Foodborne disease outbreaks in schools caused 50,000 illnesses in 25 years

Jul 31, 2002 (CIDRAP News) – Foodborne disease outbreaks that occurred in schools between 1973 and 1997 made nearly 50,000 students sick and sent 1,514 to hospitals, according to a report from the Centers for Disease Control and Prevention (CDC).

State and local health departments voluntarily reported 604 outbreaks to the CDC over the 25 years, though many more such episodes probably occurred, according to the report by Nicholas A. Daniels, MD, and colleagues. The article appears in the July issue of the Pediatric Infectious Disease Journal.

The authors analyzed outbreaks in primary and secondary schools, colleges, and universities. The annual number of outbreaks ranged from 9 in 1995 to 44 in 1981, with a median of 25, the report shows. The authors do not cite an overall trend upward or downward, but their data appear to show a slightly lower average rate of outbreaks in the 1990s than in the earlier years.

The 604 outbreaks involved 49,963 illnesses, 1,514 hospitalizations, and 1 death; the report says these accounted for about 5% of all foodborne disease outbreaks and 12% of all outbreak-associated illnesses reported to the CDC. Of the 604 outbreaks, 240 (40%) had known (laboratory-identified) causes. Bacterial pathogens caused 85% of the outbreaks with known causes, while chemical agents caused 7%, viruses 6%, and parasites 1%.

Among outbreaks with identified causes, Salmonella was the most commonly identified pathogen, accounting for 36% of the outbreaks, 37% of illnesses, and 48% of hospitalizations. Salmonella serotypes Enteritidis and Typhimurium accounted for 49% and 27% of the 77 outbreaks in which only one serotype was reported.

Other known causes of illness (with the number of outbreaks for each) included Staphylococcus aureus (60), Clostridium perfringens (25), Shigella species (9), Bacillus cereus (6), Escherichia coli O157:H7 (5), and group A Streptococcus (2). Nine outbreaks were blamed on hepatitis A virus and five were attributed to Norwalk-like viruses.

Specific foods were implicated as vehicles in 333 (55%) of the 604 episodes. Poultry dishes were blamed in 18.6% of these outbreaks to lead the list, followed by Mexican-style food, 6.0%; beef, 5.7%; and dairy products other than ice cream, 5.0%. Seventy percent of poultry-related outbreaks were blamed on turkey, but only 2 of the 38 turkey-related outbreaks occurred in the 1990s. Outbreaks related to dairy foods also declined, with only 2 of 21 occurring in the 1990s, while episodes linked to salads increased. Of the 333 outbreaks with known food vehicles, 115 were statistically linked with multiple food items.

The authors say outbreaks reported to the CDC represent only "a small proportion" of the actual total, for several reasons, including the passive nature of the surveillance, differences in pathogen incubation periods, and scarcity of testing for viral pathogens.

Local investigators who assessed the contribution of food-handling practices to the outbreaks cited improper food storage and holding temperatures in 81% of cases. Other practices cited were likely contamination by a food handler, 57%; inadequate cooking, 43%; contaminated equipment, 35%; and food obtained from unsafe sources, 11%.

To prevent outbreaks, the authors recommend training and certification of school food handlers and paid sick leave for food handlers when they have intestinal illnesses. They also say that purchase contracts for meat, poultry, and eggs should include food safety criteria, which often has not been the case. They note that the US Department of Agriculture requires that ground beef it buys for school lunch programs be tested for E coli O157:H7 and Salmonella, but most food served in schools is bought by state or local authorities (with USDA funds), not by USDA.

Broader use of irradiation would reduce the risk of outbreaks caused by bacteria and parasites, the authors say. But they add that viruses like hepatitis A and Norwalk-like viruses are more resistant to irradiation.

Daniels NA, MacKinnon L, Rowe SM, et al. Foodborne disease outbreaks in United States schools. Pediatr Infect Dis J 2002;21(7):623-8
Abstract available at (click on "Online Archive," then "July 2002," then "Original Studies"; see second item in list)

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