without diarrhea from the Tecumseh and
Cleveland studies (0.30, Table 4) to derive an overall estimate of 1.05 episodes per person per year of acute
gastrointestinal illness characterized by diarrhea, vomiting, or both.

Table 4. Frequency of gastrointestinal illness in the general population, in episodes per person per year, as determined
by three studies


--------------------------------------------------------------------------------

FoodNet Population Survey
Tecumseh Study
Cleveland Study  

--------------------------------------------------------------------------------

--------------------------------------------------------------------------------

--------------------------------------------------------------------------------

Symptom
Age adjusted
Crude
Age adjusted
Crude
Age adjusted


--------------------------------------------------------------------------------

Diarrhea or vomiting
--
0.98
0.81
1.28
0.87

Diarrhea, any
0.75
0.63
0.52
0.83
0.56

Without vomiting
0.61
0.40
0.33
0.48
0.33

With vomiting
0.14
0.23
0.19
0.35
0.23

Vomiting without diarrhea
--
0.35
0.29
0.45
0.31


--------------------------------------------------------------------------------


Previous studies have shown that some cases of acute gastrointestinal illness are accompanied by respiratory
symptoms; although the causes of these illnesses are generally unknown, such cases have traditionally been attributed
to respiratory pathogens (20,27). Data on the frequency of concomitant respiratory symptoms were not collected in the
1996-97 FoodNet survey but were 20% to 27% among patients with acute gastroenteritis in the Tecumseh and
Cleveland studies. Therefore, we adjusted downward our estimate of acute gastroenteritis by 25%, yielding a final
estimate of 0.79 (1.05 X 0.75) episodes of acute gastroenteritis per person per year. Extrapolated to a population of
267.7 million persons, the U.S. resident population in 1997 (28), this rate is equivalent to 211 million episodes each year
in the United States.

As determined previously, 38.3 million of these 211 million episodes of acute gastroenteritis are attributable to known
pathogens. A small proportion of the remaining 173 million episodes can be accounted for by known, noninfectious
agents (e.g., mycotoxins, marine biotoxins); however, most are attributable to unknown agents. Because we cannot
directly ascertain how many of these illnesses of unknown etiology are due to foodborne transmission, we used the
relative frequency of foodborne transmission for known pathogens as a guide. For illnesses of known etiology,
foodborne transmission accounts for 36% of total cases. Applying this percentage yields an estimate of 62 million cases
of acute gastroenteritis of unknown etiology (36% of 173 million) due to foodborne transmission each year.

Hospitalizations
The National Ambulatory Medical Care Survey/the National Hospital Ambulatory Medical Care Survey data were
searched for visits due to symptoms of diarrhea, vomiting, or gastrointestinal infection (reason for visit classification
(RVC) codes 1595, 1530, 1540)(17) and for visits resulting in a diagnosis of infectious enteritis (ICD-9-CM codes 001-
009.3; Table 1). Visits associated with respiratory symptoms (RVC codes 1400-1499) or a diagnosis of influenza (ICD-9-
CM code 487) were excluded. Data for the years 1992 to 1996 were combined before analysis. Overall, these criteria
yielded an average of 15,810,905 visits annually from 1992 through 1996, of which an average of 1,246,763, or 7.9%,
resulted in hospitalization. This figure is equivalent to a rate of 4.7 hospitalizations per 1,000 person-years.

The National Hospital Discharge Survey data were searched by using diagnostic codes for infectious gastroenteritis of
known cause (ICD-9-CM codes 001-008; Table 1), with the exception of the code for Clostridium difficile colitis (ICD9
008.45), a common form of nosocomially acquired diarrhea. In addition, we included the nonspecific ICD-9-CM diagnosis
codes 009 (infectious gastroenteritis) and 558.9 (other and unspecified noninfectious gastroenteritis and colitis).
Despite the description, many of the illnesses attributed to ICD-9-CM code 558.9 are likely to be either infectious or due
to agents possibly transmitted by food. For example, in the absence of laboratory testing, sporadic cases of viral
gastroenteritis may be coded as 558.9. Under the previous ICD-8 classification, these same cases would have been
assumed to be infectious and coded as 009 (29, 30). Data for the years 1992 to 1996 were weighted according to
National Center for Health Statistics criteria and averaged to derive national estimates of annual hospitalizations.
Records with a diagnosis of respiratory illness were not excluded because of the high incidence of respiratory infections
among hospitalized patients.

Considering all listed diagnoses, the National Hospital Discharge Survey data for the years 1992 to 1996 yielded an
annual average of 616,337 hospital discharges with a diagnosis of gastrointestinal illness. Included in this figure are
193,084 cases of gastroenteritis with an identified pathogen and an additional 423,293 cases of gastroenteritis of
unknown etiology (Table 5). Converted to a rate, the total number is equivalent to 2.3 hospitalizations per 1,000 person-
years. Because these data depend on the recording of a diagnosis and not just a symptom, it is likely that they
underestimate the rate of hospitalization for acute gastroenteritis. This view is supported by FoodNet population survey
data indicating a rate of approximately 7.2 hospitalizations per 1,000 person-years for diarrheal illness (H. Herikstad,
manuscript in preparation). These data were not included here because they omit hospitalizations for vomiting alone
and are not easily adjusted for concomitant respiratory symptoms. Averaging the rates from the National Ambulatory
Medical Care Survey/National Hospital Ambulatory Medical Care Survey and National Hospital Discharge Survey yields a
final estimate of 3.5 hospitalizations per 1,000 person-years, equivalent to 936,726 hospitalizations annually for acute
gastroenteritis. As noted previously, 163,153 of these hospitalizations can be attributed to known causes of acute
gastroenteritis, yielding an estimated 773,573 hospitalizations for acute gastroenteritis caused by unknown agents.
Applying the relative frequency of foodborne transmission as determined for known pathogens yields an estimated
263,015 hospitalizations (34% of 773,573) for acute gastroenteritis due to foodborne transmission of unknown agents.

