Melioidosis is a disease of humans and animals caused by the gram-negative bacterium Burkholderia pseudomallei, a soil- and waterborne saprophyte endemic to tropical and subtropical regions worldwide. Clinical signs are widely variable, as infection can occur in any tissue; the most common clinical signs are sepsis, pneumonia, and visceral and cutaneous abscesses. Morbidity and mortality are high, particularly when diagnosis and treatment are delayed.
Burkholderia pseudomallei, a gram-negative bacterium, is the causative agent of melioidosis, an infectious disease encountered predominantly in the tropics. This saprophyte can adapt to a wide range of environmental conditions, and it can infect humans and a multitude of animal hosts. The organism is classified by the US government as a Risk Group 3 pathogen and a Tier 1 select agent.
Melioidosis is often associated with suppurative or caseous lesions, comprising a mixed purulent and granulomatous response that can affect any organ. Clinically infected individuals can shed the agent via urine, feces, milk, sputum, and purulent material. The clinical presentation of melioidosis can take various forms, from fulminant septicemia, pneumonia, neurological disease, local infection, subacute illness, and chronic infection to subclinical disease.
Positive culture is the diagnostic standard for melioidosis, confirmed by MALDI-TOF mass spectrometry when available. No rapid and reliable commercially available tests exist; however, DNA-based assays, such as PCR assay and sequencing, are suitable for bacterial identification. Serological tests are less advisable because of their low sensitivity and specificity. Treatment requires long-term antimicrobial courses that are expensive and frequently unsuccessful. Human treatment guidelines for melioidosis can be followed in animals after adaptation. Vaccines for melioidosis are not available.
Etiology and Epidemiology of Melioidosis
The etiological agent for melioidosis is the saprophyte B pseudomallei, an oval, motile, gram-negative, facultative anaerobic bacillus with bipolar staining. The organism is ubiquitous throughout Southeast Asia, northern Australia, and the South Pacific. Its distribution is predominantly tropical and subtropical, with hyperendemicity in the northernmost regions of Australia and northeastern Thailand. The true boundaries of its endemicity are ambiguous, because of movement of the organism and its ability to travel to and exist in temperate regions (eg, southwestern and southeastern Australia), where it can cause sporadic disease and outbreaks.
B pseudomallei has been introduced to new environments with the export of animals; shipments of contaminated soil, water, and commercial products have also expanded the organism's range. Reports of melioidosis have also come from India, China, some Pacific islands, the southern US (Texas, Mississippi, Georgia, Puerto Rico, US Virgin Islands), Mexico, Central and South America, the Caribbean, Africa, and the Middle East (1) (see ). Given the increasing frequency of extreme weather events, it is anticipated that the geographical range of melioidosis will expand.
Map depicting evidence of melioidosis by country.
Image from Melioidosis.info. Evidence of melioidosis by country. Accessed September 16, 2026. Used under CC BY 4.0. https://www.melioidosis.info/map.aspx
B pseudomallei has been isolated from various soil types and from surface water at varying depths. Melioidosis outbreaks have coincided with heavy rainfall and flooding associated with high humidity or temperature. Major excavations and disturbances in plumbing resulting in contamination of water supplies have also caused outbreaks.
Melioidosis occurs most commonly in sheep, goats, and pigs; sheep and camelids are highly susceptible to infection. Other affected species include cattle, buffalo, horses, mules, deer, dogs, cats, dolphins, tree kangaroos, wallabies, koalas, human and nonhuman primates, birds, tropical fish, and reptiles. Laboratory animals affected by melioidosis include hamsters, guinea pigs, rabbits, mice, and rats. Host susceptibility and clinical signs vary between species.
Transmission of Melioidosis
Infection by B pseudomallei is thought to be opportunistic and primarily a result of transmission from the environment (eg, contaminated soil and surface waters) rather than from animal to animal. The most common routes of infection are percutaneous inoculation; contamination of wounds; ingestion of contaminated soil, water, or carcasses; and inhalation. Transplacental infection resulting in abortion has been observed in goats (2). Laboratory-acquired infection and iatrogenic infection via contaminated antiseptics, injections, and hospital or surgical equipment have been reported.
Pathogenesis of Melioidosis
B pseudomallei possesses a large array of virulence determinants that enable it to survive in a wide range of harsh environments and cause a variety of clinical presentations (1). Molecular-typed clonal outbreaks have produced a range of different clinical signs, which indicates that host factors and infecting dose can be just as important as isolate type in determining the severity of disease.
