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Disease detectives investigating a pair of medical mysteries in Mississippi have come to an unwelcome conclusion. The bacterium that causes a rare but dangerous disease called melioidosis is likely endemic in parts of the southern United States, along the Gulf Coast.

The bacterium, Burkholderia pseudomallei, was previously not thought to be found in the U.S., meaning people who had symptoms similar to those of melioidosis but who had no history of international travel were considered unlikely to be infected with it. That assumption is no longer sound — or safe, said experts at the Centers for Disease Control and Prevention, which on Wednesday issued a health alert to physicians.

“This is a big deal because clinicians generally speaking only consider melioidosis in patients who have recent travel to an endemic area like Thailand, South Asia, Australia. They would not consider just somebody living in the Gulf Coast area who has never traveled, that they would have melioidosis,” Julia Petras, an epidemic intelligence service officer with CDC’s bacterial special pathogens branch, told STAT.

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“So that’s really the purpose of this [alert] is to say: Look, we found it here. It is locally endemic in this region. Be on alert. Because this disease can look like a lot of different things, so it can be challenging to diagnose,” she said.

Melioidosis is generally considered a disease of the tropics. It can infect both people and a range of animals, including sheep and cows. Though infections are rare, it is dangerous enough to be seen as a potential bioterrorism weapon; it is classed as a Tier 1 select agent, the same class to which threats like Ebola and anthrax belong.

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The bacteria that cause it are found in contaminated water and soil. People can be infected through breaks in their skin if they come in contact with contaminated soil or water. It’s also believed people can contract it by inhaling contaminated dust or water droplets and by drinking or eating contaminated water or food.

The new finding stems from a case of melioidosis that was diagnosed in southern Mississippi in 2020. The man infected had not traveled outside the United States. An investigation was conducted at the time, but no possible source of his illness was identified.

Fast forward to May of this year, when a second individual who lives roughly 10 miles from the earlier case was diagnosed with melioidosis. Like the first case, this person had not traveled to a place where B. pseudomallei is found. Local health authorities, with the assistance of the CDC, conducted environmental sampling around the houses of both cases, and found three samples taken from around the first person’s home were positive for the bacteria.

Both individuals were seriously ill with their infections, but both recovered. The most recent case, though, is still undergoing treatment. People who develop melioidosis are given two weeks of intensive intravenous antibiotics, followed by three to six months of oral antibiotics.

Genetic analysis of bacterial samples taken from both cases showed they were clonal — very similar — and were from the Western hemisphere strain of B. pseudomallei. That strain of the bacterium is found in tropical parts of the Americas.

Modeling the CDC has conducted suggests multiple states along the Gulf Coast have environmental conditions that are hospitable to B. pseudomallei, Petras said, including “parts of Florida, Alabama, Texas, Louisiana, Mississippi.”

It is impossible to know how long the bacterium has been in the region, but she suggested it may have been for some time. In 2004 and again in 2018, Texas reported cases of melioidosis in people who had not traveled outside the country. The sources of those infections were not found, but Petras noted both people lived along the Gulf Coast. Bacterial samples from the Texas cases were also from the Western Hemisphere strain, and were similar but not identical to the Mississippi cases.

Changing environmental conditions may have given the bacterium a foothold in an area where it previously couldn’t thrive.

“I think that climate change could play a role. Rising temperatures could play a role in the grand scheme of things because the bacteria prefers a tropical or subtropical climate,” Petras said.

The U.S. typically sees about a dozen cases of melioidosis a year, almost always in people who have returned from traveling in Southeast Asia or northern Australia.

Even in places where B. pseudomallei is endemic, melioidosis is still a rare disease. But in people who contract it, it can be deadly; it has an estimated fatality rate of between 10% and 50%. People with certain health conditions — like diabetes, alcoholism, kidney disease, and chronic lung disease — are at greater risk of becoming seriously sick if they contract the illness.

There are four types of infection: localized, in the lungs, bloodstream infection, or disseminated infection, in which a localized infection spreads to another part of the body; symptoms vary depending on the site of infection. They may include localized pain or swelling, fever or high fever, skin ulcers or abscesses, cough, chest pain, headache, anorexia, respiratory distress, abdominal discomfort, joint pain, disorientation, weight loss, stomach or chest pain, muscle or joint pain, and seizures, the CDC website states.

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