Table 5. Average annual hospitalizations and deaths for gastrointestinal illness by diagnostic category, National Hospital
Discharge Survey, 1992–1996
--------------------------------------------------------------------------------------------------------------------------------------------
                                                                                                 1st diagnosis                        All diagnoses
                                                                                    -----------------------------------         ------------------------------------
Cause of enteritisa                                                    Hospitalizations         Deaths              Hospitalizations    Deaths
-----------------------------------------------------------------------------------------------------------------------------------------------------
Bacterial (001-005, 008-008.5)                                    27,987                      148b                    54,953             1,139

Viral (008.6-008.8)                                                       82,149                          0b                  132,332                194b

Parasitic (006-007)                                                        2,806                          82b                    5,799                 127b

Unknown etiology (009, 558.9)                                  186,537                         868b                423,293              5,148

Total                                                                          299,479                       1,898                 616,377              6,608


--------------------------------------------------------------------------------

aICD-9-CM code.

bEstimate unreliable due to small sample size.


Deaths
Multiple-cause-of-death data (16) and information on in-hospital-death data (National Hospital Discharge Survey) were
used. ICD-9-CM codes 001-008 were employed to identify deaths due to diagnosed infectious gastroenteritis and ICD-9-
CM codes 009 and 558 to identify deaths due to gastroenteritis of unknown etiology.

Death certificate data for the years 1992 to 1996 yielded an annual average of 6,195 total deaths, of which 1,432 (23%)
were due to specific causes of gastroenteritis and 4,763 (77%) to undiagnosed causes of gastroenteritis. For the same
years and ICD-9-CM codes, the average annual in-hospital deaths for all-listed diagnoses totaled 6,608, of which 1,460
were due to specific and 5,148 (77%) undiagnosed causes of gastroenteritis (Table 5). Averaging the totals for all
causes from death certificate and National Hospital Discharge Survey data and adjusting to the 1997 U.S. census
estimates, we estimated that gastroenteritis contributed to the death of 6,402 persons in the United States in 1997.

A total of 1,386 of these deaths can be explained by known causes of acute gastroenteritis (see above). Thus an
estimated 5,016 deaths from acute gastroenteritis are caused by unknown agents. Applying the relative frequency of
foodborne transmission as determined for known pathogens yields an estimated 3,360 deaths (67% of 5,016) due to
acute gastroenteritis caused by foodborne transmission of unknown agents.

Overall Food-Related Illness and Death
Figure 1  

Click to view enlarged
image
Figure: Estimated frequency of foodborne illness in the United States.  
We summed illness attributable to foodborne gastroenteritis caused by known and unknown pathogens, yielding an
estimate of 76 million illnesses, 318,574 hospitalizations, and 4,316 deaths. Adding to these figures the
nongastrointestinal illness caused by Listeria, Toxoplasma, and hepatitis A virus, we arrived at a final national estimate
of 76 million illnesses, 323,914 hospitalizations, and 5,194 deaths each year (Figure 1).

Conclusions
The nature of food and foodborne illness has changed dramatically in the United States over the last century. While
technological advances such as pasteurization and proper canning have all but eliminated some disease, new causes of
foodborne illness have been identified. Researchers have used various methods to estimate the illnesses and deaths
due to foodborne diseases in the United States. In 1985, Archer and Kvenberg coupled information on underreporting of
salmonellosis with data on other foodborne pathogens to derive estimates of 8.9 million illnesses due to known
pathogens and 24 million to 81 million illnesses due to all foodborne agents (2). In 1987, Bennett et al. computed
incidence figures for all known infectious diseases and determined the proportion of each due to various modes of
transmission. Summing these figures, they concluded that foodborne transmission of known pathogens caused 6.5
million illnesses and up to 9,000 deaths each year (3). In 1989, Todd used a combination of methods, including
extrapolation from Canadian surveillance data, to derive an estimate of 12.5 million foodborne illnesses and 522 related
deaths each year (4). Finally, in 1994, a task force convened by the Council for Agricultural Science and Technology
(CAST) reviewed available studies and estimated the overall number of food-related illnesses at 33 million cases per
year (5). These various estimates often refer to different entities. The estimates of 6.5 million and 8.9 million refer to
illness caused by known pathogens, whereas the estimate of 33 million refers to all causes of foodborne illnesses,
known and unknown, infectious and noninfectious.

Our estimates are based on data from a wide variety of sources and differ from previous estimates in several respects.
For known pathogens, our estimate of 13.8 million illnesses per year is substantially higher than the previous estimates
of 6.5 million and 8.9 million (2, 3), an increase attributable largely to our inclusion of foodborne illness caused by
Norwalk-like viruses. For foodborne illness of all etiologies, our estimate of 76 million illnesses is within the range
proposed by Archer and Kvenberg (2) but considerably higher than the point estimate of 33 million presented in the
CAST report (5). Both our estimate and the CAST estimate assume that foodborne transmission accounts for ~35% of
acute gastroenteritis cases caused by unknown agents. The disparity between the two stems from differences in the
estimated annual frequency of acute gastroenteritis overall: 211 million cases for our estimate, 99 million for the CAST
estimate.

Whereas our estimates of illness are generally higher than those of previous studies, our estimates of death are
generally lower. We estimate that foodborne illness causes 5,020 deaths annually (1,810 deaths due to known
pathogens and 3,210 deaths due to unknown agents), a total that is slightly more than half the 9,000 deaths estimated
by Bennett et al. (3). The Bennett estimate includes 2,100 deaths due to campylobacteriosis, 1,200 deaths due to
staphylococcal food poisoning, and 1,000 deaths due to trichinosis: our total for all three of these diseases is 101
deaths. Our estimated case-fatality rates for several other diseases are also lower than those used in the Bennett
report, either because better data are available or perhaps because treatment has improved.

