Oysters harvested in areas of the Chesapeake Bay close to sources of sewage or farm runoff were found to contain a parasite that could make humans ill, scientists have found.

In a study published in the March issue of the journal, Applied and Environmental Microbiology, scientists said they sought to determine if oysters harbor cryptosporidium. The parasite sickened 400,000 people after it was found in Milwaukee's drinking water in 1993.

It's transmitted only by drinking contaminated water or by swallowing something tainted with infected human or animal feces. Cooking food and boiling beverages for one minute kills the parasite.

Maryland officials said there has never been a cryptosporidium outbreak associated with oysters and other shellfish.

Because shellfish such as oysters and clams are often eaten raw, "they pose a potential public health risk when they concentrate pathogenic bacteria and viruses from polluted waters," the study said. "... No human cases of cryptosporidiosis have been linked to ingestion of raw shellfish." The scientists harvested oysters May through June and August through September last year from six sites in rivers that empty into the Bay. The sites were near where sewage is discharged, or near septic tanks and cattle farms where runoff could leach into the Chesapeake.

"It is important to emphasize that the oysters examined in the present study were from sites selected because of their close proximity to possible sources of contamination," the authors said.

The scientists said laboratory tests had shown oysters could harbor the parasite. The study was the first in natural waters, the authors said.

The study was conducted by researchers from the U.S. Department of Agriculture, the School of Hygiene and Public Health at Johns Hopkins University and the National Marine Fisheries Service.

Quentin Banks, a spokesman for the Maryland Department of the Environment, said it was the first time anyone had looked for the parasite in shellfish, and presumably it could be found in shellfish samples nationwide.

Maryland imposes strict water standards in areas where shellfish are harvested, Banks said. Most of the areas where the oysters for the study were harvested are restricted, he said.

 "There's always a risk when you eat raw shellfish," Banks said. "Our department has always advocated that shellfish should be cooked."