Penn State researchers have found that eggs from small flocks of chickens are more likely to be contaminated with Salmonella enteritidis than eggs sold in grocery stores, which typically come from larger flocks.
That conclusion was drawn from a six-month study done last year in Pennsylvania. Researchers from Penn State’s College of Agricultural Sciences collected and tested more than 6,000 eggs from more than 200 selling points across the state for the study.
Salmonella enteritidis is one of the leading foodborne pathogens in the United States, with many outbreaks being traced back to contaminated shell eggs.
The U.S. Food and Drug Administration requires that shell-egg producers from farms with 3,000 or more chickens be in compliance with the agency’s Final Egg Rule, which is aimed at restricting the growth of pathogens.
Federal regulations for these larger flocks require placement of Salmonella-“clean” chicks, intensive rodent control, cleaning and disinfecting between flocks, environmental monitoring of pullet and layer houses, continuous testing of eggs from any Salmonella-positive houses, and diverting eggs from Salmonella-positive houses for pasteurization.
However, small flocks with fewer than 3,000 laying hens are currently exempt from the rule. Eggs from these producers often are marketed via direct retail to restaurants, health food stores and farmers markets, or sold at on-farm roadside stands.
From April to September 2015, the Penn State researchers purchased two to four dozen eggs from each of 240 randomly selected farmers markets or roadside stands representing small laying flocks in 67 Pennsylvania counties.
Using standard protocols, the internal contents of the eggs and the egg shells were separately cultured for Salmonella. Recovered Salmonella were classified by serotype, and any Salmonella enteritidis isolates present were further characterized to evaluate how they might be related to isolates of the bacteria that have caused foodborne illness outbreaks.
Test results revealed that of the 240 selling points included in the study, eggs from five (2 percent) were positive for Salmonella enteritidis. Eggs sold at one of the positive selling points contained the bacteria in the shells, while the eggs from the other four selling points had Salmonella enteritidis in the internal contents.
Lead researcher Subhashinie Kariyawasam, microbiology section head at Penn State’s Animal Diagnostic Laboratory, said that is a higher prevalence of the pathogen than that found in studies of eggs from large flocks.
Kariyawasam said the study clearly demonstrated that Salmonella enteritidis is present in eggs produced by small flocks. She presented the research findings to the American Veterinary Medical Association and the American Association of Avian Pathologists at their August meeting in San Antonio, TX.
“The research highlights the potential risk posed by the consumption of eggs produced by backyard and small layer flocks. And, analysis of the Salmonella enteritidis present in the eggs from small flocks shows they are the same types commonly reported to the Centers for Disease Control and Prevention from human foodborne outbreaks,” she said.
“These findings emphasize the importance of small-producer education on Salmonella enteritidis control measures and perhaps implementation of egg quality-assurance practices to prevent contamination of eggs produced by backyard and other small layer flocks.”
Eggs from small flocks make a negligible contribution to the table egg industry in the U.S., Kariyawasam noted. But the growing demand for backyard eggs and eggs from non-farm environments — with small egg-producing flocks managed in cage-free systems and pasture situations — suggests that these production systems deserve scrutiny.
“We were curious about Salmonella contamination of eggs produced by these flocks because the prevalence of this pathogen in smaller flocks was not known. Now we know that the prevalence of Salmonella enteritidis in eggs produced by small flocks is higher than in eggs produced by larger flocks,” she said.
Salmonella enteritidis, like other foodborne bacteria, is destroyed by proper cooking. However, Salmonella infections can be very serious, even life-threatening, especially to the very young, the elderly and people with weakened immune systems.
“The bottom line is, if you buy your eggs from the small producers, you need to worry about Salmonella just as if you bought eggs produced by large flocks,” Kariyawasam said. “And, beyond the consumption of eggs, birds in these flocks can have Salmonella, so people with backyard poultry need to be aware of the dangers with pet birds — there is a risk to allowing birds to come into the house and children playing with the chickens.”
Other Penn State scientists who contributed to the study were Eva Wallner-Pendleton, senior research associate and avian pathologist, Gregory Martin, poultry science extension educator, and Thomas Denagamage, postdoctoral scholar in veterinary and biomedical sciences.
The Animal Health and Diagnostic Commission of the Pennsylvania Department of Agriculture supported the study.
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