Detecting Bacterial Pathogens in Oysters

Susan McGinley
June 5, 2001

Oysters on the half shell lose their glamour if they've got Campylobacter or Salmonella lurking in their tissues. These bacterial pathogens are leading causes of food-borne illness around the world and are a particular concern in raw or lightly cooked shellfish.

"Both of these bacteria are normal flora in chickens, but when they get into humans, they cause disease," says Lynn A. Joens, a UA veterinary microbiologist who is testing clams and oysters from both the West and East coasts of the United States for the presence of Campylobacter and Salmonella Unlike poultry and other hosts, shellfish do not usually harbor these organisms, but they acquire them by filtering water through their systems and concentrating them in their meat.

Some shellfish species reside in estuaries where fecal runoff from sewage may pollute the water and penetrate their tissues. Because shellfish feed by filtering water to gather nutrients, they also pick up fecal pathogens which can thrive in their tissues without killing them.

Joens and his colleagues, including veterinary scientist John Mar, want to determine the extent of shellfish contamination for Salmonella and Campylobacter across the U.S. With help from the USDA and the North Carolina Department of Fisheries, they have begun collecting clams and oysters from several bays and rivers in Oregon and North Carolina.

"We've found Salmonella in fifty to seventy percent of the clams and oysters we've tested," Joens says. The incidence of Campylobacter has been much lower--about 10 to 15 percent. "We're checking various types of seafood, but we're concentrating on oysters since they are most often eaten raw."

Each state regulates its own oyster farms. "Most states test the water, but only for E. coli, as defined by FDA regulations," Joens notes. Commercial shellfish farms use a method called depuration to reduce the concentration of E. coli in oysters, but it doesn't work on either of the other two bacteria, according to his findings.

The technique involves putting the oysters in a tank of water and changing the water periodically, while testing it at intervals to make sure the bacteria are gone. This gets rid of the E. coli, but unfortunately the oyster itself concentrates more and more Salmonella and Campylobacter in its tissues as the water is filtered. "You actually have to test the meat to determine the amount of contamination," Joens says.

His strategy is to bring attention to contaminated rivers and bays by testing oyster meat from different locations for the presence of the two pathogens, then tracing back up those rivers to find the sources of the pollution. "We'll report our findings to the FDA so that they can clean up the contamination sources," Joens says. He plans to expand the testing program to include samples from locations up and down both coasts and the Gulf of Mexico. "We want to find the contamination so growers and restaurants across the country can sell their oysters to the public without making them sick."

About Campylobacter and Salmonella
Although most people have never heard of Campylobacter this hardy bacterial pathogen causes plenty of trouble worldwide. Both Campylobacter and Salmonella species can survive in meat products, and in fresh and salt water for days.

In Arizona, Salmonella is the number one cause of food-borne illness; Campylobacter is third, according to Lynn Joens, UA veterinary microbiologist. More Campylobacter food poisonings have been reported since 1998. He says most people pick both of these up by eating uncooked poultry left behind on improperly washed cutting boards and counter tops. The usual scenario is the barbecue.

"People will trim the raw chicken on the cutting board, transfer it to a plate, take it to the barbecue, cook it, and then put it back on the same unwashed plate and cutting board the raw chicken was on," Joens says. "Salmonella in particular is on both the skin of the poultry and in the meat, and doesn't just wash off. Only cooking to the proper temperature of 160 degrees gets rid of it."

Shellfish don't normally harbor Campylobacter or Salmonella but they do collect these bacteria by filtering contaminants from water as they feed. Cooked shellfish poses no threat, but oysters, which are most often consumed raw, may hold these contaminants for months before harvest.

Joens says the prevalence of Campylobacter and Salmonella food poisoning may be higher than current figures show because a lot of cases probably go unreported.


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