Our analysis suggests that unknown agents account for approximately 81% of foodborne illnesses and hospitalizations
and 64% of deaths. Among cases of foodborne illness due to known agents, Norwalk-like viruses account for over 67%
of all cases, 33% of hospitalizations, and 7% of deaths. The assumptions underlying the Norwalk-like viruses figures are
among the most difficult to verify, and these percentages should be interpreted with caution (Appendix). Other important
causes of severe illness are Salmonella and Campylobacter, accounting for 26% and 17% of hospitalizations,
respectively. The leading causes of death are Salmonella, Listeria, and Toxoplasma, which together account for 1,427,
or more than 75% of foodborne deaths caused by known pathogens. Many of the deaths due to toxoplasmosis occur in
HIV-infected patients; recent advances in HIV treatment may greatly reduce deaths due to toxoplasmosis.

Of necessity, our analysis entails a number of assumptions. The first major assumption concerns the degree of
underreporting. Well-documented estimates of underreporting are not available for most pathogens; therefore, we relied
on multipliers derived for salmonellosis and other diseases. For salmonellosis, the multiplier of 38 has been
independently derived by investigators in the United States using different data sources. The U.S. figure is five to tenfold
higher than multipliers for Salmonella and Campylobacter recently derived in Great Britain (31). However, this difference
is nearly or wholly offset by far higher per capita rates of reported infections in Great Britain. Nevertheless, when
extrapolated to other pathogens, these multipliers may result in under- or overestimates, and clearly studies such as
those conducted for Salmonella are needed to develop better multipliers for these other diseases. However, in our
analysis, changing the multipliers for individual diseases has a minimal effect on the overall estimate of foodborne illness.

Our second set of assumptions concerns the frequency of foodborne transmission for individual pathogens. We have
used published studies when available, but these are rare. As with underreporting multipliers, errors affect estimates for
individual pathogens but have minimal effect on the estimate of overall illness and death from foodborne diseases. The
one notable exception is the estimate for Norwalk-like viruses. Because these viruses account for an especially large
number of illnesses, changes in the percentage attributed to foodborne transmission have a major effect on our overall
estimates. For example, if the actual number of infections due to foodborne transmission were 30% rather than 40%, the
overall estimate would decrease from 76 million to 63 million illnesses per year. Interestingly, our overall estimate is
influenced far less by the Norwalk-like virus case estimate itself. It would require a 100-fold reduction in the estimated
number of Norwalk-like virus cases to reduce the overall estimate from 76 million to 63 million.

A third assumption concerns the frequency of acute gastroenteritis in the general population. The rate we used is based
in part on recent data from the FoodNet population survey, a retrospective survey involving more than 9,000
households. The overall rate of diarrhea as recorded by the survey was 1.4 episodes per person per year; however, we
used the survey's far lower rate of 0.75 episodes of diarrheal illness per person per year. Furthermore, we limited our
definition of acute gastroenteritis to symptoms of diarrhea or vomiting and reduced the rate to account for concomitant
respiratory symptoms. As a result, our final assumed rate of 0.79 episodes of acute gastroenteritis per person per year
is very similar to respiratory-adjusted estimates derived from the prospectively conducted Tecumseh (0.74) and
Cleveland (0.71) studies (27). All three studies are based on household surveys, and thus the rates of illness are not
influenced by changes in health-care delivery. Compared with rates of diarrheal illness from studies conducted in Great
Britain, our estimated rate is higher than in one recent study (31) but lower than another (32).

In addition to these assumptions, our analysis has several limitations. Differences in available surveillance information
prevented us from using the same method to estimate illness and death from bacterial, parasitic, and viral pathogens.
Furthermore, because of a paucity of surveillance information, we did not include specific estimates for some known,
occasionally foodborne pathogens (e.g., Plesiomonas, Aeromonas, or Edwardsiella), nor did we develop specific
estimates for known noninfectious agents, such as mushroom or marine biotoxins, metals, and other inorganic toxins.
However, many of these agents cause gastroenteritis and are therefore captured in our overall estimate of foodborne
illness. With the exception of a few important pathogens (Appendix), we have not estimated the number of cases of
chronic sequelae, although these may be part of the overall burden of foodborne diseases. Finally, future research will
refine our assumptions and allow for more precise estimates.

Methodologic differences between our analysis and previously published studies make it difficult to draw firm
conclusions regarding overall trends in the incidence of foodborne illness. In general, the differences between our
estimates and previously published figures appear to be due primarily to the availability of better information and new
analyses rather than real changes in disease frequency over time. For example, E. coli O157:H7 was estimated to cause
10,000 to 20,000 illnesses annually, based on studies of patients visiting a physician for diarrhea. Recent FoodNet data
have allowed a more detailed estimation of mild illnesses not resulting in physician consultation. Our estimate of nearly
74,000 illnesses per year incorporates these milder illnesses and should not be misconstrued as demonstrating a recent
increase in E. coli O157:H7 infections. Whatever the limitations on retrospective comparisons, the estimates presented
here provide a more reliable benchmark with which to judge the effectiveness of ongoing and future prevention efforts.

Further refinements of foodborne disease estimates will require continued and improved active surveillance. Beginning
in 1998, the FoodNet population survey was modified to capture cases of vomiting not associated with diarrhea; further
enhancement to capture concomitant respiratory symptoms should refine the FoodNet survey data. Expansion of
laboratory diagnostic capacity could lead to better detection of certain pathogens, estimates of the degree of
underreporting for additional diseases, and estimates of the proportion of specific diseases transmitted through food.
Heightened surveillance for acute, noninfectious foodborne diseases, such as mushroom poisoning and other illnesses
caused by biotoxins, could further improve estimates of illness and death from foodborne illness. Emergency
department-based surveillance systems (33) or poison control center-based surveillance might provide such
information. Finally, identifying new causes of enteric illness and defining the public health importance of known agents
(e.g., enteroaggregative E. coli) would improve foodborne disease prevention efforts.

