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

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 estimateda illnesses, frequency of foodborne transmission, and hospitalization and case-fatality rates for known foodborne pathogens, United States
		Estimated total cases	Reported 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.