Salmonella enteritidis on the rise in chickens

Nov 22, 2006 (CIDRAP News) – Sampling by the US Department of Agriculture (USDA) over the last 5 years has shown a fourfold increase in the number of broiler chicken carcasses contaminated with Salmonella enterica serotype enteritidis, a strain previously associated mainly with eggs.

The findings, published yesterday in the December issue of Emerging Infectious Diseases (EID), do not necessarily signal an overall increase in the risk of chicken-related Salmonella infection, but they appear to reinforce other evidence about the emergence of S enteritidis in chicken.

The authors, led by Sean F. Altekruse of the USDA, note that two recent US case-control studies from the Foodborne Diseases Active Surveillance Network (FoodNet) associated eating chicken with sporadic human infections with S enteritidis. Though the overall incidence of human salmonellosis was lower in 2005 than in the mid 1990s, FoodNet surveillance indicated the incidence of S enteritidis infections was about 25% higher.

The USDA researchers tested rinse water samples collected from 2000 through 2005 at plants that slaughter broiler chickens. Eligible poultry processors were randomly selected each month for sampling, which involved collecting rinse water used on one chilled broiler chicken carcass per day for 51 days. Samples were sent for analysis to USDA Food Safety and Inspection Service (FSIS) laboratories in Georgia, California, and Missouri.

Over the 6-year study, researchers identified 280 S enteritidis isolates from 51,327 broiler rinses; the annual number of isolates rose from 23 in 10,057 samples in 2000 to 120 in 9,592 samples in 2005. As a proportion of all Salmonella strains found, S enteritidis increased from 2.5% in 2000 to 7.7% in 2005.

The proportion of establishments that had positive tests increased from 17 of 197 (9%) in 2000 to 47 of 187 (25%) in 2005. In addition, the number of states where the strain was found increased from 14 in the 2000-2002 period to 24 in the ensuing 3 years. Two S enteritidis phage types accounted for most isolates from broiler rinse water: PT 8 and PT 13.

The researchers write that the sampling program is not designed to estimate national prevalence of poultry contamination, because it doesn't consider production volume or regional or seasonal effects, but the findings are significant.

"Enteritidis in broilers is noteworthy given the increase in human Salmonella enteritidis infection rates in the United States and recent findings that eating chicken is a new and important risk factor for sporadic infection," they state.

They point to a recent FoodNet report that showed a strong association between infection with S enteritidis phage types 8 and 13 and eating chicken. "The possible emergence of these two phage types in broiler chickens suggests that industry should implement appropriate Salmonella enteritidis controls for broiler chickens," the authors write.

Craig Hedberg, PhD, a foodborne disease expert and associate professor of environmental and occupational health at the University of Minnesota in Minneapolis, said the identification of a different serotype in broiler chickens by itself doesn't mean that eating or handling chicken is becoming more dangerous.

"The implications are mostly for how we evaluate our surveillance system," he told CIDRAP News. "There are about 50 different serotypes, and the clinical illness is the same [for all serotypes]."

Hedberg said the findings should prompt renewed attention to S enteriditis control programs in egg production and that it's difficult to disentangle the risk factors for the organism in chickens. The egg and broiler industries are very different and their disease-control strategies vary, he said.

Earlier this year, the USDA reported evidence of a steady increase in overall Salmonella contamination in broiler chickens since 2002, with 16.3% of samples testing positive in 2005. The trend prompted the FSIS to announce plans to report test results faster and increase monitoring of processing plants that have high numbers of positive samples. The Salmonella initiative is patterned after a recent FSIS program aimed at ground beef, which the agency says led to a 40% reduction in the number of Escherichia coli O157:H7 illness cases.

Meat and poultry producers that haven't reduced the percentage of positive Salmonella tests to no more than half the FSIS performance standard by July 2007 will face consequences, the USDA authors report. For example, the FSIS may post test results, including processing plant names, on the Web for products that have not made sufficient progress.

However, they say that voluntary quality-assurance programs enacted by the egg industry in the 1990s were enough to control S enteritidis in eggs. Many of the interventions are adaptable to broiler chickens, the authors write, including monitoring and sanitation of breeding flocks, hatcheries, broiler flocks, and slaughtering facilities.

Altekruse SF, Bauer N, Chanlongbutra A, et al. Salmonella enteritidis in broiler chickens, United States, 2000-2005. Emerg Infect Dis 2006;12(12):1848-52 [Full text]

See also:

Mar 6 CIDRAP News article "USDA aims to reduce Salmonella in meat and poultry"

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