Appendix
Methods, assumptions, and references for pathogen-specific estimates

Bacterial Pathogens

Pathogen: Bacillus cereus

Reported cases: Cases not routinely reported. Because it is a mild illness, reported cases assumed to be 10 times the
average annual number of outbreak-related cases reported to CDC, 1983-1992 (10,25).

Total cases: Assumed to be 38 times the number of reported cases by extrapolation from studies of salmonellosis.

Hospitalization rate: Determined from outbreaks reported to CDC, 1982-1992 (10,25) and (CDC, unpub. data).

Case-fatality rates: Determined from outbreaks reported to CDC, 1982-1992 (10,25), including those associated with
nursing homes (34).

Percent foodborne: Although infection occasionally occurs through other routes, case estimates presented are based
on foodborne outbreaks and are therefore assumed to reflect only foodborne transmission.

Pathogen: Clostridium botulinum

Reported cases: Average annual number of cases of foodborne botulism reported to CDC, 1992-1997 (7).

Total cases: Because it is a severe illness, assumed to be two times the number of reported cases.

Hospitalization rate: Determined from outbreaks reported to CDC, 1982-1992 (10,25) and (CDC, unpub. data).

Case-fatality rate: Based on outbreaks reported to CDC, 1982-1992 (10,25).

Percent foodborne: 100% by definition.

Pathogen: Brucella spp.

Reported cases: Average annual number of cases reported

to CDC, 1992-1997 (7).

Total cases: Assumed to be 14 times reported cases, based on published estimates that 4% to 10% of cases are
reported (35).

Hospitalization rate: Determined from outbreaks reported to CDC, 1982-1992 (10,25) and (CDC, unpub. data).

Case-fatality rate: Historically 2% to 5% (36)

Percent foodborne: Overall, consumption of milk or cheese products from Mexico implicated in 45% of cases reported
from California from 1973 to 1992 (37). Because the proportion of cases due to foodborne transmission was higher in
the latter half of this period, we assumed that currently 50% of cases are foodborne.

Comments: Reports from California or Texas account for most of cases in recent years.

Pathogen: Campylobacter spp.

Reported cases: Outbreak-related cases based on reports to CDC, 1983-1992 (10,25). Passive surveillance estimate
based on average number of cases reported to CDC, 1992-1994 (CDC, unpub data). Active surveillance estimate
based on extrapolation of average 1996-1997 FoodNet rate (24.1 cases per 100,000 population) to 1997 U.S.
population (23).

Total cases: Assumed to be 38 times the number of reported cases, based on studies of salmonellosis. Resulting
estimate is roughly comparable with midpoint rate estimate from Tauxe (38) for C. jejuni (1,020 cases per 100,000
population), applied to 1997 population. Assumes minimal contribution from non-jejuni Campylobacter.

Hospitalization rate: Based on hospitalization rate for culture-confirmed cases reported to FoodNet, 1996-1997 (23,24).

Case-fatality rate: Based on case-fatality rate for culture-confirmed cases reported to FoodNet, 1996-1997 (23,24).

Percent foodborne: Although waterborne outbreaks occur, foodborne transmission accounts for most of the sporadic
cases (38).

Comments: Guillain-Barré syndrome (GBS) is an acute flaccid paralysis that can occur several weeks after infection with
various agents, including Campylobacter. The incidence of GBS has been estimated at 1.7 cases per 100,000
population, and serologic studies suggest that ~30% of patients with GBS have evidence of recent infection with
Campylobacter (39). Based on these figures, we estimate that ~1,360 cases of Campylobacter-associated GBS
occurred in the United States in 1997.

Pathogen: Clostridium perfringens

Reported cases: Cases not routinely reported. Because it is a mild illness, number of reported cases assumed to be 10
times the average annual number of outbreak-related cases reported to CDC, 1983-1992 (10,25).

Total cases: Assumed to be 38 times the number of reported cases, by extrapolation from studies of salmonellosis.

Hospitalization rate: Determined from outbreaks reported to CDC, 1982-1992 (10,25) and (CDC, unpub. data).

Case-fatality rate: Based on reported outbreaks, 1983-1992 (10,25).

Percent foodborne: 100% (40). Case estimates presented are based on foodborne outbreaks and therefore reflect
foodborne transmission of C. perfringens, type A.

Pathogen: Escherichia coli O157:H7

Reported cases: Passive surveillance estimate based on average number of cases reported to CDC through the
National Electronic Telecommunications System for Surveillance (NETSS), 1995-1998; data from the Public Health
Laboratory Information System (PHLIS) were used for those states not reporting to NETSS during this time period (7).
Passive surveillance data for 1998 are provisional. Active surveillance estimate based on an extrapolation of a weighted
average of the FoodNet rate for the years 1996-1997 to the 1997 U.S. population (23,24). A weighted average was
used because the overall FoodNet rate is disproportionately influenced by a high rate in a single northern state with a
relatively small population. Because the incidence of infection is thought to be generally higher in northern states (41),
we weighted the crude rate derived from FoodNet by the total population of each participating state. The weighted rate
(1.34 cases per 100,000 population) was used when extrapolating the FoodNet rate to the total U.S. population.

Total cases: Studies conducted in FoodNet sites suggest that 13-27 cases of E. coli O157:H7 infection occur in the
community for each confirmed case that is reported (22). To estimate total cases, we multiplied the number of reported
cases, as determined through active surveillance, by 20, the midpoint of this estimate.

