United States Department of Agriculture
Report to Congress

FoodNet: An Active Surveillance System
for Bacterial Foodborne Diseases
in the United States

Food Safety and Inspection Service
United States Department of Agriculture
Washington, D.C.
March 1999

Table of Contents

Preface

Executive summary

Background

Objectives

Methods

Results

Cases reported

Seasonality

1998 Rates

1996-1998 Rates

Rates by site

Rates by age

Rates by gender

Rates by age and gender

Hospitalizations

Deaths

HUS

Outbreaks

Additional studies

Burden of illness

Causes of foodborne disease

Future activities

Presentations

1998 FoodNet Working Group

Preface

The U.S. Department of Agriculture has been directed by Congress to provide an annual report on the incidence of foodborne illness in the United States. This report is to be submitted to the House and Senate Committees on Appropriations.

This report includes a description of the foodborne Diseases Active Surveillance Network (FoodNet) and data on the incidence of foodborne illness for calendar year 1998. Data already collected are being analyzed by the Centers for Disease Control and Prevention and the seven surveillance sites.

Questions about this report or about FSIS may be directed to the Food Safety and Inspection Service, U.S. Department of Agriculture, Washington, DC 20250.

Executive summary

FoodNet is the principal foodborne-disease component of the Centers for Disease Control and Prevention's (CDC's) Emerging Infections Program (EIP). FoodNet is a collaborative project among CDC, the eight EIP sites, the Food Safety and Inspection Service (FSIS) of the United States Department of Agriculture (USDA), and the Food and Drug Administration (FDA).  FoodNet augments, but does not replace, longstanding activities at CDC, FSIS, FDA, and in states to identify, control, and prevent foodborne disease hazards.
 

The following are key findings of FoodNet surveillance activities during 1998:

• The rate of Campylobacter, Salmonella, and Cryptosporidium infections declined.  Although this decline might reflect simple annual fluctuations in foodborne illness, it may also reflect disease prevention efforts, particularly for campylobacteriosis and salmonellosis.  These efforts include changes in meat and poultry processing plants in the United States mandated by the USDA HACCP Rule.  Similarly, the percentage of meat and poultry products sampled at processing plants that yielded Salmonella declined compared to baseline rates.
• The rate of Salmonella Enteritidis (SE) infections declined in all states except Oregon.  The possible reasons are under investigation.  SE infections, particularly outbreaks, have been commonly associated with eating undercooked eggs.  Implementation of an egg quality assurance program with microbiologic testing and diverting eggs intended for retail stores to egg breaker plants when SE is found on a farm may have contributed to the decline in reported human illnesses.
• The large reduction in cyclosporiasis is associated in part with increased public awareness of and restrictions on the importation of raspberries into the United States.
• The sustained increase in Vibrio rates is a reflection of multistate outbreaks of Vibrio parahaemolyticus in 1997 and 1998.
• The rate of E. coli O157 infections increased in 1998 to slightly above the 1996 levels, reversing a decline in 1997; the reasons for the fluctuations are unknown.
• FoodNet conducted case-control studies of Salmonella infections.  Eating chicken and undercooked eggs was associated with sporadic SE and sporadic Salmonella Heidelberg infections.  Antimicrobial use in the month prior to illness was associated with multi-resistant sporadic Salmonella Typhimurium DT104 infections.  Breast-feeding was found to be protective against infant salmonellosis.  Salmonellosis was also associated with pet reptile contact.
• FoodNet surveillance contributed to the detection and investigation of a large, multistate outbreak of listeriosis.  In FoodNet sites, Listeria infections had the highest hospitalization rate and caused nearly half of the reported deaths.  FoodNet will conduct additional studies of Listeria infections to identify food sources and potential control points in 1999.

