CDC: Leafy green, dairy outbreaks up slightly

Covering more than a decade of foodborne disease outbreaks, researchers from the US Centers for Disease Control and Prevention (CDC) found an uptick in events linked to leafy vegetables and dairy products and a drop in illnesses associated with eggs.

The group's analysis of all outbreaks reported through the electronic Foodborne Disease Outbreak Reporting System (eFORS) from 1998 through 2008—which includes instances of two or more cases of similar illnesses resulting from ingesting a common food—could help health officials better gauge food safety risks and craft new prevention strategies, the authors said.

The investigators published their findings in Morbidity and Mortality Weekly Report (MMWR).

A small number of foodborne illnesses, hospitalizations, and deaths occur as part of recognized outbreaks, the team noted. However, the CDC's review of 11 years of reports submitted by states and federal jurisdictions provides a different look at foodborne threats than what the public might see from news about high-profile multistate outbreaks. During the study period there were 128 multistate outbreaks.

Overall, the CDC received reports of 13,405 foodborne disease outbreaks, which resulted in 273,120 illnesses, 9,109 hospitalizations, and 200 deaths. Of all the outbreaks, the cause was suspected or known in 7,998. Viruses and bacteria were each linked to 45% of the outbreaks, with 9% from chemical or toxic agents and 1% from parasites.

Norovirus (43%) topped the list of most common causes, followed by Salmonella (18%). Outbreaks linked to Salmonella, however, were related to the largest number of hospitalizations.

The most deadly pathogens in terms of numbers were Salmonella, followed by Listeria and Shiga toxin-producing Escherichia coli. Though the percentage of Salmonella illnesses was fairly constant over the study period, the percentage from Salmonella Enteritidis—the type often linked to eggs—decreased by almost half.

When researchers looked at the settings linked to foodborne outbreaks, they found that 68% originated in restaurants or delis, with 9% associated with private homes and 7% connected to catering or banquet facilities. The rest involved other settings. The most common settings for norovirus outbreaks were restaurants or delis. Private homes were the most common site for only one cause—ciguatoxin.

Not all outbreaks could be traced to a specific food, but when illnesses were, the top ones were poultry (17%), leafy vegetable (13%), beef (12%), and fruits and nuts (11%).

Their analysis of pathogen-commodity pairs found some notable patterns. Compared with the first 2 years of the study period, the percentage of outbreaks connected to leafy vegetables was higher during the last 3 years, a pattern that was also seen for dairy product outbreaks.

On the other hand, the percentage of outbreaks linked to eggs fell from 6% during the first 2 years to 2% during last 3 years. For E coli the largest percentage of foodborne disease outbreaks was linked to beef, which was also responsible for the highest percentage of outbreaks caused by Clostridium perfringens.

Some of the group's findings also showed the impacts of enhanced outbreak surveillance through PulseNet, better testing tools, and even public health funding. For example, they found that the average number of outbreaks reported to CDC during the study period was more than double the number reported from 1973 through 1997. "…the increase was largely a surveillance artifact because of the transition to electronic reporting rather than a true increase in the number of foodborne illness outbreaks," they wrote.

The team found that although multistate outbreaks represented only 1% to 2% of those with known causes, the events contributed a disproportionate number of deaths and hospitalizations than other types of outbreaks.

The median number of outbreaks reported by each state was highest in 2004, a time that represented a peak in federal emergency preparedness funding, according to the report.

Researchers predicted that the number of multistate outbreaks will continue to increase as molecular subtyping methods become more advanced and accessible. "However, the vast majority of recognized outbreaks are still local, requiring epidemiology, laboratory, and environmental health capacity at the local and state levels," they wrote.

Gould LH, Walsh KA, Vieira AR, et al. Surveillance for foodborne disease outbreaks—United States, 1998-2008. MMWR 2013 Jun 28;62(SS02):1-34 [Full text]

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