Hospitalization rate: Based on the hospitalization rate for culture-confirmed cases reported to FoodNet, 1996-1997
(23,24).

Case-fatality rate: Case-fatality rate based on mortality associated with sporadic cases reported to FoodNet, 1996-1997
(23,24).

Percent foodborne: Based on outbreaks of known source reported to CDC, 1982-1997 (CDC, unpub. data). Person-to-
person transmission assumed to be secondary to foodborne transmission (2).

Comments: Our estimate of total cases is considerably higher than previous estimates based on patients seeking care
for diarrhea. Our estimate includes patients with far milder illness and should not be interpreted as indicating an
increase in incidence. Hemolytic uremic syndrome (HUS) occurs in ~4% of all reported cases. Based on our estimate of
total cases and active surveillance cases, between 2,954 and 147 patients are expected to contract HUS each year.

Pathogen: E. coli, Shiga toxin-producing serogroups other than O157 (STEC)

Reported cases: Cases not routinely reported; many clinical laboratories cannot identify.

Total cases: Assumed to be half as common as infection with E. coli O157:H7. Early studies suggest that the incidence
of non-O157 STEC infections is 20%-30% that of E. coli O157:H7 in North America (42, 43); however, more recent
studies using different techniques suggest that this figure should be 50% (44,45).

Hospitalization rate: Assumed to be comparable with E. coli O157:H7, but may be lower (46).

Case-fatality rate: Assumed to be comparable with E. coli O157:H7, but may be lower (46).

Percent foodborne: Assumed to be comparable with E. coli O157:H7.

Comment: Although non-O157 STEC can cause hemolytic uremic syndrome, the relative frequency of this complication
is unknown. Reports from Canada suggest that non-O157 STEC are the cause of at least 7% (47) and possibly as many
as 20% (48) of HUS cases.

Pathogen: E. coli, enterotoxigenic

Reported cases: Not routinely reported. Outbreak-related cases based on average for 18 outbreaks reported to CDC
from 1975 through 1997 (CDC, unpub. data). Reported cases assumed to be 10 times the number of outbreak-related
cases.

Total cases: Assumed to be 38 times the number of reported cases by extrapolation from studies of salmonellosis.

Hospitalization rate: Low; assumed to be 0.5% of cases.

Case-fatality rate: Serious illness is generally restricted to infants in developing countries. Based on experience with
reported outbreaks, assumed to be 1 in 10,000 cases in the United States.

Percent foodborne: Nearly all outbreaks reported to CDC from 1975 through 1997 have been foodborne (CDC, unpub.
data); many sporadic cases are associated with travel to other countries where both water and foodborne exposures are
likely.

Pathogen: E. coli, other diarrheogenic

Reported cases: Not routinely reported. Assumed to be at least as common as enterotoxigenic E. coli (ETEC) based on
limited information from studies in North America and Europe (49).

Total cases: Assumed equal to ETEC.

Hospitalization rate: Assumed equal to ETEC.

Case-fatality rate: Assumed equal to ETEC.

Percent foodborne: Very little data available. As few foodborne outbreaks have been reported, it is assumed that only
30% of cases are foodborne.

Comment: This category includes enteropathogenic, enteroaggregative, and enteroinvasive E. coli, as well as poorly
defined pathogenic groups (50). Although little is known about the incidence of these infections in the United States,
these pathogens have been linked to both outbreaks and sporadic illnesses. Limited studies suggest that the
importance of some of these organisms in the United States is seriously underestimated (see Nataro and Kaper [49]).
Although clearly a heterogeneous collection of organisms, we assume that these pathogens as a group have similar
modes of transmission and mortality rates as ETEC.

Pathogen: Listeria monocytogenes

Reported cases: Rates from FoodNet, 1996-1997, (23,24) and comparable sentinel site surveillance (51), extrapolated
to the 1997 U.S. population.

Total cases: Because it is a severe illness, assumed to be 2 times the number of reported cases.

Hospitalization rate: Based on hospitalization rate for culture-confirmed cases reported to FoodNet, 1996-1997 (23,24).

Case-fatality rate: Based on published reports (51), 1996-1997 FoodNet data (23,24), and recent outbreaks (CDC,
unpub. data).

Percent foodborne: Although foodborne transmission accounts for all reported domestic outbreaks (52), the potential for
nosocomial transmission has been demonstrated (53).

Comments: Figures include both perinatal and nonperinatal disease. FoodNet data on hospitalization indicate that
nearly 90% of reported cases result in hospitalization (24).

Pathogen: Salmonella Typhi

Reported cases: Average number of cases reported to CDC, 1992-1997 (7).

Total cases: Because it is a severe illness, assumed to be two times the number of reported cases.

Hospitalization rate: Rate of hospitalization based on published outbreak reports (54,55).

Case-fatality rate: Based on outcomes of 2,254 cases reviewed by Mermin (56).

Percent foodborne: Although waterborne outbreaks have been reported in the United States, foodborne transmission is
believed to account for most cases (3).

Comments: Over 70% percent of reported cases are associated with foreign travel (56).

A. Pathogen: Salmonella, nontyphoidal

B. Reported cases: Outbreak-related cases based on reports to CDC, 1983-1992 (10,25). Passive surveillance estimate
based on average number of cases reported to CDC, 1992-1997 (57). Active surveillance estimate based on
extrapolation of the average 1996-1997 FoodNet rate to the 1997 U.S. population (23).

Total cases: Assumed to be 38 times the number of reported cases based on FoodNet data (Voetsch, manuscript in
preparation) and the "sequential surveillance artifact" multiplier derived by Chalker and Blaser (21).

Hospitalization rate: Based on hospitalization rate for culture-confirmed cases reported to FoodNet, 1996-1997 (23,24).