 
Background

Foodborne infections are an important public health challenge. The Centers for Disease Control and Prevention (CDC), the Food Safety and Inspection Service (FSIS) of the United States Department of Agriculture (USDA), the Food and Drug Administration (FDA), and the eight Emerging Infections Program (EIP) sites are actively involved in preventing foodborne diseases. Recently, the interagency national Food Safety Initiative was established to meet the public health challenge of foodborne diseases.  CDC's principal role in the Food Safety Initiative has been to enhance surveillance and investigation of infections that are usually foodborne.  This mission is being accomplished through several activities including FoodNet.
 

Objectives
The objectives of FoodNet are to determine the frequency and severity of foodborne diseases; determine the proportion of common foodborne diseases that result from eating specific foods; and describe the epidemiology of new and emerging bacterial, parasitic, and viral foodborne pathogens. To address these objectives, FoodNet uses active surveillance and conducts related epidemiologic studies. By monitoring the burden of foodborne diseases over time, FoodNet can document the effectiveness of new food safety initiatives, such as the USDA HACCP Rule, in decreasing the rate of foodborne diseases in the United States each year.

Methods
In 1998, FoodNet conducted population-based active surveillance for confirmed cases of Campylobacter, Cryptosporidium, Cyclospora, Shiga toxin-producing Escherichia coli O157, Listeria, Salmonella, Shigella, Vibrio, and Yersinia infections in Connecticut, Minnesota, and Oregon and selected counties in California, Georgia, Maryland, and New York (total population 20.5 million).  To identify cases, FoodNet personnel contact each of the more than 300 clinical laboratories within the catchment areas, either weekly or monthly, depending on the size of the clinical laboratory.  FoodNet also conducts surveillance for hemolytic uremic syndrome through pediatric nephrologists, and surveillance for foodborne disease outbreaks.

Results

Cases reported
In 1998, a total of 9787 confirmed cases of infections caused by the pathogens under surveillance were identified in the seven sites. Of these, 9213 were bacterial, including 4031 cases of Campylobacter infection, 2849 Salmonella infections, 1483 Shigella infections, 508 E. coli O157 infections, 186 Yersinia infections, 106 Listeria infections, and 50 Vibrio infections (Table 1A). Of the 2670 Salmonella isolates that were serotyped, the most commonly identified serotypes were Typhimurium (808 cases), Enteritidis (406), Heidelberg (168), Newport (88), and Braenderup (61). In addition, 574 cases of parasitic diseases were reported, including 565 cases of Cryptosporidium infection and 9 cases of Cyclospora infection (Table 1B).

Table 1A. Cases of infections caused by specific bacterial pathogens, reported by FoodNet sites, 1998

Table 1B. Cases of infections caused by specific parasitic pathogens, reported by FoodNet sites, 1998

Seasonality
Isolation rates for several pathogens showed wide seasonal variation; 68% of Vibrio, 46% of E. coli O157, 41% of Campylobacter, and 35% of Salmonella were isolated during June through August  (Figure 1).  Forty-four percent of cyclosporiasis cases and 33% of cryptosporidiosis cases were identified during the summer months.  Yersinia infections were more likely to have occurred in winter months with 41% of cases being reported during January, February, or December.
 

1998 Rates
To compare the number of cases among sites with different populations, preliminary annual incidence rates were calculated (incidence is the number of cases divided by the population).  All 1998 rates reported here use 1997 population estimates and are therefore considered preliminary.  Final incidence data will be available once 1998 population estimates are available in mid-1999.  Overall incidence rates were highest for infections with Campylobacter  (19.7/100,000 population), Salmonella (13.9/100,000), and Shigella (7.2/100,000).  Lower overall incidence rates were reported for Cryptosporidium (2.3/100,000), E. coli O157 (2.5/100,000), Yersinia (0.9/100,000), Listeria (0.5/100,000), Vibrio (0.2/100,000), and Cyclospora (0.04/100,000).

