Surveillance data on foodborne disease outbreaks in 2007 revealed that norovirus and salmonella contamination were the leading causes, with poultry, beef, and leafy greens the most common foods involved, the U.S. Centers for Disease Control and Prevention reported.
The analysis also indicated that no cause was ever found for about one-third of outbreaks and a quarter of the victims, according to a report from CDC researchers in the Aug. 13 issue of Morbidity and Mortality Weekly Report.
Nearly 1,100 outbreaks involving 21,244 individual illnesses were covered by the data, supplied by public health laboratories in all 50 states, the District of Columbia and Puerto Rico.
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The CDC researchers noted that these were just a handful of the estimated 76 million illnesses occurring in the U.S. annually from contaminated food.
Of the 734 outbreaks with known etiologies in 2007, 320 involved bacterial pathogens, 324 were traced to viruses, 49 involved chemical contamination (mostly of microbial origin), and five were parasitic infections. Another 36 had more than one cause.
All but seven of the viral outbreaks stemmed from norovirus, which gets into food products when infected workers fail to wash their hands.
In an accompanying commentary, the MMWR's editors said the 2007 figures for norovirus outbreaks were actually less than in previous years, perhaps because of growing population immunity.
The average annual number of outbreaks in which norovirus was confirmed or suspected from 2002 to 2006 was 338 with a mean of 10,854 illnesses. In 2007, the corresponding totals were 317 and 8,024.
On the other hand, the editors indicated that norovirus outbreaks tend to surge worldwide every two to three years. Hence, 2007 may simply represent a trough between spikes.
Salmonella accounted for 142 of the bacterial outbreaks in 2007, including two of the three largest, the CDC researchers reported. Those outbreaks included 802 illnesses traced to tainted hummus and 401 illnesses from frozen pot pies.
The Salmonella totals were close to the averages for 2002 to 2006.
Rodents in food packaging and distribution facilities are the most common source of Salmonella contamination.
Clostridium perfringens and Shiga toxin-producing E. coli were the next most common bacterial pathogens, accounting for 45 and 42 outbreaks, respectively. Both of these totals were up from the averages of previous years (34 and 28, respectively), though the number of people affected by C. perfringens infections was somewhat lower.
The MMWR editors suggested that the food industry could be doing more to prevent foodborne illnesses.
"Enhanced food safety training for food employees that work with ready-to-eat foods, and the presence of a certified food protection manager in food service and retail establishments, as recommended by [FDA's] Food Code, might help to reduce the number of outbreaks and outbreak-related illnesses resulting from contamination in food service establishments, if adopted by all states and territories," they wrote.
The editors indicated that nearly all states and territories have adopted food-safety codes, "but the specific components of individual state regulations vary."
They also recommended that potential outbreaks continue to be reported and investigated.