Case-fatality rate: Average case-fatality rate among cases reported to FoodNet, 1996-1997 (23,24). This rate is lower
than the previously published rate of 1.3% (58).

Percent foodborne: Although occasionally associated with exposure to pets, reptiles, and contaminated water,
salmonellosis is primarily a foodborne disease (59).

Pathogen: Shigella spp.

Reported cases: Outbreak-related cases based on reports to CDC, 1983-1992 (10,25). Passive surveillance estimate
based on average number of cases reported annually to CDC, 1992-1997 (57). Active surveillance estimate based on
extrapolation of average 1996-1997 FoodNet rate to the 1997 U.S. population (23).

Total cases: Because Shigella frequently causes bloody diarrhea, total cases assumed to be 20 times the number of
reported cases, based on similarity to E. coli O157:H7.

Hospitalization rate: Based on hospitalization rate for culture-confirmed cases reported to FoodNet, 1996-1997 (23,24).

Case-fatality rate: Average case-fatality rate among cases reported to FoodNet, 1996-1997 (23,24).

Percent foodborne: Assumed to be 20%. Although most cases are due to person-to-person transmission (60),
foodborne outbreaks are responsible for a substantial number of cases (61).

Pathogen: Staphylococcus aureus (enterotoxin)

Reported cases: Not routinely reported. Assumed to be 10 times the number of foodborne outbreak-related cases
reported to CDC, 1983-1992 (10,25).

Total cases: Assumed to be 38 times the number of reported cases, by extrapolation from studies of salmonellosis.

Hospitalization rate: Determined from outbreaks reported to CDC, 1982-1992 (10,25), (CDC, unpub. data), and
published reports (62).

Case-fatality rate: Determined from reported outbreaks to CDC, 1977-1992 (10,25,63).

Percent foodborne: 100% by definition. Case estimates presented are based on foodborne outbreaks and therefore
reflect foodborne transmission.

Comment: The number of outbreak-associated cases of staphylococcal food poisoning reported to CDC has decreased
substantially since 1973 (Bean and Griffin, 1990). This decrease is unlikely to be an artifact of decreased recognition:
there has been no compensatory increase in the number offoodborne outbreaks of unknown etiology with an incubation
period consistent with staphylococcal food poisoning (CDC, unpub. data).

Pathogen: Streptococcus, Group A

Reported cases: Not routinely reported. Assumed to be 10 times the number of foodborne outbreak-related cases
reported to CDC, 1982-1992 (10,25).

Total cases: Assumed to be 38 times the number of reported cases, by extrapolation from studies of salmonellosis.

Hospitalization rate: Determined from outbreaks reported to CDC, 1982-1992 (10,25) and CDC, unpub. data.

Case-fatality rate: Determined from outbreaks reported to CDC, 1982-1992 (10).

Percent foodborne: 100% foodborne by definition. Case estimates presented are based on foodborne outbreaks and
therefore reflect foodborne transmission.

Pathogen: Vibrio cholerae, toxigenic O1 or O139

Reported cases: Based on cases reported to CDC, 1988-1997 (7).

Total cases: Assumed that the number of clinically significant illnesses is two times the number of reported cases.

Hospitalization rate: Based on cases reported to CDC, 1992-1994 (64).

Case-fatality rate: Based on cases reported to CDC, 1992-1994 (64).

Percent foodborne: Assumed to be primarily foodborne. Most reported cases linked to foodborne outbreaks, and at
least 65% of sporadic cases may be foodborne (64).

Comments: 96% of cases acquired abroad (64).

Pathogen: Vibrio vulnificus

Reported cases: Cases reported to CDC from 22 states, 1988-1996 (65).

Total cases: Because it is a severe illness, assumed to be two times the number of reported cases.

Hospitalization rate: Based on overall rate among cases reported to CDC, 1988-1996 (65).

Case-fatality rate: Based on overall rate among cases reported to CDC, 1988-1996; death rate higher among cases
due to foodborne transmission (65).

Percent foodborne: Based on Shapiro et al. (65).

Comment: Most cases are reported by Gulf States (Florida, Alabama, Louisiana, Texas).

Pathogen: Vibrio, other spp.

Reported cases: Passive surveillance estimate based on cases reported to CDC, 1988-1996 (CDC, unpub. data). Active
surveillance estimate based on 1996 FoodNet rate extrapolated to the 1997 U.S. population (23). FoodNet data from
1997 not included because of a large outbreak of Vibrio parahaemolyticus infections that could falsely elevate the
overall rate.

Total cases: Because it is a moderately severe illness, total cases assumed to equal 20 times the reported cases, a
degree of underreporting comparable with E. coli O157:H7 infections.

Hospitalization rate: Based on rate among non-vulnificus, non-cholerae O1 cases reported by Hlady (66).

Case-fatality rate: Based on rate among non-vulnificus, non-cholerae O1 cases reported by Hlady (66).

Percent foodborne: Based on history of shellfish consumption for cases reported by Hlady (66)

Comment: Because of larger sample size, data from Hlady (66) used in preference to FoodNet data for hospitalization
and death rates.

Pathogen: Yersinia enterocolitica

Reported cases: Active surveillance estimate based on extrapolation of average 1996-1997 FoodNet rate to the 1997 U.
S. population (23,24).

Total cases: Assumed to be 38 times the number of reported cases, based on studies of salmonellosis.

Hospitalization rate: Based on the hospitalization rate for culture-confirmed cases reported to FoodNet, 1996-1997
(23,24).

Case-fatality rate: Low, assumed to be 0.5% (23).

Percent foodborne: Assumed to be 90%. Nearly all reported outbreaks in United States have been linked to
contaminated foods, and pork is specifically believed to be the source of most infections (67).

