Vibrio on the rise as climate warms the Chesapeake Bay

One day last July, Vernon Spear scratched his forearm on a crab pot as he pulled it out of the Choptank River from his pier in Cambridge, MD. He put hydrogen peroxide on the tiny cuts, thinking that would disinfect them.

That night, his forearm swelled up, and he spiked a high fever. The next morning, the fever was gone, and his arm didn’t hurt. But the wounds were bright red and oozing. Alarmed, his wife and son insisted that Spear go to the local emergency room.

The physician who saw him there immediately recognized that Spear was in imminent danger of losing his arm — and maybe his life — from a Vibrio vulnificus infection.

He was rushed by helicopter to the R Adams Cowley Shock Trauma Center in Baltimore, which specializes in treating severe injuries and critical illnesses.

Spear was fortunate: He survived and kept his arm. But he’s spent most of the past year undergoing treatment, including skin grafts, to restore his arm from what’s often called the “flesh-eating bacteria.”

Cases like Spear’s remain relatively uncommon, but their number is increasing because of climate change, scientists say. And as the Chesapeake Bay warms along with the rest of the planet, its mildly salty and nutrient-gorged waters offer prime habitat for the potentially deadly microbe.

Naturally occurring Vibrios are naturally occurring bacteria found in warm, brackish coastal waters, including the Chesapeake and its tributaries.

There are more than 80 vibrio species, and many play an ecological role. They recycle carbon and nitrogen in the estuary by breaking down chitin, the tough substance in the shells of blue crabs, insects and even tiny zooplankton.

But about a dozen species can make people ill, either through swallowing them or getting them in a wound. Nationwide, the U.S. Centers for Disease Control and Prevention (CDC) estimates there are about 80,000 vibrio infections each year.

The majority are food-borne illnesses, mostly from eating uncooked oysters or other shellfish. In those cases, people can suffer diarrhea, stomach cramps, nausea, vomiting, fever and chills. The symptoms usually abate within a few days, but they pose more serious health risks for individuals with immune deficiencies.

V. vulnificus, the type that infected Spear, can also enter the body through open wounds or even scratches exposed to bacteria-infested waters. When that happens, the infection can lead within hours to sepsis, a dangerous overreaction of the immune system that can kill human tissue, shut down organs and cause death if not treated promptly.

There were more than 400 vibrio cases — from all species — reported in Maryland and Virginia in 2025, according to the states’ health departments. But they are likely undercounted. Many people who get sick from eating vibrio-infested shellfish or fish probably weren’t ill enough to be hospitalized or even see a doctor.

“Food-borne illness overall is vastly underreported,” said Kelsey Holloman, food-borne disease epidemiology program manager for the Virginia Department of Health.

Reported cases increasing

In recent years, the number of vibrio infections reported in Maryland has more than doubled — from 100 in 2018 to 260 in 2025, according to the state Department of Health. This year, 72 cases had been reported by late June.

The vibrio case count in Virginia, though lower overall, has nearly doubled in the same period, from 86 infections reported in 2018 to 151 in 2025, according to the state Department of Health. By late June this year, 42 cases had been reported.

Improved testing technology accounts for some but not all of the increase, experts say.

“I think environmental conditions have changed,” said Dr. Clifford Mitchell, environmental health bureau director of the Maryland Department of Health.

“Vibrio do thrive in warmer waters,” Holloman noted, “so as the temperatures rise and stay warm longer, we would expect [cases] to increase.”

Over the past 30 years, driven by warming air, the average water temperature in the Chesapeake Bay has increased by 1.8 degrees, according to the National Oceanic and Atmospheric Administration.

V. vulnificus, the so-called “flesh-eating” species, thrives when the water is warmer than 64 degrees and has salinity between 10 and 20 parts per thousand. Those conditions are met for at least part of the year throughout most of the Bay and the lower portions of its tributaries.

Add to that mix the elevated levels of nutrient pollution throughout the Bay, and conditions are optimum for vibrio to thrive.

“Vibrios are present in very, very low numbers in traditionally unpolluted waters. They’re part of the natural flora,” said Rita Colwell, a microbiologist and marine scientist with the University of Maryland who’s spent much of her six-decade career studying the bacteria.

“But as soon as we create eutrophic [low oxygen] conditions and with increased temperature of the water,” she added, “that’s sufficient to change the composition of the microbial population.”

Temperature major driver

Warming water is the major driver of increased vibrio. The prime season for vibrio traditionally has been from May through October, but with coastal waters trending warmer, infections have been popping up earlier and later in the year. A V. vulnificus case occurred in April this year, the Maryland Department of Health reported.

Colwell’s research has focused in part on V. cholerae, which is responsible for cholera, a potentially life-threatening illness caused by consuming contaminated water or food. According to the CDC, 1 to 4 million people worldwide get cholera annually, and 20,000 to 140,000 die.

There are other illness-causing microbes in the water, too. Oysters can pick up norovirus and salmonella from contaminated water. People who eat them might experience stomach cramps, diarrhea and vomiting.

But vibrios are a special concern because they can proliferate in oysters and other seafood harvested from otherwise untainted waters, and in places considered safe for swimming. Also, the consequences of an infection can be more severe, especially for people with underlying medical conditions.

