Four years after federal scientists found it in Mississippi soil and water, the bacterium that causes melioidosis is still quietly establishing itself along the Gulf Coast. New laboratory work suggests that the strains living there may not behave like the ones the world has spent decades studying.
In the August issue of the CDC journal Emerging Infectious Diseases, C. P. Klimko and colleagues report lethal dose analyses in BALB/c and C57BL/6 mice exposed to small-particle aerosols of Burkholderia pseudomallei isolates from the Western Hemisphere and Africa.
The isolates grew similarly in culture. In the animals, they did not behave similarly at all. The researchers describe a variety of virulence patterns, including rapidly lethal disease in some cases and delayed-onset fatal disease in others, alongside major differences in biofilm formation among isolates.
The finding matters because almost everything clinicians know about melioidosis comes from Southeast Asia and northern Australia.
Declared Endemic in 2022 After Two Mississippi Cases
The bacterium had never been documented in the environment of the continental United States until an investigation into two unrelated patients living close to one another in southern Mississippi. Neither had traveled outside the country recently.
Sampling of soil, water, and plant matter on and near their properties turned up B. pseudomallei. According to the CDC health advisory issued July 27, 2022, three samples taken from soil and water on the property of the patient infected in 2020 tested positive at the CDC by both polymerase chain reaction and culture.
Isolates from both patients and from the environment were genetically similar to each other and distinct from previously known isolates, indicating a local environmental source present since at least 2020.
CDC declared melioidosis locally endemic in areas of the Gulf Coast region of Mississippi. It also stated something that has shaped every response since: once well established in soil, B. pseudomallei cannot feasibly be removed. Public health efforts, the agency said, should focus primarily on improving case identification so that appropriate treatment can be administered.
Melioidosis became a nationally notifiable disease that same year following a favorable vote at the Council of State and Territorial Epidemiologists conference.
A 2026 review in PLOS Neglected Tropical Diseases by Bart Currie, Mirjam Kaestli, and Ella Meumann concluded that the predicted establishment of the bacterium in the southern United States has now happened, with melioidosis endemic in Mississippi and likely endemic in Georgia and Texas.
Strains from Different Continents Did Not Behave the Same
The new laboratory work matters because it undercuts an assumption that has quietly underpinned US preparedness: that a B. pseudomallei isolate is a B. pseudomallei isolate.
Nearly all clinical understanding of melioidosis, including case fatality estimates and expected time course, was built in Southeast Asia and northern Australia. When the bacterium turned up in Mississippi soil, that body of knowledge was the only reference available.
The August analysis suggests it may not transfer cleanly. Isolates from the Western Hemisphere and Africa produced markedly different outcomes in mice, ranging from rapidly lethal to fatal with delayed onset, and differed substantially in biofilm formation despite growing similarly in culture.
Biofilm capacity influences how a bacterium persists in the environment and how it resists clearance during infection.
This is mouse work, and mouse virulence does not directly translate to human outcomes. What it establishes is that geographic origin is a variable worth tracking, not a footnote.
The Diagnosis American Doctors Are Not Trained to Consider
Melioidosis is difficult to recognize even where it is common, and close to invisible where it is not expected.
The infection can present as pneumonia, as a bloodstream infection, as abscesses in multiple organs, or as a chronic illness emerging months after exposure. It mimics other severe bacterial infections closely enough that patients are routinely worked up for something else first. The median incubation period is about four days, with a range of 1 to 21 days.
Historically, most US cases occurred in people who had traveled to countries where the bacterium is common, so a clinician's mental checklist for an unexplained severe pneumonia has not included soil exposure in Mississippi. Before the Gulf Coast finding, a handful of domestic cases were traced to imported household products, including a 2021 cluster of four patients in four states, two of whom died, linked to a contaminated aromatherapy spray.
Treatment requires prolonged antibiotic therapy in two phases: an intensive acute phase followed by a longer eradication phase, and relapses occur. The bacterium is naturally resistant to many antibiotics commonly used for sepsis, so an empiric regimen chosen without suspecting melioidosis may not cover it. There is no approved vaccine.
Rain, Hurricanes and Who Is Actually at Risk
Exposure happens through three routes: ingestion, inhalation, and inoculation through broken skin. Incidence rises sharply after heavy rainfall, and cases climb during rainy seasons and after severe weather.
That seasonality is the reason Gulf Coast endemicity is more than a curiosity. The Atlantic hurricane season runs June 1 through November 30, and CDC and Georgia investigators reported four geographically linked melioidosis cases in a single Georgia county between 1983 and 2024, two of which began within days of Hurricane Helene making landfall as a Category 4 storm. Sequencing showed all four isolates were highly related despite spanning four decades.
Most people exposed never develop melioidosis. CDC guidance identifies the risk factors that change the odds: diabetes, excessive alcohol use, chronic lung disease, chronic kidney disease and immunosuppressive conditions, along with work or recreation involving soil.
Diabetes is the most overrepresented risk factor worldwide and is prevalent across the southeastern United States, part of why researchers have flagged the region.
The practical guidance is modest. People with diabetes, chronic kidney or liver disease, or weakened immune systems along the Gulf Coast can reduce exposure by wearing waterproof boots and gloves during gardening, agricultural work, or flood cleanup, and by covering open wounds before contact with soil or standing water.
The more consequential audience is clinical. CDC has urged clinicians to consider melioidosis in patients with a compatible illness who live in or have traveled to the Gulf Coast region, particularly those with the recognized risk factors, and to report suspected cases to state and local health departments. Anyone with a severe or unexplained respiratory illness who has had significant exposure to soil or floodwater should mention that history to their physician, because it is the detail that prompts the right test.
Key Questions Answered
What is melioidosis? It is a bacterial infection caused by Burkholderia pseudomallei, an organism that lives in soil and water in tropical and subtropical regions. It spreads to people through contact with contaminated soil, water, or airborne droplets, and can present as pneumonia, bloodstream infection, organ abscesses, or chronic illness.
What did the new laboratory study find? Researchers exposed two strains of laboratory mice to aerosolized B. pseudomallei isolates from the Western Hemisphere and Africa. The isolates grew similarly in culture but produced a range of virulence patterns in animals, from rapidly lethal disease to fatal disease with delayed onset, and differed substantially in biofilm formation.
Does that mean Gulf Coast strains are more dangerous to people? No. This is animal research, and mouse virulence does not directly translate to human outcomes. The finding establishes that geographic origin is a variable worth tracking, not that any particular strain poses greater risk to patients.
Where in the United States is the bacterium established? In 2022, the CDC declared melioidosis locally endemic in areas of the Gulf Coast region of Mississippi. A 2026 review concluded the disease is endemic in Mississippi and likely endemic in Georgia and Texas.
Who is most at risk? Most exposed people never develop melioidosis. Recognized risk factors include diabetes, excessive alcohol use, chronic lung disease, chronic kidney disease and immunosuppressive conditions, along with occupational or recreational soil contact.
Why do cases rise after hurricanes? Heavy rainfall and severe weather bring the bacteria to the soil surface and into standing water and airborne droplets. Investigators documented Georgia cases beginning within days of Hurricane Helene's landfall.
What should a patient tell a doctor? Anyone with a severe or unexplained respiratory illness who has had significant soil or floodwater exposure should mention that history. Melioidosis requires prolonged two-phase antibiotic therapy, and the bacterium resists many drugs commonly used empirically for sepsis.