Parasitic Pathogens

Pathogen: Cryptosporidium parvum

Reported cases: Passive surveillance estimate based on the average annual number of cases reported to CDC, 1995-
1997 (7). Active surveillance estimate based on extrapolation of the average 1997-98 FoodNet rate to the 1997 U.S.
population (6,24).

Total cases: Published studies suggest that ~2% of all stools tested for Cryptosporidium are positive (68, 69). We
assume this rate of infection applies to all patients visiting a health-care provider for acute gastroenteritis. Using an
estimate of ~15 million physician visits for diarrhea each year (see text), we estimate there are approximately 300,000
cases of cryptosporidiosis per year. This figure is 45-fold higher than the estimated number of reported cases based on
FoodNet active surveillance, a multiplier only slightly larger than the one used for salmonellosis.

Hospitalization rate: Based on the hospitalization rate for culture-confirmed cases reported to FoodNet, 1997-1998
(6,24).

Case-fatality rate: Average case-fatality rate among cases reported to FoodNet, 1997-1998 (6,24).

Percent foodborne: Based on very limited information (70-72), we assume that 10% of cases are attributable to
foodborne transmission, with the rest due to consumption of contaminated water or person-to-person transmission.

Comment: Cryptosporidiosis in AIDS is associated with a severe protracted course of diarrhea (73).

Pathogen: Cyclospora cayetanensis

Reported cases: Passive surveillance estimate based on average annual number of cases reported to CDC, 1995-1997
(7). Active surveillance estimate based on extrapolation of average 1997-1998 FoodNet rate to the 1997 U.S. population
(6,24).

Total cases: Assumed to be 38 times the number of reported cases based on studies of salmonellosis.

Hospitalization rate: Based on the hospitalization rate for culture-confirmed cases reported to FoodNet, 1997 (24).

Case-fatality rate: Very low (74,75). Assumed to be 0.05%, comparable with Clostridium perfringens.

Percent foodborne: Assumed 90% foodborne, based on recent reported outbreaks (74,75).

Pathogen: Giardia lamblia

Reported cases: Not routinely reported.

Total cases: Sensitive surveillance in two sites (Vermont and Wisconsin) suggests a rate of 40 cases per 100,000
persons per year (76,77). In addition, an estimated 5% of all cases are reported. Thus, approximately 100,000 cases will
be detected each year, representing 2,000,000 actual cases.

Hospitalization rate: An estimated 5,000 cases per year are severe enough to require hospitalization.

Case-fatality rate: Exceedingly low. Assumed to be no more than 10 deaths annually.

Percent foodborne: Assumed to be 10%. Recreational water is probably the major source of transmission (76-78);
however, several foodborne outbreaks have been reported (79,80).

Pathogen: Toxoplasma gondii

Reported cases: Not routinely reported.

Total cases: Based on national serologic data collected during the 1994 NHANES, approximately 40% of persons 60
years old are seropositive for toxoplasmosis (CDC, unpub. data). Assuming equal rates of infection over time, at least
0.6% of the population experiences an acute infection each year, representing approximately 1,500,000 infections per
year. Approximately 15% of infections are symptomatic.

Hospitalization rate: Varies widely according to host immune status. Data from NHDS indicate that from 1992 to 1996,
toxoplasmosis was the first listed diagnosis for approximately 5,000 hospital discharges each year. We have used this
figure as a conservative estimate of the number of actual hospitalizations.

Case-fatality rate: Varies widely according to host immune status. Of the approximately 5,000 hospital discharges
annually for which toxoplasmosis is the first listed diagnosis, approximately 750 involve a deceased patient. We have
used this figure as a conservative estimate of the number of actual deaths.

Percent foodborne: Although the proportion associated with eating contaminated food varies by geographic region, we
assume an overall average of 50%. Recent unpublished data from Europe suggest that 60% of acute infections are
from contaminated food (Ruth Gilbert, pers. comm.).

Comment: Typically, infection with Toxoplasma gondii produces an asymptomatic illness or a mild viral-like febrile illness
with lymphadenopathy. Acute diarrhea is not commonly associated with acute infection. Estimates from the
Massachusetts Department of Health suggest that one case of congenital toxoplasmosis occurs for every 10,000 births
(81). Extrapolating to 4,000,000 live births in the United States, an estimated 400 children are born with congenital
toxoplasmosis. Based on calculations by investigators from Stanford University, each year approximately 6,000 women
who experience an acute infection during pregnancy and who do not receive treatment give birth to a child with
congenital toxoplasmosis, which results in chronic sequelae (82). During an outbreak of toxoplasmosis in British
Columbia, of an estimated 2,900-7,700 infections, 19 cases of retinitis were reported. If there are at least 150,000
symptomatic cases annually, from 300 to 1,050 cases (0.2% to 0.7%, respectively) of ocular toxoplasmosis could occur.
If there are 300,000 cases, from 600 to 2,100 ocular cases could occur. Thus, there could be from 300 to 2,100 ocular
cases of toxoplasmosis annually. An estimated 4,000 persons with AIDS develop Toxoplasma encephalitis annually. In
summary, from (400+300+4,000) = 4,700 to (6,000+2,100+4,000) = 12,100 persons develop chronic sequelae due to
toxoplasmosis each year.

Pathogen: Trichinella spiralis

Reported cases: Based on NETSS surveillance data, approximately 40 cases are reported annually.

Total cases: Because it can be a severe illness, assumed to be two times the number of reported cases.

Hospitalization rate: Based on outbreak-related cases reported to CDC, 1982-1992 (10).

Case-fatality rate: Assumed to be 0.3% based on data from a large series in Europe.

Percent foodborne: 100% (83)

Comment: Clinically, acute trichinosis may be asymptomatic or may have acute gastrointestinal symptoms, followed by a
parenteral phase of fever and myalgias. In 10% to 20% of cases neurologic or cardiac symptoms develop, many severe
and potentially leading to chronic illness.

