“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 annually conducts tests on nearly 13,000 environmental samples. About 60 per cent of the samples originate from Iowa egg farms and the remainder from sites located in more than a dozen other states.

The US Food and Drug Administration requires facilities housing more than 3,000 laying hens, comprising more than 98 per cent 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 to check for the presence of Salmonella. Samples are submitted to the ISU lab to be tested for the Salmonella bacterium.

An analysis shows the percentage of environmental samples testing positive declined from 24.5 per cent in 2010 to 2.5 per cent 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 Mr Xin.

The supply of vaccine since 2010 has jumped dramatically, with the number of doses produced under USDA license reaching over 200 million in some years, more than quadruple what was produced in 2010. There also has been heightened awareness and training in Salmonella enteritidis prevention, he added.

When an environmental sample does test positive, the FDA requires testing of shell eggs from that 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 timeframe of a 2010 national egg recall.

“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,” Dr Sato said. “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,” said Mr Xin.