Neurological melioidosis

Neurological abnormalities have long been recognised in animals with melioidosis, including laboratory rodents and sheep in the first Australian outbreak in 1949. Autopsies in animals have shown microabscesses and lymphocytic infiltration to be present on occasion in the same animal, but Burkholderia pseudomallei is usually able to be grown from central nervous system (CNS) tissue. In humans CNS melioidosis is unusual, but both macroscopic brain abscesses and encephalitis occur. There has been a recently recognised syndrome of meningoencephalitis with varying involvement of brainstem, cerebellum and spinal cord. The prospective melioidosis study at Royal Darwin Hospital has documented 12 cases of CNS melioidosis over 9 years out of a total of 232 cases of melioidosis (5%). Prominent features on presentation were unilateral limb weakness (6), predominant cerebellar signs (2), mixed cerebellar and brainstem features with peripheral weakness (2) and flaccid paraparesis (2). Eight patients had unilateral VIIth nerve palsy and six bulbar palsy, with five requiring prolonged ventilation. Brain CT scans are usually normal initially, but MRI shows dramatic changes. Three patients died and only three made a full recovery. In two patients with predominant mononuclear CSF pleocytosis, B. pseudomallei was cultured from CSF and autopsy in one of these showed necrotising encephalitis with microabscesses. Although it has been postulated that a neurotropic exotoxin may account for melioidosis encephalomyelitis, the recent findings and comparison with the animal data suggest that direct organism spread within the CNS may be primarily responsible. Preliminary molecular typing of isolates shows no evidence of a specific strain of B. pseudomallei responsible for CNS melioidosis and further studies are required to determine if the apparent higher rate of CNS disease in Australia is due to true regional differences or is from increased ascertainment.