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The paper published in the journal Annals of Internal Medicine (AIM) described five cases where patients had come in contact with the waters of Delaware Bay. There, Vibrio vulnificus infections are traditionally rare because it is relatively cool compared with nearby Chesapeake Bay, where cases are commoner. Of the five patients, one died, while another had his leg amputated.

The doctors wrote they wanted to alert other clinicians that the bacteria appeared to be operating outside of "traditional risk areas." They argued climate change is likely partly to blame.

Over the past three decades, climate change has caused sea surface temperatures in many parts of the U.S. to climb, according to the authors of the AIM study.

Between 2009 and 2017, just one case of V. vulnificus was diagnosed at Cooper University Hospital near Delaware Bay. But in the summers of 2017 and 2018, the figure rose to five.

Professor John Glenn Morris, a director at the Emerging Pathogens Institute University of Florida who was not involved in those cases, explained to Newsweek: "Vibrio species happen to be very temperature-sensitive, and grow much better as water temperatures rise. During cold weather, they will almost completely disappear from the water."

"As water warms up, they will start growing—and, with climate change, even the degree or two of increase in water temperature being seen at multiple locations globally is enough to increase the number of Vibrios in the water—which, in turn, increases the risk that a human will come in contact with the microorganism and become sick, either by eating contaminated seafood, or through exposure of a wound to the water," said Morris.

"Environmental warming—the most obvious and pervasive environmental manifestation of climate change—increases both the numbers of these bacteria in the environment and the geographical range over which they are found," Craig Baker-Austin is a senior research scientist at the Centre for Environment Fisheries and Aquaculture Science told Newsweek. Baker-Austin researchers the bacteria and the threat they pose due to climate change.

Kimberly Reece, chair of Aquatic Health Sciences at the Virginia Institute of Marine Science, told Newsweek the effect is a longer Vibrio season from late spring to early fall in the Mid-Atlantic region, for example.

The "tropical" bug appears to be spreading northwards from areas like the Gulf of Mexico, explained Baker-Austin.

As such, over the past two decades incidents of Vibrio have doubled in the U.S., he continued.

The trend is not isolated to the U.S., according to Reece. "Scientists around the world have reported Vibrio species expanding their range into areas that previously had waters that were too cold," she said.

And as the temperature climbs, more people are likely to interact with the ocean in activities like swimming or fishing, said Baker-Austin. There were huge increases in infections in Europe during heatwave years of 1994, 2003, 2006, 2010, 2016, he said.

However, at the same time, the spike in cases could also be due to better surveillance and reporting of infections, and the rise in people living by and interacting with the coast, he added.

Yet, despite the bugs making headlines recently, Baker-Austin stressed Vibrio infections are "very rare." In the USA, there are around 500 hospitalizations and 100 fatalities yearly, he said.

"Vibrio vulnificus seldom infects healthy individuals, although some people who become infected may not realize that they have a condition that makes them more susceptible," said Reece. People whose health or immune systems are compromised are more susceptible to Vibrio vulnificus infections, for example those with liver disease, cancer, taking immunocompromising drugs and with HIV.

Reece advised those worried about catching the condition to remember to check local resources before heading into the water, and those without open cuts or wounds to stay out.

"If someone does get an injury while in the water they should seek medical treatment and inform their physician about the exposure to natural water," she said.