Six years of Iowa State University testing show a dramatic decrease in the number of environmental samples taken from egg facilities that test positive for the Salmonella enteritidis (SE) bacterium that causes human food poisoning.

“The test data also show that the likelihood of a positive environmental test translating into contaminated eggs is extremely low,” said Hongwei Xin, director of the Egg Industry Center at Iowa State. “It’s a very positive outcome of the industry implementing the federal egg safety rules that went to effect in July 2010.”

The Veterinary Diagnostic Laboratory in Iowa State’s College of Veterinary Medicine conducts tests on nearly 13,000 environmental samples annually. About 60% of the samples originate from Iowa egg farms and the remainder from sites located in more than a dozen other states.

The U.S. Food & Drug Administration requires facilities housing more than 3,000 laying hens — which comprise more than 98% of the nation’s flocks — to take environmental samples during various stages of production. Environmental samples are taken from the surfaces of egg conveyor belts, floors and poultry manure and are submitted to the Iowa State lab to be tested for the presence of the salmonella bacterium.

An analysis showed that the percentage of environmental samples testing positive declined from 24.5% in 2010 to 2.5% in 2015. Potential reasons for the significant drop in positive samples may include an increase in flocks that are vaccinated for the salmonella bacterium, according to Xin. The supply of vaccine since 2010 has jumped dramatically, with the number of doses produced under a U.S. Department of Agriculture license reaching more than 200 million in some years — more than quadruple what was produced in 2010. There also has been heightened awareness and training in SE prevention, he added.

When an environmental sample does test positive, FDA requires testing of shell eggs from the facility — four consecutive tests of 1,000 eggs each done at specified intervals. Once received by the Veterinary Diagnostic Laboratory, eggs are separated into egg pools; a pool consists of the contents and shells of 20 eggs. Scientists culture samples from the egg pools to detect the presence of the salmonella bacterium.

Following these FDA protocols, the Veterinary Diagnostic Laboratory tested more than 35,000 egg pools from 2010 to 2015. In that time period, only one positive egg pool was identified, which occurred during the time frame of a national egg recall in 2010.

“Over the past year, egg safety testing has been continuous and ongoing,” said Dr. Yuko Sato, assistant professor and extension poultry veterinarian at Iowa State. “Environmental sampling and testing continued throughout Iowa’s avian influenza crisis and its aftermath, which claimed more than 30 million birds.

“The FDA’s Egg Safety Rule requires the farms to test and then to act on those tests if there is the possibility of contamination,” Sato added. “From the test results we are seeing, the rules are functioning as they were meant to — to ensure egg safety.”

“While continued efforts are being made to ensure egg safety in the supply chain, consumers also must continue to be vigilant in how they obtain, handle, store and prepare eggs to reduce the potential for contamination,” Xin said.