1996-1998 Rates
For the five original sites, overall incidence rates of illness caused by pathogens under surveillance declined from 1996 to 1998 (Table 2).  Infections caused by Salmonella decreased from 14.5/100,000 in 1996 to 12.4/100,000 in 1998.  This decrease was particularly pronounced for serotype Enteritidis, which dropped from 2.5/100,000 to 1.4/100,000.  Although Campylobacter rates increased slightly from 1996 to 1997 (23.5/100,000 to 25.2/100,000), 1998 rates experienced a substantial decline (21.7/100,000).  After showing a decline from 1996 to 1997, E. coli O157 increased in 1998 to 2.8/100,000.  The incidence of Vibrio infections, which increased substantially from 1996 to 1997, remained elevated in 1998.  Incidence rates for Listeria and Yersinia infections were similar for the 3 years.  The incidence of illness caused by Cryptosporidium dropped from 2.8/100,000 in 1997 to 2.5/100,000 in 1998, and Cyclospora decreased from 0.3/100,000 in 1997 to 0.05/100,000 in 1998.

Compared with 1997, Georgia reported an overall increase in the incidence of illnesses caused by the pathogens under surveillance while California, Connecticut, Minnesota, and Oregon reported decreases.

  Table 2. Cases per 100,000 of specific bacterial foodborne pathogens for the five original sites, FoodNet Sites, 1996-1998

Rates by site
Incidence rates for many of these pathogens varied substantially among the sites (Figure 2). The incidence rates for Campylobacter infection varied from 10.2/100,000 in Maryland to 37.7/100,000 in California and for Shigella infections, from 2.2/100,000 in Connecticut to 16.0/100,000 in Georgia. Although incidence rates for Salmonella infection were similar among the sites, the rates for Salmonella serotype Enteritidis infection varied dramatically, from 0.7/100,000 in Georgia and New York to 5.1/100,000 in Maryland.  Rates of infection with Salmonella Typhimurium varied from 3.1/100,000 in California and New York to 4.2/100,000 in Minnesota.  Incidence rates for E. coli O157 infection varied from 1.1/100,000 in Maryland to 4.5/100,000 in Minnesota. Infections caused by Yersinia varied from 0.4/100,000 in New York to 1.6/100,000 in California and Georgia.  Incidence rates of Cryptosporidium cases ranged from 0.6/100,000 in Maryland to 3.7/100,000 in Minnesota.  Reasons for these regional differences in incidence rates are being investigated; for example, most laboratories do not test specimens routinely for all pathogens.
 

Rates by Age
Annual incidence rates of foodborne illness varied by age, especially for Campylobacter and Salmonella infections (Figure 3). For children <1 year of age, the rate of Salmonella infection was 128.9/100,000 and the rate of Campylobacter infection was 54.1/100,000, rates substantially higher than for other age groups.
 

Rates by gender
Incidence rates varied significantly by sex (Table 3).  Overall, males were more likely than females to be infected with one of these pathogens.  In particular, rates of Campylobacter infection were 30% higher among males than among females.
 

Table 3. Sex-specific incidence rates per 100,000 population, by pathogen, FoodNet sites,  1997

Rates by age and gender
The incidence rate of Campylobacter infection was higher for males than for gender females in all age groups. In contrast, although rates of Salmonella infection were higher for male infants and persons aged 1-19 years compared with females, rates were higher among females for all other age groups.

Hospitalizations
Information on hospitalization is still being collected.  Preliminary data show that overall, 16% of culture-confirmed persons were hospitalized; hospitalization rates differed markedly by pathogen (Figure 4).  The rate of hospitalization was highest for persons infected with Listeria (90%) followed by those infected with E. coli O157 (32%), Yersinia (26%), Vibrio (24%), Salmonella (20%), Shigella (11%), and Campylobacter (11%).
 

Deaths
Information on deaths is still being collected.  Twenty-nine persons died; of those, 11 were infected with Listeria, 10 with Salmonella, two with E. coli O157, two with Campylobacter, two with Cryptosporidium one with Vibrio, and one with Yersinia. The pathogen with the highest case-fatality rate was Listeria; 12% of persons infected with Listeria died.

