Melioidosis case clusters in a tropical urban setting: association with soil type and geomorphology

Abstract

Geospatial analysis of the distribution of clinical cases of melioidosis, an often fatal tropical disease, in the Townsville region indicated case clustering in distinct geomorphic settings and characteristic soil associations. Two significant clusters were identified. Cluster 1 is associated with piedmont slopes adjacent to granitic hill and mountain slopes. The soils developed on colluvium are typically Kandosols with dark grey-brown loamy sand to silty loam A horizon, grading into dark red or yellow sandy clay loam to sandy clay subsoils. Cluster 2 is associated with Pleistocene floodplains, levees and channel-fill. The duplex soils typically grade from acidic to alkaline at depth. These soils are poorly draining due to a shallow impermeable B horizon in Sodosols. It is postulated that the two geomorphic positions and soil types are predisposed to soil wetness following periods of intense rainfall. Cluster 1 is located where large amounts of runoff are received from the adjacent granitic hill, whereas Cluster 2 associates with poorly drained soils at the lower edges of poorly drained alluvial plains. This preliminary study has generated the hypothesis that melioidosis distribution in the Townsville region is controlled by environmental factors, specifically soil type, geomorphic position and drainage. A detailed multidisciplinary field-based study investigating soil physico-chemical features, field isolates of Burkholderia pseudomallei, and epidemiological considerations is now underway to test this hypothesis.