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Salmonella carriers worry scientists.

It is well known that pigs infected with Salmonella typhimurium bacteria can be treated with antibiotics and recover.

But what worries microbiologist Paula J. Cray and immunologist Thomas J. Stabel of the National Animal Disease Center in Ames, Iowa, is that some pigs appear to recover but still carry the disease.

"Pigs can be infected but show no signs of disease. And even after careful administration of antibiotics, small pockets of the bacteria may still remain in the pig's internal organs," says Cray.

So the object of her research is to eliminate or reduce as much as possible the cartier state.

"To this end, our approach is to reduce the number of infected pigs, as well as decrease the number of bacteria carried by infected pigs. Doing either of these successfully will decrease the risk of human exposure," says Cray.

Risks can be minimized, however, by following established food safety recommendations. [See box.]

In market pigs, shipping stress appears to reactivate Salmonella bacteria in those pigs that are infected but show no signs of disease. Once reactivated, the organisms are shed in fecal matter. Bacteria quickly spread among pigs crowded together in trucks, says Cray.

She would like to understand more about how the bacterium is passed from infected pigs to those that have never been exposed. The most significant finding in her work so far has been that pigs can become infected soon after being exposed to very low levels of the bacteria. Cray found that uninfected pigs can shed Salmonella bacteria within 2 days of exposure to infected pigs. And piglets can become infected after birth and before weaning, if they are nursing a sow that's shedding Salmonella.

"This was a surprise and it changes our approach to using new vaccines," says Cray.

She has been evaluating the effectiveness of a dozen genetically modified vaccines in which the bacteria have part of the virulent region of DNA missing. Roy Curtiss, III, of Washington University in St. Louis, Missouri, created the vaccines that Cray has tested in pigs.

"The altered vaccine strain allows an animal's immune system to develop a response, but does not overwhelm the animal and cause disease. Ultimately, the bacteria are cleared from the animal," says Cray.

"Two of these modified vaccines look promising. In tests, they didn't cause disease or weight loss in pigs receiving oral doses," says Cray.

An effective Salmonella vaccine is badly needed. At best, current vaccines may modify the course of the disease and the severity of symptoms, but they haven't been shown to eliminate the disease, says Cray.

The researchers have yet to determine when and how to vaccinate pigs to achieve a level of immunity that's effective against a small number of bacteria.--By Linda Cooke, ARS.

Paula J. Cray and Thomas J. Stabel are in the ARS-USDA Physiopathology Research Unit, National Animal Disease Center, P.O. Box 70, Ames, IA 50010. Phone (515) 239-8672, fax number (515) 239-8458.

Food Safety Recommendations

Salmonellosis is one of the most common types of foodborne illnesses.

Its common symptoms are nausea, vomiting, fever, and diarrhea. Most people recover fairly quickly from the illness, but it can be fatal to those with weakened immune systems. Most at risk are pregnant women, infants, the elderly, and AIDS patients.

Salmonellosis is often caused by improper storage and cooking of meats that may have been contaminated. Therefore, it is important that all food products be handled and cooked properly.

The USDA's Food Safety and Inspection Service recommends:

* Refrigerate raw meat promptly and store for no more than 3 to 5 days; or freeze, if you do not intend to use soon after purchase.

* Wash hands thoroughly, both before and after handling raw meat.

* Use a plastic, dishwasher-safe cutting board--not wood.

* Wash countertops, cutting board, and any utensils that have come in contact with raw meat with hot, soapy water.

* Don't prepare, cut, or carry raw meat on the same plate that you later use for cooked meat.

* Don't let juices from raw meat drip on other foods.

* Cook raw pork and beef to an internal temperature of 160[degrees]F at its thickest point and poultry to 185[degrees]F.
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Author:Cooke, Linda
Publication:Agricultural Research
Date:Jan 1, 1993
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