The incubation period ranges from a few days to months or even years. B pseudomallei is a facultative intracellular pathogen that can remain dormant for many years before initiating clinical disease.
Clinical Findings of Melioidosis
Clinical signs of melioidosis in animals can vary widely within a species depending on the site of infection. Fever, anorexia, or swollen glands can be noted. Subclinical infection is common. Infection can be associated with single or multiple suppurative or caseous nodules or abscesses, which can be located in any organ. Disease is acquired by percutaneous inoculation or inhalation and often spreads to distant sites, with or without evidence of active infection at the inoculation site. The organs most commonly affected include the lungs, spleen, liver, and associated lymph nodes.
Goats with melioidosis frequently develop mastitis or pneumonia; aortic aneurysms respiratory system is involved preferentially in sheep; clinical signs can include fever, severe coughing, respiratory distress, and mucopurulent nasal and ocular discharge. CNS disease, with clinical signs that include circling, incoordination, blindness, nystagmus, and spasms, has been observed in cattle, horses, sheep, and goats (2).
Pigs with melioidosis often have subclinical lesions on the spleen that are incidental findings at slaughter. Lameness because of septic arthritis and osteomyelitis can occur (see ). Fatalities often occur in association with acute fulminating infections or when vital organs are affected.
Infected joint containing caseous, greenish pus in a pig with melioidosis.
Courtesy of Dr. Cathy Shilton, Berrimah Veterinary Laboratories Darwin, Northern Territory, Australia.
Various forms of melioidosis have been reported in horses (2); clinical signs can include weakness, edema and lymphangitis of the limbs, mild colic, diarrhea, coughing, or nasal discharge.
Skin infections (see ) can resemble fungal eczema initially, progressing to become papular.
Multiple facial abscesses and skin ulcerations in a capuchin with melioidosis.
Courtesy of Dr. Cathy Shilton, Berrimah Veterinary Laboratories Darwin, Northern Territory, Australia.
In dogs and cats, melioidosis can be acute, subacute, or chronic. In acute cases, septicemia with fever, severe diarrhea, and fulminant pneumonia are common. Subacute cases can present as a skin lesion with lymphangitis and lymphadenitis; untreated cases can progress to septicemia. Chronic disease can occur in any organ, with clinical signs that include anorexia, myalgia, edema of the limbs, and skin abscesses. Neurological melioidosis with ataxia and seizures can occur. Cats can also present with localized ocular disease, pyothorax, or rhinosinusitis.
Lesions
Melioidosis abscesses that contain thick, caseous, greenish-yellow to off-white material are noted at necropsy. The organs most commonly involved are the lungs, spleen, lymph nodes, liver, and subcutaneous tissues (see ).
These photographs from a goat with melioidosis show (A) numerous multifocal abscesses in the lung associated with consolidation and hemorrhage and (B) an abscess in a grossly enlarged submandibular lymph node.
Courtesy of Dr. Cathy Shilton, Berrimah Veterinary Laboratories Darwin, Northern Territory, Australia.
Exudative bronchopneumonia, consolidation, and abscesses can be found in the lungs of animals with respiratory disease caused by melioidosis. Nodules and ulcers can be found on the nasal mucosa and septum and on the turbinates, and they can coalesce into irregular plaques. Meningoencephalitis, unilateral ocular infection ("red eye"), severe enteritis, suppurative polyarthritis, and other syndromes also have been reported (2, 3).
Diagnosis of Melioidosis
Culture
PCR assay and sequencing
The clinical signs of melioidosis are not diagnostic, because of the variable nature of the disease. For a definitive diagnosis, the organism must be isolated and identified from lesion or discharge samples.
It is possible to culture B psuedomallei on routine diagnostic media such as McConkey, chocolate, or sheep-blood agar. Colonies can take up to 48 hours to appear and can be overgrown by commensal organisms. Ashdown agar is preferred, as it contains gentamicin and crystal violet as selective agents and promotes the characteristic colony morphology of B pseudomallei (see ). Burkholderia cepacia–selective agar containing crystal violet, polymyxin B, gentamicin, and vancomycin is also suitable.
Micrograph of the saprophyte B pseudomallei, growing on Ashdown medium. After approximately 72 hours of incubation, the colonies are flat, wrinkled, and dry, with a slight metallic sheen, and between 2 and 5 mm in diameter. Colonies appear pink because they have absorbed neutral red.
