That conclusion - which flies in the face of conventional consumer wisdom that eggs from backyard poultry and small local enterprises are safer to eat than "commercially produced" eggs - was drawn from a six-month study done last year in Pennsylvania. Researchers collected and tested more than 6,000 eggs from more than 200 selling points across the state.

From April to September 2015, the researchers purchased two to four dozen eggs from each of 240 randomly selected farmers markets or roadside stands representing small layer flocks in 67 counties of Pennsylvania. Two per cent of the selling points sold eggs that were positive for Salmonella enteritidis, test results revealed.

That is a higher prevalence of the pathogen than that found in studies of eggs from large flocks, noted lead researcher Subhashinie Kariyawasam, microbiology section head at Penn State's Animal Diagnostic Laboratory. Eggs from large flocks, of more than 3,000 birds, are subject to federal regulations aimed at reducing Salmonella contamination.

These regulations require measures such as placement of Salmonella-"clean" chicks, cleaning and disinfecting between flocks, continuous testing of eggs from any Salmonella-positive houses, and diverting eggs from Salmonella-positive houses for pasteurisation.

"The research highlights the potential risk posed by the consumption of eggs produced by backyard and small layer flocks," Subhashinie Kariyawasam said. "These findings emphasise 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 US, Ms Kariyawasam noted. But she added that the growing demand for backyard eggs and eggs from smaller producers suggests these production systems deserve some scrutiny.