Another Reason to Avoid Shellfish

A shrimp cocktail is a culinary delight, but scientists warn that cooked and ready-to-eat shrimp bought at supermarkets may still harbor dangerous food pathogens.

Douglas Marshall, a professor of food science at Mississippi State University was surprised that he could isolate so many pathogens on a popular ready-to-eat product. The pathogens should have been killed when the shrimp were processed as ready-to-eat food, he said.

Marshall speculated that the problem could be caused by inadequate cooking of shrimp at plants, poor sanitation, contamination by workers or poor water quality in Third World countries where some of the products were processed.

Some of the pathogens he found, he explained, are not normally found in seafood, but are commonly found carried by humans or animals.

The Institute of Food Technologists earlier this year warned that supermarket trends towards ready-to-eat products could lead to outbreaks of food-borne diseases. Some scientists recommend that, as a precaution, prepared foods be re-cooked to kill any pathogens that might have developed.

Marshall and graduate student Gianna Duran launched their study to see if shrimp might have antibiotic-resistant bacteria. They purchased 13 packages of ready-to-eat shrimp from different manufacturers at local grocery stores. Some were refrigerated products, and others frozen. The frozen shrimp were then thawed for the study.

The scientists concluded that 42 percent of the bacteria cultured from the shrimp were drug-resistant. Marshall said fish and shrimp farms use drugs to treat diseases in their stock, and that it is not surprising there should be drug-resistant strains in the shrimp sold at supermarkets.

But the study also found pathogens that should have been killed in the preparation process, including a waterborne agent called vibrio vulnificus. Cases of diarrhoea and even death have been linked to raw oysters and clams tainted with the pathogen.

Other pathogens, like E. coli, staphylococcus and shigella, were also found in the shrimp, although they are carried in the intestines of warm-blooded animals. The study said that this suggests fecal contamination of shrimp, probably after cooking by workers using unsanitary practices.

Marshall suggested that people with compromised immune systems should re-heat prepared foods before consuming them.

Annual Meeting of the American Society for Microbiology in Salt Lake City May 2002

PHILLIP DAY'S COMMENT: Regularly during my talks, I advise against the consumption of shellfish because of contamination and also because these creatures are carrion-eaters or filter feeders. Aside from parasitic or bacterial infections, shellfish can contain unacceptable levels of mercury and other heavy metal contamination from the surrounding seas.

Best avoid.