Salmonella can decrease egg shell quality.

While performing research on S. enteritidis in poultry, USDA-ARS veterinary medical officers came across an interesting phenomenon. Not only is Salmonella present inside seemingly uncracked chicken eggs, but other bacteria are there as well.

These other bacteria are usually found in eggs that have cracks in the shell. But researchers questioned how they could get into unbroken eggs. The investigators also noticed a decrease in the quality of egg shells among infected birds in some tests they had undertaken. They theorized that the reason other bacteria besides Salmonella gained entry into the egg was that shell integrity was being compromised.

To test their theory, the scientists consulted other USDA-ARS colleagues on mechanical methods that could be used to measure the strength of an egg shell. The investigators inoculated chickens with S. enteritidis and evaluated eggs with an Instron, a standard laboratory instrument used to test food firmness by means of compression.

The shells were compressed until a hairline crack appeared. Eggs from Salmonella-infected hens cracked more easily than those from non-infected hens. S. enteritidis seems to target the hen's reproductive tract, which sometimes results in an egg with a less resilient shell.

Other experiments that used a high dose of bacteria in hens confirmed that S. enteritidis targeted the avian reproductive tract. The reproductive tract organs involuted, or shrank, after exposure, even though the hen continued to appear healthy. At a low-dose infection, researchers 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. 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, the researchers wonder whether the decline is occasionally worsened by an active infection from a pathogen. Either way, the discovery could inadvertently lead to a way to detect Salmonella-infected birds by testing eggshell integrity. The scientists' next step is to develop techniques to use as a sensitive assay of bird health in general, leading to the potential for controlling S. enteritidis infection and a way to detect other pathogens that result in poor shell quality. In the meantime, producers themselves can review their own production records for unusual and unexpected increases in production that might signal a need for further evaluation of the flock.

Further information. Jean Guard Bouldin, USDA-ARS Southeast Poultry Research Laboratory, 934 College Station Rd., Athens, GA 30605; phone: [phone]; fax: [phone]; email: [e-mail].