Viral Pathogens

Pathogen: Rotavirus

Reported cases: Not routinely reported.

Total cases: Because every child has at least one symptomatic infection (84-86), the number of cases is assumed to
equal the 1997 U.S. birth cohort (3.9 million).

Hospitalizations: 50,000 (87,88).

Case-fatality rate: Very low: 20 to 40 deaths per year (89).

Percent foodborne: probably very low (<1%) (90).

Pathogen: Astrovirus

Reported cases: Not routinely reported.

Total cases: Because every child has at least one symptomatic infection, the number of cases is assumed to equal the
1997 US birth cohort (3.9 million).

Hospitalizations: Assumed to equal 25% of number of hospitalizations for rotavirus (= 12,500) (91).

Case-fatality rate: Very low (<10 deaths per year).

Percent foodborne: Probably very low (<1%) (91).

Pathogen: Norwalk-like viruses (NLV).

Reported cases: Not routinely reported.

Total cases: Very few data are available for assessing the disease burden associated with Norwalk-like viruses, and
very few studies have been conducted using the most sensitive diagnostics for NLVs. One community-based study from
the Netherlands found 17% of cases of acute gastroenteritis were associated with Norwalk-like viruses, compared with
6% of controls, using reverse transcriptase polymerase chain reaction (RT-PCR) for detection of NLVs (92). An
Australian study detected NLVs in 15% of hospitalized patients using immune electron microscopy (93). Studies have
generally been conducted exclusively among young children or used less sensitive detection methods (electron
microscopy); in these studies, NLVs have been detected in ~1% to 5% of participants (94-98). However, a recent study
incorporating RT-PCR for viral detection among children 2 months to 2 years of age found that 21% of cases of acute
gastroenteritis were associated with NLVs (99). Given these data, we assume that 11% of all episodes of acute primary
gastroenteritis are due to NLVs (using the data from the best of the studies) (92).

Hospitalizations: NLV assumed to account for 11% of 452,000 annual hospitalizations for viral gastroenteritis (100).

Case-fatality rate: Low. NLV assumed to account for 11% of an estimated 2,800 fatal cases of viral gastroenteritis each
year (100).

Percent foodborne: We assume that the proportion of all NLV-associated illness that is foodborne is 40%. This estimate
is based on a recent report which found that 47% of NLV-associated acute gastroenteritis outbreaks in the United States
in which the modes of transmission were known were foodborne (101). Since we would assume that foodborne-
associated outbreaks might be more likley to be reported than Norwalk-like virus-associated outbreaks with other
mechanisms of spread, the proportion was lowered to 40%. This estimate is in general agreement with other reviews
(102-104). No data are available to directly determine the proportion of cases of NLV-associated disease attributable to
foodborne transmission.

Pathogen: Hepatitis A

Reported cases: Based on cases reported to CDC, 1992-1997 (7).

Total cases: Assumed to be three times the number of reported cases (105).

Hospitalizations: Thirteen percent; based on data from CDC Sentinel Counties Studies (106);

Case-fatality rate: 0.3%; based on data from the viral Hepatitis Surveillance Program and the CDC Sentinel Counties
Studies (105,107). Deaths calculated by applying the case-fatality rate to reported cases.

Percent foodborne: Foodborne transmission accounts for approximately 5% of outbreaks of known source (105). Note
that the source is not determined in approximately 50% of hepatitis A outbreaks, and foodborne transmission could
account for a far higher percentage of cases.

Acknowledgments
We thank Fred Angulo, Beth Bell, Thomas Breuer, Cindy Friedman, Roger Glass, Eric Mintz, Steven Ostroff, Morris
Potter, David Swerdlow, Tom Van Gilder, and two anonymous reviewers for their comments.

Dr. Mead is a medical epidemiologist with the Foodborne and Diarrheal Diseases Branceh, CDC, in Atlanta, Georgia. His
professional interests include infectious diseases surveillance, outbreak investigations, and interventions to prevent
foodborne illness.

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Address for correspondence: Paul S. Mead, Division of Bacterial and Mycotic Diseases, Centers for Disease Control
and Prevention, Mail Stop A38, 1600 Clifton Road, Atlanta, GA 30333, USA; fax: 404-639-2205; e-mail: pfm0@cdc.gov.




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This page last reviewed September 15, 1999

Emerging Infectious Diseases Journal
National Center for Infectious Diseases
Centers for Disease Control and Prevention




URL: http://www.cdc.gov/ncidod/eid/vol5no5/mead.htm
Table 1. ICD-9-CM codes and
associated conditions
--------------------------------------------------
Code                     Condition
--------------------------------------------------
001                        Cholera
002                        Typhoid fever
003                         Salmonella
004                          Shigellosis
005.0                Staphyloccocal food
                            poisoning
005.1                       Botulism
005.2-005.3        Other Clostridia
005.4              Vibrio parahaemolyticus
005.8-005.9     Other and unspecified
                   bacterial  food poisoning
006                          Amebiasis
007.1                        Giardiasis
007.0, 007.2-007.9  Other protozoal
                            intestinal infections
008.00, 008.09   Misc. Escherichia coli
008.01             Enteropathogenic E.
coli008.02               Enterotoxigenic E.
coli
008.03                Enteroinvasive E. coli
008.04          Enterohemorrhagic E. coli
008.43              Campylobacter
008.44                  Yersinia
008.41-2, 008.46-9,  Misc. bacterial
008.5, 008.61        Rotavirus
008.62                  Adenovirus
008.63                  Norwalk virus
008.64               Other small round
                     structured viruses
008.65                    Calicivirus
008.66                    Astrovirus
008.67                    Enterovirus
008.69, 008.8 Other virus
009.        Ill-defined intestinal infections
558.9                Other noninfectious
                         gastroenteritis