The British Columbia Provincial Laboratory detected Vibrio parahaemolyticus from nine patients, more than twice the expected number, in July. The patients had reported that they had eaten oysters within 24 hours prior to becoming ill. Between July and September additional illnesses were reported. Traceback of the oysters consumed by the ill showed that the oysters had been harvested from the coastal waters of British Columbia. These harvesting areas were closed until mid September.

Beginning in May, reports of illness in Washington state were traced to the consumption of oysters harvested from Washington state beaches.

In July, the Washington state Department of Health (WDOH) advised the public to avoid eating uncooked oysters. In August, the Pacific Oyster Growers' Association voluntarily halted shipments of shell oysters from Washington and the WDOH closed the oyster beds in major shellfish harvesting areas. In August, Oregon Health Division began receiving reports of illness. The oysters consumed by these persons had originated from British Columbia, Washington state, Oregon, and California. The implicated oyster bed in Oregon was closed to sale of uncooked oysters until September.

Between May and July, the San Francisco Department of Public Health began to note an increase in illness. In August, a public advisory was issued to avoid eating oysters from Washington and British Columbia. Some persons had reported eating oysters from Tomales Bay, California. Illnesses in California were noted through early December.

The Vibrio isolated from ill persons were a variety of serotypes, some of which matched the serotypes found in the oysters. Mean Pacific coastal sea surface temperatures for this year were 2F to 9F above the recorded temperatures for 1996. Elevated water temperatures may have contributed to this outbreak.