HUS
In 1998, FoodNet began surveillance for hemolytic uremic syndrome (HUS).  A total of 50 cases were reported from FoodNet sites.  The overall rate of HUS among residents in the FoodNet sites for persons less than 15 years of age was 8.1 per million, ranging from no cases reported in Connecticut and New York to 16.8 per million in Minnesota.

Outbreaks
A foodborne disease outbreak is defined as an incident with two or more persons ill caused by ingestion of a common food (Table 4).  The overall rate of foodborne disease outbreaks in which 10 or more persons become ill reported in FoodNet sites was 3.8 outbreaks per million population, ranging from 0.8 outbreaks per million in Georgia to 8.1 outbreaks per million in New York.
 

Additional Studies

Burden of illness

Cases reported through active surveillance represent a fraction of the number of  cases in the community. To better estimate the number of cases of foodborne disease in the community, we have conducted surveys of laboratories, physicians, and the general population in the FoodNet sites (Figure 5).  Using these data, we can determine the proportion of people in the general population with a diarrheal illness and from among those, the number who seek medical care for the illness.  We can estimate the proportion of physicians who ordered a bacterial stool culture for patients with diarrhea, and we can evaluate how variations in testing for bacterial pathogens influence the number of culture-confirmed cases.

Salmonella
Data from this model suggest that there are 1,400,000 Salmonella infections, resulting in 113,000 physician office visits, and 37,200 culture-confirmed cases each year in this country.  Culture-confirmed cases alone resulted in an estimated 8500 hospitalizations and 300 deaths; additional hospitalizations and deaths occur among persons who are not culture-confirmed.

Causes of foodborne disease

As part of FoodNet, case-control studies are conducted to determine the proportion of foodborne diseases that are caused by specific foods or food preparation and handling practices.  By determining this proportion, prevention efforts can be made more specific and their effectiveness documented.
 
E. coli O157 case-control studies
A case-control study of E. coli O157:H7 infections conducted at FoodNet sites in 1997 found that undercooked ground beef was the principal food source of these infections.  A follow-up case-control study in 1999, which will also include subtyping of isolates by pulsed-field gel electrophoresis (PFGE), will again evaluate the role of undercooked ground beef and examine risk and prevention factors for E. coli O157 infections.
Salmonella case-control studies
Eating chicken and undercooked eggs was associated with sporadic Salmonella Enteritidis and Salmonella Heidelberg infections.  Antimicrobial use in the month before illness was associated with multiresistant Salmonella Typhimurium DT104 infections.  Breast-feeding was found to be protective against infant salmonellosis.  Reptile contact was associated with salmonellosis.
Campylobacter case-control study
In 1998, a FoodNet case-control study to determine risk and prevention factors for Campylobacter infection enrolled more than 1200 cases and 1200 controls.  Analysis is ongoing.  A pilot study in four FoodNet sites showed that domestically acquired fluoroquinolone-resistant Campylobacter has emerged in the United States.
Listeria case-control study
To determine sources and risk factors for listeriosis, a FoodNet case-control study will begin in 1999.
Cryptosporidium case-control study
A FoodNet case-control study is being conducted to determine sources and risk factors for Cryptosporidium infection.

Future Activites
 

• Continue population-based surveillance for Campylobacter, Cryptosporidium, Cyclospora, Salmonella, Shigella, shiga-toxin producing Escherichia coli, Listeria, Yersinia, and Vibrio infections and for hemolytic uremic syndrome (HUS).
• Conduct surveillance for all foodborne disease outbreaks of any cause that occur within the FoodNet sites and pilot electronic reporting for outbreaks.
• Expand the population under active surveillance by including additional counties in Georgia, New York, and Tennessee. In 1999, the population within the catchment areas will include 32.2 million persons or 12% of the U.S. population.
• Conduct an additional case-control study of E. coli O157 infections, which will include PFGE subtyping of isolates.
• Conduct a case-control study of Cryptosporidium infections.
• Conduct a case-control study of Listeria infections.
• Repeat a survey of microbiology laboratories in FoodNet sites to determine changes in laboratory practices.
• Repeat the survey of the general population in the FoodNet catchment area beginning July 1999 to help determine the burden of illness in the community.