A 2024 study found a 39% increase in the per capita incidence of vibrio infections in Maryland from 2006 through 2019, with the greatest increase from V. vulnificus.

All along the East Coast, another study estimated that wound infections from V. vulnificus grew eightfold, from 10 cases in 1988 to 80 in 2018. Florida has the most cases, but the northernmost case reported has shifted gradually northward. As sea waters continue to warm and vibrio spread to New York and beyond, the study warned that V. vulnificus cases could double in 20 to 40 years. In 2024, a Rhode Island resident died from an infection, that state reported.

Nationwide, 150 to 200 V. vulnificus infections are reported to the CDC annually. About one in five infected people die, sometimes within a day or two of becoming ill.

A handful of deaths each year

In Maryland last year, there were 14 confirmed cases of V. vulnificus, including an outbreak in Wicomico County tied to a crab feast. Three deaths were attributed to the infections. Virginia reported 11 confirmed cases in 2025, with one death. Infections from other vibrio species accounted for four other deaths combined in the two states.

To put those numbers in perspective, people are much more likely to die from a motor vehicle accident than from vibrio. The nationwide traffic fatality rate in 2024 was 11.5 per 100,000 people, compared with a total of eight vibrio deaths in 2025 in a combined population of roughly 15 million in Maryland and Virginia.

While still relatively uncommon, the pain and suffering from a vibrio death are significant. The costs to society are also substantial. Infections from eating seafood contaminated by Vibrio parahaemolyticus impose about $20 million annually in health care costs and lost income, according to one study.

Because V. vulnificus has such a high mortality rate, the health care costs and estimated lost income from illness and premature death are much greater — about $238 million, the study added.

Shellfish safety precautions

Foodborne illnesses from vibrio and other microbes are also an economic risk for oyster growers, whose business depends on customers trusting their products’ safety. Federal and state agencies have laid out rules for the harvesting and handling of shellfish to prevent transmission of bacteria and other pathogens.

Farmed oysters can be harvested year-round, even in warm weather. But they must be landed in the morning from May through October, and they must be refrigerated within two to five hours of being pulled from the water, depending on time of year, to limit the chance that vibrio might proliferate.

JD Blackwell, owner of 38° North Oysters, said he purposely selected the site in rural St. Mary’s County where he raises oysters to steer clear of sewage spills and other contaminants. He said he finds government rules on harvest and handling quite reasonable, and he does his best to go beyond those requirements by shortening the time to land his shellfish and getting them quickly iced down.

“Anything I can do to get behind the issue, we execute on,” he said.

Blackwell said he’s only had one of his employees get a suspected and relatively mild case of V. vulnificus in the past 15 years, but it wasn’t clear how or where he acquired it.

All of his staff wear wet suits and gloves when handling the oysters, he said.

Forecasting vibrio levels

Given the consequences of vibrio infections, scientists and industry experts have developed computer models that can forecast where and when vibrio levels might increase.

“We can’t predict what number [of infections] will occur,” Colwell said, “but we can predict the risk.”

Working with the U.S. Food and Drug Administration, state agencies, and university scientists, the National Oceanic and Atmospheric Administration has developed tools for assessing vibrio risks along the nation’s coasts, including in the Chesapeake Bay.

In the Chesapeake, one model aims to help oyster farmers gauge the best time to harvest their product. It uses water temperature and salinity data to predict concentrations of V. parahaemolyticus around the Bay up to 48 hours in advance. Another tool tracks air temperatures and projects how quickly the vibrio will proliferate in oysters once removed from the water. That can give growers an idea how quickly they should refrigerate their shellfish.

For the public at large, a third tool aims to assess levels of V. vulnificus around the Bay and in its tributaries. Drawing on two decades’ worth of water sampling by state agencies and molecular testing for vibrio DNA in the water, it estimates concentrations of the bacteria based on temperature and salinity for the previous six days, the current day and the next one. To view it, search the web for “NOAA vibrio forecast.”

“It’s an inherent risk of recreating in a natural body of water,” said Ava Ellett, a marine biologist with NOAA’s National Ocean Service in Oxford, MD. “We’re not trying to prevent people from recreating in the water,” she added, but “just provide that context.” To take it one step further, she and others are working on yet another model they hope can provide a long-range forecast of expected vibrio illnesses every summer in the Chesapeake region.

Henry Sage, one of Colwell’s graduate students looking for V. cholerae in the Potomac River this summer, said the point of his and others’ work is not to scare people away from eating oysters or going in the water.

“I do recreate on the Potomac at times,” he said. “I want to know if I’ll be affected.”

There is no vaccine to prevent vibrio infections, so risk reduction is up to each individual. Cooking oysters and other fish kills the bacteria, for instance. Covering up cuts or wounds or staying out of the water until they heal avoids that infection pathway. 

After his ordeal with V. vulnificus, Vernon Spear isn’t taking any chances. His crab pots washed away while he was undergoing treatment and, with his focus on recuperating, he hasn’t replaced them.

Meanwhile, he said, “I love raw oysters, but I’ll never eat another.”