Melioidosis is an infection caused by the gram-negative bacillus Burkholderia pseudomallei. The disease is most commonly seen in tropical climates. The pathogen is generally found in soil, and transmission can occur when heavy rains after a dry spell result in increased presence of the pathogen in soil and surface water, and even aerosolization (in the setting of heavy monsoon rains in certain geographies for example). It is endemic to Southeast Asia and northern Australia with most cases occurring during the rainy season. It may also be found in the South Pacific, Africa, India, the Middle East, and Central and South America.

Note: In the United States, consider melioidosis in patients with a compatible illness even if they do not have a travel history to a disease-endemic country. Between March and July 2021, the US Centers for Disease Control & Prevention (CDC) identified a multistate cluster of nontravel-associated B pseudomallei infections in 4 patients from Georgia, Kansas, Texas, and Minnesota. Infection was likely a consequence of long dry spells followed by heavy rains aerosolizing the pathogen.

Melioidosis is usually acquired through skin abrasions or inhalation, although ingestion and materno-fetal, sexual, and person-to-person transmission have been described. Nearly every body system can become infected. There are 4 clinically distinct types of melioidosis: localized, pulmonary, bacteremia (septicemic), and disseminated. Common presentations include skin abscesses, pneumonia, and sepsis.

Melioidosis is considered a Category B bioterrorism agent by the CDC. As an agent of bioterrorism, the most likely method of dispersal would be in an aerosolized form. Melioidosis is usually nonfatal, except in the septicemic form. In a bioterrorism attack, mortality rates could be higher due to the higher initial bacterial exposure.

Localized
Acute localized melioidosis usually presents with a skin nodule or pustule. The skin lesions typically occur 1–5 days following an initial bacterial inoculation through a break in the skin, from contaminated water or soil. Lymphadenitis and regional lymphadenopathy are common. Associated systemic symptoms may include fever, chills, and myalgias. Localized infections often progress to septicemia, especially in immunocompromised patients or the chronically ill. Uncomplicated localized infections are relatively rare. Children may present with isolated parotitis.

Pulmonary
The lung is the most commonly affected organ in melioidosis, accounting for over half of cases. Pulmonary involvement may be from inhalation or bacteremic seeding of the lung. Clinical presentations can vary from a mild bronchitis to a severe pneumonia. The average incubation time is 9 days. Symptoms include sudden onset of high fever, chills, productive or non-productive cough, chest pain, headache, anorexia, and myalgia. This is the only form of melioidosis that presents with a cough. Skin abscesses may be seen, even months after infection, and it may produce late chronic lung abscesses that can be mistaken for cavitary TB.

Bacteremia (Septicemic)
Bacteremia due to B pseudomallei (septicemic melioidosis) usually results in septic shock and is most likely to occur in the immunocompromised and those with diabetes and renal insufficiency. Usually, there is no obvious infected wound. Abscesses may occur throughout the body, and symptoms may reflect the organ systems affected. Symptoms include headache, fever, chills, diarrhea, disseminated abscesses, myalgia, skin pustules, disorientation, and respiratory distress. Mortality rates are around 40%, and death may occur within 48 hours, even with therapy. Patients with diabetes, cirrhosis, lung disease, renal disease, or cystic fibrosis, the immunocompromised, and those who consume Kava root are predisposed to septicemic infection.

Disseminated
Disseminated or chronic suppurative melioidosis usually has a delayed onset and presents with abscesses in organs such as the skin, brain, liver, lungs, and spleen. It may also produce abscesses in the lymphatics, kidney, prostate, bones, and joints. It can produce a protracted wasting illness.

The incubation period of melioidosis can vary from between 2 days and years. Peak incidence is in the fourth and fifth decades, which may represent new disease or reactivation of suppressed disease due to a waning immune system. Currently, there is no vaccine for melioidosis. Although attempts to develop a vaccine are under way, it will be a difficult task. Antibodies from repeated natural immunization are not protective against future infection.

The disease is most commonly seen in tropical climates. It is endemic to Southeast Asia and Northern Australia, with most cases occurring during the rainy season. It may also be found in the South Pacific, Africa, India, the Middle East, and Central and South America. Few cases occur in the United States (less than 5 each year); therefore, an outbreak of melioidosis should be highly suspect for a bioterrorism attack.

Individuals in endemic areas, military personnel, agricultural workers, farmers, gardeners, and tourists are at higher risk for contracting melioidosis. Healthy individuals may have asymptomatic infections; seroprevalence in endemic areas may be as high as 80%. Uncontrolled diabetes and alcoholism are the most common risk factors for serious infection.