Courtesy of Dr. Mandy Elschner.
Gram-stained smears of exudate or pus can sometimes identify bipolar “safety pin”–shaped, gram-negative rods. Serological tests for melioidosis such as complement fixation and indirect hemagglutination must be evaluated with caution and should not be used as the sole diagnostic tool. PCR assay and targeted gene sequencing (eg, 16S rRNA) are recommended for bacterial identification and typing. Matrix-assisted laser desorption/ionization time-of-flight (MALDI-TOF) mass spectrometry can also be used for bacterial identification, provided that sufficient reference spectra are in the database.
Pearls & Pitfalls
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Treatment and Prevention of Melioidosis
Antimicrobials
Prevention of exposure
Antimicrobial treatment for melioidosis should be based on culture and susceptibility results. B pseudomallei is intrinsically resistant to penicillin, ampicillin, amoxicillin, aminoglycosides, first- and second-generation cephalosporins, macrolides, and polymyxins. Treatment can be expensive, prolonged, and possibly unsuccessful, with the risk of recrudescence after treatment is discontinued. Underlying immunosuppressive conditions should be considered. Surgical drainage of abscesses can also be required.
Treatment recommendations from Australia for canine and feline melioidosis, which are on the basis of the treatment guidelines for humans updated in 2024, are as follows: For the initial phase, use one of the following drugs: Meropenem (preferably) or imipenem or ceftazidime or ceftiofur (the latter after successful testing for susceptibility) with trimethoprim sulfadoxine or amoxicillin and clavulanic acid. For the eradication phase, monotherapy with trimethoprim sulfadoxine, or, if not tolerated, a combination of two of the following drugs: amoxicillin clavulanate, doxycycline monohydrate, or, after successful susceptibility testing, pradofloxacin or enrofloxacin (only dog) and cefovecin (3).
Preventive measures for melioidosis are more practical and economical in intensive farming environments; these include raising animals off the soil, avoiding exposure to muddy or water-inundated regions, and providing clean drinking water via chlorination and filtration. Minimizing environmental contamination by diseased animals is also an important control measure. There is no effective vaccine for melioidosis.
Zoonotic Risk of Melioidosis
Melioidosis is an infectious disease affecting humans and animals after environmental exposure (soil, water) to B pseudomallei. Infection can occur percutaneously or via inhalation or ingestion. Human-to-human and zoonotic transmission are very rare. Introduction of infected animals into nonendemic regions can lead to establishment of the organism in the environment if the conditions are suitable. B pseudomallei has been isolated from goat milk, prompting the requirement for pasteurisation of commercial goat milk in the tropics. Infected animal carcasses are condemned at the abattoir.
Key Points
Melioidosis is an infectious disease affecting a broad range of animal species and humans.
Though they do occur, zoonotic infections are extremely rare.
Clinical diagnosis is difficult because of the multitude of clinical signs and syndromes. Isolation and identification of the organism are required.
DNA-based assays are suitable for isolate identification.
Treatment is prolonged and not always successful; however, selection of appropriate antimicrobials can decrease the mortality rate.
In endemic areas, prevention of exposure to contaminated environments is the best management strategy.
For More Information
Wiersinga WJ, Virk HS, Torres AG, et al. Melioidosis. Nat Rev Dis Primers. 2018;4:art.17107.
Centers for Disease Control and Prevention: Melioidosis
Lee K, Shilton C, Benedict S, et al. Treatment of melioidosis in cats and dogs: antimicrobial therapy and clinical guidance for Burkholderia pseudomallei infections. Aust Vet J. 2026:1-4.
Also see pet owner content regarding dogs, cats, and horses.
References
Meumann EM, Limmathurotsakul D, Dunachie SJ, Wiersinga WJ, Currie BJ. Burkholderia pseudomallei and melioidosis. Nat Rev Microbiol. 2024;22(3):155-169. doi:10.1038/s41579-023-00972-5
Sprague LD, Neubauer H. Melioidosis in animals: a review on epizootiology, diagnosis and clinical presentation. J Vet Med B Infect Dis Vet Public Health. 2004;51(7):305-320. doi:10.1111/j.1439-0450.2004.00797.x
Lee K, Shilton C, Benedict S, et al. Treatment of melioidosis in cats and dogs: antimicrobial therapy and clinical guidance for Burkholderia pseudomallei infections. Aust Vet J. 2026; 1–4. doi:10.1111/avj.70125
