Melioidosis

Melioidosis is an infectious disease caused by the bacterium Burkholderia pseudomallei (previously known as Pseudomonas pseudomallei) and is a multisystem disorder which may affect the lungs, brain, visceral organs, or musculoskeletal system.

Epidemiology

Melioidosis is a disease of the monsoon season in the tropics with a greatest prevalence in the northeastern provinces of Thailand and the 'Top End' of the Northern Territory and Queensland in Australia ⁶. It is unknown in temperate zones, other than when seen in returned travelers, and as such is not well known to many physicians despite the fact that in hyperendemic areas (see above) up to 20% of community-acquired septicemia is due to Burkholderia pseudomallei ⁶.

Risk factors

Numerous risk factors have been identified most of which are associated with a degree of impaired host defenses (either immunological or structural) ⁶:

• metabolic/immunological
  • diabetes mellitus
  • excessive alcohol consumption
  • chronic renal failure
  • glucocorticosteroid excess (iatrogenic or Cushing syndrome)
  • HIV/AIDS
  • leukemia and lymphoma
• chronic structural pulmonary diseases
  • chronic obstructive pulmonary disease
  • idiopathic pulmonary hemosiderosis
  • cystic fibrosis
• congestive cardiac failure
• neoplasms
• kava (Piper methysticum) ingestion
• occupation, e.g. rice farmer

Clinical presentation

As many different organ systems can be affected, presentation is similarly variable, and patients may present with acute, subacute or chronic illness, each with different radiological findings.

The lungs are the most commonly affected organ, and most commonly patients present with an acute pulmonary illness, often dramatic and often clinically more pronounced than imaging or physical findings would suggest ^4,6.

Pathology

The bacterium Burkholderia pseudomallei is an environmental saprophyte found in soil and stagnant water ⁶. The organism can enter the body directly though cuts/wounds or be inhaled in dust ⁶.

The organism survives within the cytoplasm of macrophages which ingest it, and it may thus remain dormant for many years ⁶.

Although the majority of infective changes visible on imaging are due to abscess formation, clinical presentation has been thought to be due, in part at least, to an endotoxin. This would explain how the clinical presentation can be more severe than would be expected from physical or imaging findings alone, although animal models have thus far failed to identify such a toxin ^4,6.

Radiographic features

Pulmonary manifestations

Acute

In acute disease, imaging may demonstrate multiple small pulmonary nodules (hematogenous spread) and multilobar consolidation, typically starting in the upper lobes. This may rapidly progress resulting in cavitation or lung abscess formation ⁶. 

Subacute to chronic

In subacute and chronic forms, the radiological features are similar (mixed nodular or patchy opacities), although in the chronic form, progression is slower.

Rupture of a cavity or lung abscess into the pleural space may result in a pneumothorax or hydropneumothorax. Pleural effusions are uncommon and often associated with lower lobe involvement.

Head and neck manifestations

Suppurative parotitis is seen particularly in Thai children and presents as an abscess ⁶.

Abdominal manifestations

Melioidosis can also occur in the visceral organs of the abdomen, most commonly in the liver and spleen. Appearances can range from a large 'honeycomb' type abscess to a multitude of microabscesses. It can also occur, albeit far less commonly, in the pancreas, kidneys and even prostate gland (higher incidence in Australia than elsewhere) ^3,6.

Central nervous system manifestations

Central nervous system involvement (see neuromelioidosis) is uncommon and can take many forms, ranging from cerebral abscesses / cerebritis / encephalitis to cranial nerve palsies and even dural venous sinus thrombosis ⁴.

Treatment and prognosis

Treatment depends on the location of the infection and the severity of systemic symptoms. Intravenous antibiotics are usually required and care must be taken as the organism is resistant to many antibiotics. Choices include ceftazidime or meropenem with co-trimoxazole ⁶.

Intensive care treatment will be required for cases of septicemia and any abscess needs to be drained surgically.

The prognosis depends on the clinical presentation and organs involved, and ranges from very high to very low mortality ⁶:

• septicemia (disseminated): 87% mortality
• septicemia (non-disseminated): 17% mortality
• localized infection: 9% mortality
• transient bacteremia: 0% mortality

History and etymology

The English pathologist Alfred Whitmore (1876-1946) and the Indian bacteriologist C.S Krishnaswami first described melioidosis in opium addicts in Yangon, Burma in 1912 ^5,6. 

Differential diagnosis

The differential depends entirely on the location and is too diverse to be listed here. In general terms it falls into two broad groups:

• non-infective conditions which mimic abscesses (e.g. cavitating lung cancer, high grade gliomas)
• other infections (e.g. tuberculosis, amoebic abscess): based on the fact that the lungs are the most commonly affected organ, and shared epidemiology, tuberculosis is the most significant differential diagnosis