For centuries, oyster lovers have slurped down raw shellfish, enjoying the taste plus the fact they are low in calories and high in protein, iron, and other nutrients. But as ocean temperatures have warmed, some of the shellfish have become incubators for various species of genus Vibrio bacteria, some of it harmless, some of it causing serious illness or even death in those with compromised immune systems.
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According to the Molluscan Shellfish Institute of North America, Americans eat about 2.5 billion oysters each year, farmed rather than native-grown, as the latter populations have declined due to disease and other factors over the years.
Though Vibrio occur naturally in ocean saltwater and around estuaries and brackish water, their escalation in oysters, which filter feed and thus build up the bacteria, didn’t start until the 1970s when growers who had avoided selling the shellfish during the summer months to give them a chance to reproduce instead began selling them year-round, says Robert Tauxe, MD, MPH, deputy director, CDC’s Division of Foodborne, Waterborne, and Environmental Diseases based in Atlanta.
“The oyster harvests in the old days were suspended in the summer to let the oysters breed,” says Dr. Tauxe. “That changed in the 1970s and that is when Vibrio surfaced.”
He also notes while Vibrio occurs naturally, there is some suspicion that its movement from the Gulf of Mexico to northern U.S. oceans may be the result of its transfer in ballast water on ships, especially those in oil ports.
“Warm water increases the prevalence of Vibrio infections,” Dr. Tauxe says. Though infections are rare, they are most prevalent within the Gulf of Mexico coast from April to October. “It’s particularly important for people in the Gulf area during warm seasons to stay out of the water. Vibrio are champions among multipliers. They can multiply every 15-18 minutes.”
Up to 45,000 Vibrio cases occur a year, with most causing watery diarrhea, vomiting, abdominal pain, and even death. Of the 1,252 cases of vibriosis recorded in 2014, there were 326 hospitalizations and 27 deaths, according to the CDC’s Cholera and Other Vibrio Illness Surveillance system.
CDC surveillance epidemiologist Erin Burdette, MPH, adds that V. parahaemolyticus, the most common species of Vibrio, has been reported as far north as Maine in recent years. In 2004, an outbreak occurred in Alaska that was linked to oysters raised locally during one of the state’s warmest summers. In other cases, oysters are imported from other areas of the country and eaten elsewhere: therefore, harvest origins need to be traced if there’s a disease outbreak. During the winter, Vibrio cannot multiply but instead become dormant and drop to very low levels until the water warms again.
A Sentinel for a Changing World?
Dr. Tauxe likened Vibrio to a sentinel in the foodborne illness world. “There are other organisms that are impacted by an increase in heat to the water,” he says. “We expect to see new problems emerge as the landscape of foodborne infections changes.” This includes movement of pathogens like Vibrio into warming northern oceans and global food sourcing.
The three most frequently seen Vibrio are: 1) V. parahaemolyticus, which is the most common and is creeping up both U.S. coasts and their northern borders, 2) V. alginolyticus, which is the second-most common and whose infectiousness is associated with direct contact with sea water, and 3) V. vulnificus, which has the highest death rate, related to the consumption of raw shellfish and exposure to open wounds.