Ecology of Burkholderia pseudomallei and the interactions between environmental Burkholderia spp. and human-animal hosts

Early workers thought that melioidosis was a zoonosis with a reservoir in r odents, but we now know that Burkholderia pseudomallei is a widely distribu ted environmental saprophyte. In northeast Thailand, two thirds of paddy fi elds yield the organism, and 80% of children have antibodies by the time th ey are 4 years old. However, interpretation of these results has been compl icated by the recent recognition of avirulent, antigenically cross-reacting environmental organisms for which the name B. thailandensis has been propo sed. We still know very little about the climatic, physical, chemical and b iological factors which control the proliferation and survival of Burkholde ria spp. in the environment, although epidemiological studies show space-ti me clustering of melioidosis. It is assumed that most human and animal meli oidosis arises through exposure to contaminated soil or muddy water, althou gh only 6% of human cases have a clear history of inoculation, and a furthe r 0.5% of cases follow near-drowning. Laboratory animals have also been inf ected by ingestion, inhalation and insect bites, but evidence of infection acquired naturally by these routes remains anecdotal. Sporadic cases have r esulted from iatrogenic inoculation, laboratory accidents, and person-to-pe rson or animal-to-person spread. Whether exposure to B. pseudomallei will r esult in disease probably depends on the balance between the virulence of t he strain, the immune status of the host (e.g. diabetes mellitus) and the s ize of the inoculum. (C) 2000 Elsevier Science B.V. All rights reserved.