Parasite found in oyster beds

Scientists discover Cryptosporidium in several bay areas

Report is for 1997, 1998

Organism was source of Milwaukee illnesses

no outbreak in Md.

August 31, 1999|By Joel McCord | Joel McCord,SUN STAFF

Scientists have detected the presence of the parasite that caused the largest outbreak of water-borne illness in U.S. history in oysters taken from commercially harvested beds in Chesapeake Bay and its tributaries.

The oysters containing Cryptosporidium, which can cause vomiting and severe diarrhea, were taken from beds in the Wicomico, Nanticoke, Potomac and Patuxent rivers, Fishing Bay and Tangier Sound in the fall of 1997 and winter and fall of last year, the scientists reported.

The scientists from the U.S. Department of Agriculture in Beltsville, the National Oceanographic and Atmospheric Administration laboratory at Oxford on the Eastern Shore, the Johns Hopkins University School of Public Health and the Centers for Disease Control in Atlanta reported their findings in the September/October issue of the CDC's journal, Emerging Infectious Diseases.

The organism contributed to more more than 100 deaths in Milwaukee in 1993.

The Maryland study was a follow-up to a 1997 study that found Cryptosporidium in oysters in beds near sewer plant outfalls and cattle farms that have been closed to commercial harvesting.

"We wanted to continue this work to find out if Crypto was also present at commercial sites," said Thaddeus K. Graczyk of the Johns Hopkins School of Public Health.

In both studies, the incidences of the microorganism varied from bed to bed and sample to sample with no discernible pattern, said USDA scientist James Trout.

The parasite, nicknamed Crypto, replicates in the intestines about a week before a victim experiences diarrhea, fever, vomiting and stomach cramps. It is especially dangerous to infants, the elderly and cancer patients receiving chemotherapy. It can be fatal to acquired immune deficiency syndrome patients, whose immune systems have been damaged by the disease, Trout said.

A treatment hasn't been found, but people with healthy immune systems usually recover on their own within a few days.

The scientists who examined the Chesapeake Bay sites warned against eating raw oysters, but said frying, steaming or stewing them would kill the parasite.

No recent outbreaks of Cryptosporidiosis have been reported from people eating raw oysters, said Graczyk.

Oysters feed by filtering water through their gills. The parasite, from humans and other larger mammals, washes into the water from fields and storm drains, stays on gills and spreads disease.

The organism, about the size of a human red blood cell, settled into the Milwaukee water supply, causing an outbreak that sickened more than 400,000 people in addition to the deaths.

The cause was never pinned down, but health officials in Wisconsin say it might have been related to problems with filtration at a water treatment plant and run-off from farms into Lake Michigan, where the city draws its water.

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