As Oceans Warm, Vibrio Increases

V. parahaemolyticus falls in the same family of bacteria that causes cholera. The bacteria do not change the taste, smell, or look of an oyster, so it’s hard for consumers to tell if the oyster has the bacteria or whether the strain it has is indeed infectious.

Species such as the potent V. vulnificus can enter the body through a wound. Those who have liver disease, chronic illnesses like diabetes, or are otherwise immune-compromised are particularly susceptible and should either stay out of the water or cover their wound with a waterproof bandage, according to Dr. Tauxe. That being said, V. vulnificus infections are uncommon—with just a couple 100 cases a year, half from wounds and half from raw oyster consumption.

Image Credit: CDC/Foodnet

Over the past 20 years, E. coli 0157, Yersinia, Campylobacter, and Listeria culture-confirmed infection rates have decreased significantly while Salmonella showed no change. Comparatively, Vibrio cases showed a sharp rise (see Chart 1), according to the CDC’s Foodborne Diseases Active Surveillance Network (FoodNet). In 2015, Vibrio cases were up 34 percent compared to 2006-2008. Relatively, E. coli 0157 was down 30 percent over the period.

Within the Vibrio types, V. Vulnificus has been decreasing in recent years, likely the result of increased control of post-harvest time and temperature requirements of the shellfish industry, Jason Strachman-Miller, an FDA spokesman, said in an email.

“A decrease of over 30 percent has been observed for 2013 and 2014,” he wrote. “While the illness records for 2015 have not been finalized, it is expected that this decrease will continue.”

However, annual illnesses for V. parahaemolyticus in 2013 and 2014 almost doubled, Strachman-Miller noted, likely because of the emergence of the Pacific Northwest type of V. parahaemolyticus in the Mid and North Atlantic. He stated preliminary numbers for 2015 indicate a return to the baseline, although illness data is not finalized.

Dr. Tauxe says the CDC supervises the human health aspects of Vibrio while the FDA supervises shellfish sanitation programs. State health departments send out alerts in an attempt to ensure consumer safety.

Testing the Waters

Currently, says Dr. Tauxe, the needed markers and tests for Vibrios are not in place.

Cheryl Whistler, PhD, researcher and associate professor of microbiology and genetics, University of New Hampshire, Durham, N.H., and her colleagues are working to change that. They are using whole genome analysis to assess genetic characteristics of distinct Vibrio populations in the Atlantic. The hope is that the information will help with more accurate trace-back. So far, their analyses are allowing identification of strain-specific genetic loci they can use to develop strain-specific quantitative polymerase chain reaction, or PCR, detection assays for rapid quantification of the strains causing the most concern.

“We can quantify the total degree of all strains [in the water],” Dr. Whistler says. “While we can detect the total number of Vibrio, we want to be able to quantify which of them are pathogens.” Not all Vibrio are pathenogenic. “It’s like looking at football spectators and trying to find only those with red hair, blue eyes, and no freckles.”

Dr. Whistler says she is interested in whether validating specific strains will be a better predictive tool in modeling the combination of conditions that increase risk and warns of cross-contamination. “The first case of Vibrio was a case of cooked oysters put back on the ice the raw product was on,” she says.

So while her tests can tell there is Vibrio in the water and its prevalence, ultimately it’s up to consumers to choose what they eat and how they eat it.

“Anybody who wants to eat an oyster should have the choice to cook it or not,” she says. “Raw oysters are a delicacy. Cooking loses the flavor and texture.”

Making Oysters Safe

There are some methods that have helped diminish harm from V. parahaemolyticus. For example, harvesters need to refrigerate oysters as soon as possible after catch to decrease the temperature, Dr. Tauxe explains. He says that approach has worked well in Japan, which had a big problem with the bacterium. The cooling doesn’t kill the oysters, which would alter their taste and texture, but prevents future growth of the bacterium.