Where does V. parahaemolyticus come from and How are Vibrio infections prevented?

Never eat oysters in a month that does not contain the letter R. Or so they say, and they may be right, at least partly. In the summer of 1997, a lot of people ignored this advice, and a lot of people got sick.

It began in July in British Columbia, where the British Columbia Center for Disease Control (BCCDC) got some samples from nine patients who’d come down with gastroenteritis, the usual nasty combination of diarrhea, abdominal cramps, nausea, vomiting, fever, and bloody diarrhea.

All the samples contained a gram-negative bacterium called Vibrio parahaemolyticus. They wanted to learn where this was coming from, so they tracked down eight of the nine people who’d given them samples to find out what they’d been eating. Seven of the eight had eaten raw oysters. By the end of July, the BCCDC issued a warning telling people not to eat raw or undercooked oysters.

By mid-August, they’d figured out where the contaminated oysters were coming from, and the Federal Department of Fisheries and Oceans closed all British Columbia coastal waters to oyster harvesting. They kept them closed until September 12, and no further cases of vibrio infection were reported.

In the meantime, the Washington Department of Health (WDOH) was getting similar reports. About two weeks after the British Columbia infections were reported, the WDOH advised commercial harvesters to refrigerate oysters within four hours after gathering them, and a week later issued a public advisory to cook all oysters thoroughly before eating them. When the WDOH interviewed people who were infected, they found that 90 percent of them had eaten raw oysters.

The WDOH closed Washington oyster beds on August 28, then reopened them on September 15, after which no additional cases of infection were reported in these states, and issued similar warnings and oyster bed closing orders there. Oregon closed their infected oyster beds on August 26, and no further cases were reported. But California didn’t bother to close their beds, which was too bad for the citizens of California, because infections there continued through December. Eighty-three Californians got sick. By the time the whole thing was over, 209 confirmed cases of V. parahaemolyticus infection had been reported in the Northwest. Two people had to be hospitalized, and one died.

V. parahaemolyticus loves warm water, and it is possible that the increasing number of outbreaks of disease, there have been four multistate outbreaks since 1997, is one more unpleasant result of global warming. The surface temperature of the Pacific coastal waters between May and September 1997 ranged from 54°F to 66°F, two to nine degrees warmer than during the same period in 1996.

The same observation was made in an outbreak in summer 1998 in Long Island Sound: the mean temperature of the water that summer was 77.2°F, compared to 74.1° in 1997 and 69.4° in 1996. These epidemics also called into question the level of V. parahaemolyticus that is safe to consume. It is known that the more you eat, the sicker you get, and that small amounts are harmless. But the level of contamination that caused illness in these outbreaks was less than 200 cfu per gram, when the level at which closing oyster beds is recommended is 10,000 cfu per gram.

CFU stands for colony forming unit, or a single cell capable of reproducing to form a colony. The CDC has affirmed that this standard must be revised.

Other species of vibrio have also been implicated in gastroenteritis outbreaks. V. vulnificus, for example, causes most of the food-related deaths in Florida (almost all of these people have some underlying illness, usually liver disease, or are immunocompromised). These infections also come almost exclusively from raw oysters, and typically in the summer months.

Vibrio infections don’t come from eating oysters that grew in “dirty” water. Vibrio is a naturally occurring bacterium in seawater, not a result of pollution. Raw oysters are delicious. Eating them holds a small chance of making you sick. If you’ve got liver disease or are otherwise ill or immunocompromised, it’s probably a good idea just to stay away from raw oysters completely.

But the risk that a healthy person will get sick from eating raw oysters is very small indeed, and even if you do get sick, the illness is likely to be mild and pass quickly. Many healthy oyster lovers will probably be willing to take their chances.