Effect of Salmonella on eggshell quality

Agricultural Research, Oct, 2004 by Sharon Durham

While researching Salmonella enteritidis in poultry, ARS veterinary medical officer Jean Guard Bouldin (nee Petter) found an interesting phenomenon. Not only is Salmonella present inside seemingly uncracked chicken eggs, but other bacteria are there too.

These other bacteria are usually found in eggs that have cracks in the shell. How could they get into unbroken eggs?

Bouldin and her colleagues at the Southeast Poultry Research Laboratory in Athens, Georgia, noticed a decrease in shell quality among infected birds in some experiments. They guessed that the reason other bacteria besides Salmonella gained entry into the egg was that shell integrity was being compromised.

To test her theory, Bouldin consulted with Jeff Buhr, of the ARS Poultry Processing and Meat Quality Research Unit in Athens, Georgia, on mechanical methods that could be used to measure the strength of the shell. They inoculated chickens with S. enteritidis and evaluated eggs with an Instron, a standard laboratory instrument used to test food firmness through compression. The eggs were compressed until a hairline crack formed. Eggs from Salmonella-infected hens cracked more easily than those from non-infected hens. "Salmonella enteritidis seems to target the hen's reproductive tract, which sometimes results in an egg with a less resilient shell," says Bouldin.

Other experiments that used a high dose of bacteria in hens confirmed that S. enteritidis targeted the avian reproductive tract, because the reproductive tract organs involuted, or shrank, after exposure, even though the hen continued to appear healthy.

At a 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--calcium--a bit too thin, literally. Other diseases of chickens can also decrease shell quality, but they usually decrease egg production and cause symptoms of illness.

Though eggshell quality normally decreases over the lifespan of a laying hen, Bouldin wonders whether the decline also occurs from pathogen infection. Either way, her discovery could inadvertently lead to a way to detect Salmonella-infected birds--by testing eggshell integrity.

"Our next step is to develop methods to use as a sensitive assay of bird health in general, leading to a possible control of S. enteritidis infection and a way to detect other pathogens that result in poor shell quality," says Bouldin.

Jean Guard Bouldin is with the USDA-ARS Southeast Poultry Research Laboratory, 934 College Station Rd., Athens, GA 30605; phone (706) 546-3446, fax (706) 546-3161, e-mail jbouldin@seprl.usda.gov.