The following are available at the FoodNet web site:
site:http://www.cdc.gov/ncidod/dbmd/foodnet/foodnet.htm.

CDC. 1996 Final FoodNet Surveillance Report. Atlanta: Centers for Disease Control and Prevention; 1998.
CDC. 1997 Final FoodNet Surveillance Report. Atlanta: Centers for Disease Control and Prevention; 1998.
CDC.  The Catchment. Atlanta: Centers for Disease Control and Prevention; Vol.1, No.1, Fall 1998.
CDC.  The Catchment. Atlanta: Centers for Disease Control and Prevention; Vol.1, No.2, Spring 1999.

The following MMWR articles about FoodNet are available at this web site:
http://www.cdc.gov/epo/mmwr/mmwr.html

CDC. The Foodborne Diseases Active Surveillance Network, 1996. Morbidity and Mortality Weekly Report. 1997; 46(12):258-61.
CDC. Incidence of foodborne illness-FoodNet, 1997. Morbidity and Mortality Weekly Report. 1998; 47(37):782-86.  
CDC. Incidence of foodborne illness-FoodNet, 1998. Morbidity and Mortality Weekly Report. 1999; 48(9):189-194.
    

Presentations

Angulo FJ, Voetsch AC, Vugia D, et al. Determining the burden of human illness from foodborne diseases: CDC's Emerging Infectious Disease Program Foodborne Diseases Active Surveillance Network (FoodNet). Microbial Foodborne Pathogens, Veterinary Clinics of North America 14:165-72.

Angulo FJ, Voetsch AC, Swerdlow DL, et al. Determining the burden of foodborne illness: FoodNet 1996-97 [Abstract].  In: Program and abstracts of the International Conference on Emerging Infectious Diseases, Atlanta, March 8-11, 1998. Washington, D.C.: American Society of Microbiology, 1998;84.
 
Bender J, Mead P, Voetsch D, et al.  Hemolytic uremic syndrome (HUS) cases identified in the 1996 FoodNet Escherichia coli O157:H7 surveillance. [Abstract]  In: Program and abstracts of the International Conference on Emerging Infectious Diseases, Atlanta, March 8-11, 1998. Washington, D.C.: American Society for Microbiology, 1998;116.

Deneen V, Wicklund JH, Marcus R, et al. The impact of physician knowledge of laboratory practices on surveillance for E. coli O157:H7. [Abstract] In: Program and abstracts of the International Conference on Emerging Infectious Diseases, Atlanta, March 8-11, 1998. Washington, D.C.: American Society for Microbiology, 1998;117.
 
Friedman CR, Yang S, Rocourt J, et al. Fluoroquinolone-resistant Campylobacter Infections in the United States: A Pilot Case-Control Study in FoodNet Sites. [Abstract] In: Program and abstracts of the Infectious Diseases Society of America 36th Annual Meeting, Denver, November 12-15, 1998. Alexandria, VA: Infectious Diseases Society of America; 1998;179.

Hennessy T, Cheng L, Kassenborg H, et al. Eggs identified as a risk factor for sporadic Salmonella serotype Heidelberg infections: a case-control study in FoodNet sites. [Abstract] In: Program and abstracts of the Infectious Diseases Society of America 36th Annual Meeting, Denver, November 12-15, 1998. Alexandria, VA: Infectious Diseases Society of America; 1998;178.

Hennessy T, Deneen V, Marcus R, et al.  The FoodNet Physician Survey: implications for foodborne disease surveillance. [Abstract] In: Program and abstracts of the International Conference on Emerging Infectious Diseases, Atlanta, March 8-11, 1998. Washington, D.C.: American Society for Microbiology, 1998;49.
 
