Strengthening and Broadening the Base of Surveillance

Under the National Food Safety Initiative, CDC is building epidemiologic and laboratory capacity in state and local health departments so they can quickly and effectively identify foodborne hazards, investigate and control outbreaks, and develop effective programs to prevent additional illnesses from the same source. Routine surveillance and outbreak response, critical to the proper functioning of health departments, have been neglected at the state and local levels because of limited resources and competing priorities. Therefore, CDC is providing training and equipment needed by the health departments to update surveillance systems and diagnostic capability for bacterial, parasitic, and viral foodborne diseases. In addition, CDC, state epidemiologists, and state public health laboratory directors are working to define the elements of core capacity for effective response to food safety problems. The results of this evaluation will provide guideposts for efficient allocation of initiative resources in 1999 and beyond to construct a truly effective national early warning system for foodborne diseases.

CDC's response to foodborne hepatitis A virus infections, supported in 1998 by the National Food Safety Initiative, illustrates the value of better surveillance. Existing hepatitis A surveillance data document food as a vehicle of transmission in only 2% to 5% of cases. On the basis of these data, economic analysis suggests that routine immunization of food workers to prevent foodborne transmission of hepatitis A virus (with a vaccine already available) is not a cost-effective nationwide public health response. However, existing surveillance data document a source of exposure (food or otherwise) for only half of the reported cases of hepatitis A. Therefore, CDC is collaborating with health departments in three states to more fully describe the epidemiology of hepatitis A infections for which routine investigation fails to determine a source. In addition, CDC will evaluate the effectiveness of the algorithm used to determine which patrons of food service establishments with infected food workers will receive immune globulin for postexposure prophylaxis and will establish a genetic sequence-based library of hepatitis A virus strains circulating in the United States and other countries to permit the effective use of molecular epidemiology in investigations of hepatitis A outbreaks and sporadic cases.

Diarrheal disease results in large numbers of physician visits and hospitalizations each year in the United States. However, the infectious cause of the illness is frequently not determined, which prevents good surveillance and impedes development of effective prevention strategies. Therefore, CDC is embarking on studies of mild and severe diarrhea to determine to the greatest extent possible the bacterial, parasitic, and viral agents involved. Similarly, applied research at CDC that is improving diagnostic and subtyping methods for parasitic foodborne diseases and caliciviruses will support future surveillance and outbreak response for these foodborne pathogens, as well as help define the extent of their contribution as causes of sporadic foodborne disease.

Establishing Electronic Networks and Generating Quantitative Data

Another aspect of CDC's response to foodborne disease under the National Food Safety Initiative is the development of an electronic network that links public health laboratories for rapid data sharing, including the digitalized images of pulsed-field gel electrophoresis (PFGE) patterns. These patterns can be used to distinguish related cases that may be part of an outbreak from background disease so that risk factors can be identified and control measures can be established. By the end of 1997, CDC had laboratories in 16 state health departments trained and equipped to perform PFGE subtyping of Escherichia coli serotype O157:H7 with standardized techniques so the results could be compared among laboratories. Microbiologists in federal regulatory laboratories were also trained in these standardized techniques so E. coli O157:H7 isolated from food and animal samples could be directly compared with clinical isolates from human cases. Laboratories in four states were established as regional resources to make PFGE subtyping available to surrounding states, and one was directly linked to CDC's PFGE library of strains so it could query CDC's database in addition to being able to submit electronic data for analysis. The other three regional laboratories and laboratories in USDA and FDA will be directly linked to CDC's database early in 1998; the remaining six FoodNet sites will be linked later in the year. In addition, in 1998 six more states will be trained and equipped to perform PFGE subtyping of E. coli O157:H7, training will be provided to health department laboratories in two foreign countries, PFGE subtyping of Salmonella Typhimurium will begin, and methods for subtyping Salmonella Enteritidis and other foodborne pathogens will be developed.

Changes in food safety regulations (e.g., regulatory use of Hazard Analysis and Critical Control Points systems and international requirements for quantitative risk assessment) and increasing emphasis on the cost-effectiveness of prevention programs require epidemiologic data on foodborne disease that cannot be provided by routine passive surveillance systems based on laboratory and physician diagnoses. In response, CDC collaborated with U.S. Department of Agriculgure, Federal Drug Administration, and the five state health departments enrolled in CDC's emerging infections active surveillance program in 1995 to create FoodNet. FoodNet expands the quantitative database on foodborne diseases, expedites response to certain food safety concerns, and provides a test bed for evaluating new prevention strategies. FoodNet activities include 1) comprehensive documentation of foodborne infections through active laboratory-based surveillance, 2) determination of health and food-handling practices of the population under surveillance, 3) determination of health-care practices by surveys of the physicians and diagnostic laboratories in the surveillance areas, and 4) implication of specific foods as vehicles for various foodborne pathogens by conducting case-control studies of sporadic infections.

These efforts have shown that Campylobacter is the most commonly diagnosed cause of diarrheal disease, that great geographic variations in the frequency of E. coli O157:H7 infections and other foodborne pathogens suggest a need for targeted control measures, and that approximately 1.4 diarrheal illness episodes occur per person per year in the United States.

FoodNet began with active surveillance of sporadic infections with seven bacterial pathogens in the five sites. In 1997, two new sites were added, and in 1998 an eighth active surveillance site will be added; active surveillance for Cryptosporidium, Cyclospora, and Calicivirus infections will be included, and intensive investigation of foodborne disease outbreaks will be initiated. Many bacterial pathogens, including those that are foodborne, are becoming resistant to antibiotics. To measure this change in resistance, CDC, in collaboration with FDA, USDA, and the states, began monitoring resistance levels of several foodborne bacteria. This information is being used to promote the prudent use of antibiotics in human health and in agriculture.