A decrease in eggshell quality is a trait that may be used to detect chickens infected with salmonella, according to Agricultural Research Service scientists.

Veterinary medical officer Jean Guard Bouldin, at the ARS Southeast Poultry Research Laboratory in Athens, Georgia, found that not only was salmonella present inside chicken eggs, but other bacteria were there also. Since these bacteria are usually seen in eggs that have been contaminated through cracks in the shell, Bouldin theorised that poor eggshell quality allowed the bacteria to enter the egg.

Salmonella enteritidis is hard to detect in chickens because there are no symptoms. This poses a significant problem, because salmonella, found inside the egg, is an important cause of human food-borne illness.

Bouldin and Jeff Buhr, of the ARS Poultry Processing and Meat Quality Research Unit in Athens, conducted tests in which chickens were inoculated with S. enteritidis. Eggs were then tested for hardness by compressing them until a hairline crack formed. Eggs from salmonella-infected hens cracked easier than those from non-infected hens. Other research has shown that some strains of salmonella seem to target the hen's reproductive tract, which appears to result in an egg with a less resilient shell, according to Bouldin.

At low-dose infection, Bouldin found that S. enteritidis actually stimulated egg production, particularly in older hens. This increased production may have stretched the limited eggshell material a bit too thin.

Other poultry diseases can also decrease shell quality, but usually they result in a decrease in production and illness in hens. Changes to eggshell quality over the lifespan of a laying hen are to be expected, and thus a hen's age could be an additional risk factor.

The Agricultural Research Service is the US Department of Agriculture's chief scientific research agency.