Herikstad H, Vugia D, Hadler J, et al. Population-based estimates of the burden of diarrheal illness: FoodNet 1996-97. [Abstract] In: Program and abstracts of the International Conference on Emerging Infectious Diseases, Atlanta, March 8-11, 1998. Washington, D.C.: American Society of Microbiology, 1998;49.

Kassenborg H, Hedberg C, Evans M, et al.  Case-control study of sporadic Escherichia coli O157:H7 infections in 5 FoodNet sites (CA, CT, GA, MN, OR). [Abstract] In: Program and abstracts of the International Conference on Emerging Infectious Diseases, Atlanta, March 8-11, 1998. Washington, D.C.: American Society of Microbiology, 1998;50.
 
Ladd-Wilson S, Yang S, Deneen V, et al. High risk food consumption, handling and preparation practices in the FoodNet Sites:1996-97.  [Abstract] In: Program and abstracts of the International Conference on Emerging Infectious Diseases, Atlanta, March 8-11, 1998. Washington, D.C.: American Society of Microbiology, 1998;83.
 
Marcus R, Fiorentino T, Evans M, et al. FoodNet active surveillance for Salmonella Enteritidis 1996. [Abstract]  In: Program and abstracts of the International Conference on Emerging Infectious Diseases, Atlanta, March 8-11, 1998. Washington, D.C.: American Society of Microbiology, 1998;117.
 

Mermin J, Hutwagner L, Vugia D, et al. Salmonella infections from reptiles in FoodNet sites: the resurgence of a preventable illness. [Abstract] In: Program and abstracts of the Infectious Diseases Society of America 36th Annual Meeting, Denver, November 12-15, 1998. Alexandria, VA: Infectious Diseases Society of America; 1998;179.

Ray S, Voetsch D, Segler S, et al. FoodNet active surveillance for Yersinia enterocolitica infection, 1996. [Abstract] In: Program and abstracts of the International Conference on Emerging Infectious Diseases, Atlanta, March 8-11, 1998. Washington, D.C.: American Society of Microbiology, 1998;82.

Reilly K, Shallow S, Angulo FJ, et al. Laboratory-based active surveillance for Campylobacter infections: CDC's Emerging Infections Program sites, 1996. [Abstract]  In: Program and abstracts of the International Conference on Emerging Infectious Diseases, Atlanta, March 8-11, 1998. Washington, D.C.: American Society of Microbiology, 1998;83.

Villar R, Bardsley M, Reddy S, et al. Chart review of deaths with enteric pathogens occurring in the FoodNet sites, 1996. [Abstract] In: Program and abstracts of the International Conference on Emerging Infectious Diseases, Atlanta, March 8-11, 1998. Washington, D.C.: American Society of Microbiology, 1998;84.

Voetsch D, Angulo F, Farley M, et al. Estimate of the burden of illness caused by nontyphoidal Salmonella infections in the United States from FoodNet. [Abstract] In: Program and abstracts of the American Public Health Association 126th Annual Meeting and Exposition, Washington, D.C., November 15-18, 1998. Washington, D.C.: American Public Health Association; 1998:Session 3226.
 
Vugia DJ, Shallow S, Farley M, et al. Salmonella bacteremia incidence and characteristics,  FoodNet 96. [Abstract] In: Program and abstracts of the International Conference on Emerging Infectious Diseases, Atlanta, March 8-11, 1998. Washington, D.C.: American Society of Microbiology, 1998;83.

Yang S, Rocourt J, Shiferaw B, et al. Breast feeding decreases risk of salmonellosis among infants in FoodNet sites. [Abstract] In: Program and abstracts of the Infectious Diseases Society of America 36th Annual Meeting, Denver, November 12-15, 1998. Alexandria, VA: Infectious Diseases Society of America; 1998;215.