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Deadly Tropical Bacteria Found in Continental U.S. Soil for the First Time

Gizmodo logo Gizmodo 7/28/2022 Ed Cara
Colonies of Burkholderia pseudomallei in an agar plate. © Image: Shutterstock (Shutterstock) Colonies of Burkholderia pseudomallei in an agar plate.

A bacteria endemic to tropical areas that causes a life-threatening infection has just been detected within the soil and waters of the continental U.S. for the first time. On Wednesday, the Centers for Disease Control and Prevention reported that they recently isolated Burkholderia pseudomallei bacteria in southern Mississippi. At least two local cases of the serious infection it causes, known as melioidosis, have been traced back to the bacteria found there as well, though both victims have since recovered.

The discovery was announced in a health advisory released to doctors Wednesday afternoon by the CDC.

CDC and local health officials had been conducting an investigation into the two cases, one documented in July 2020 and the other in May 2022. Both patients had no recent travel history to endemic areas, but did live fairly close to one another in the Gulf Coast region of Mississippi. With the patients’ permission, health officials were allowed to sample the soil and water near their properties. In three samples that were collected from the 2020 patient’s home in June 2022, the investigators found B. pseudomallei. Further testing showed that the bacteria found in the environment had a clear genetic resemblance to the bacteria found in both patients, and that none of the samples looked quite like the strains found in other areas of the world. The patients were hospitalized and developed sepsis, but did eventually beat back the infection.

Given the overwhelming evidence, the CDC said in its advisory, “the bacteria in the environment was the likely source of infection for both patients and has been present in the area since at least 2020.”

The rod-shaped B. pseudomallei is abundantly found in the soil and water of warmer regions. Humans exposed to it don’t always become sick, but it’s a very difficult infection to diagnose even when symptoms do happen. Many signs of illness are nonspecific and can vary depending on where the infection begins or migrates to. The bacteria can lay dormant in people as well, not causing illness until years later when a person’s health declines for other reasons. Standard diagnostic tests can also mistake the bacteria for other infections, further delaying care. Though melioidosis can be treated with antibiotics, its fatality rate can range from 10% to 50%, and it’s especially dangerous for people with underlying health problems.

Melioidosis is a serious problem in endemic areas (one estimate has found that it may kill nearly 90,000 people every year), but it’s been rare in the U.S. Until recently, nearly all cases here have been linked to travel, with the infection only being diagnosed once people returned home. But that trend has started to change as of late. Outbreaks in recent years have been tracked back to contaminated products imported from endemic areas, such as aquariums and aromatherapy sprays. And researchers have warned that there are parts of North America and the U.S. where the bacteria could easily establish a new local reservoir if it had the chance.

Once it’s established a home in the soil environment, the CDC noted, the bacteria “cannot feasibly be removed.”

The only thing surprising about the CDC’s discovery, according to Alfredo Torres, a microbiologist and immunologist at the University of Texas Medical Branch who studies melioidosis, is that it took as long as it did to find the bacteria here.

“We and others have documented the presence of the bacteria in northern Mexico, Puerto Rico, and the Caribbean,” Torres said in an email to Gizmodo. He noted that there have been several recent cases with no travel history documented in Texas that may have been linked to a natural reservoir that was never found.

Melioidosis can theoretically be spread like an airborne disease, though this seems to require special circumstances, like heavy storms that massively kick up the surrounding soil or the use of certain medical procedures on infected patients. Otherwise, the bacteria primarily infects people through direct contact with contaminated soil or water (usually through open skin wounds), and there have been few cases of documented person-to-person transmission. That means that its potential for further spread isn’t likely to be on par with other emerging diseases like the ongoing covid-19 pandemic. But given its high lethality and difficult-to-trace nature, it has been deemed a biological threat on par with germs like anthrax or Ebola. And once it’s established a home in the soil environment, the CDC noted in its advisory, the bacteria “cannot feasibly be removed.”

B. pseudomallei doesn’t usually live on the soil surface, Torres said, so even that exposure route isn’t guaranteed for those unlucky enough to live nearby these reservoirs. But it’s possible that changes in temperature, humidity, and erosion—aided by climate change as a whole—are making the bacteria migrate to the surface and allowing it to more commonly infect humans, he added.

In sending the advisory, the CDC stated that it wanted to make sure doctors would be on the lookout for melioidosis. That’s a goal Torres firmly agrees with, but much more will have to be done to quantify its threat in North America. A 2016 paper, for instance, predicted that the soil environment in not just Mississippi, but Florida, Alabama, and other parts of the southern U.S., would be able to sustain the bacteria comfortably.

Scary as all that sounds, Torres said that confirming its presence in the U.S. should now allow scientists like him the chance to better understand and develop effective interventions against melioidosis.

“We have been doing a lot of studies in my lab and others to understand how these bacteria cause disease, but the isolates that we used are from Thailand or Australia, and some from Latin America,” he said. “Now we have a unique opportunity to study U.S. bacterial isolates and to define how infectious they are, how they cause disease in humans, and what therapeutic interventions (in addition to the vaccines that my lab have already developed) will be effective against these U.S. isolates.”

Other important questions will include figuring out the potential range and distribution of melioidosis in the U.S., as well as when and how exactly the bacteria ended up here.

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