Investigation of an outbreak of Scrub Typhus in a military population in North Queensland

Abstract

Scrub typhus, a mite-borne rickettsiosis (Orientia tsutsugamushi) endemic to parts of the Asia-Pacific region, has historical significance for military populations. It also poses a current threat to some Australian Defence Force (ADF) training activities. At the conclusion of a 2011 training exercise at Cowley Beach Training Area, near Innisfail, a large proportion of ADF soldiers fell ill with fever, rash, headache and polyarthralgia. This prompted an outbreak investigation and recall of all exposed soldiers for treatment with doxycycline, the appropriate agent for scrub typhus, known to be endemic to the area. In light of the fact that many affected soldiers reported compliance with the prophylactic doxycycline regimen, and also because of reports of a resistant strain in Thailand, a susceptibility assay was conducted on the isolated organism. A total of 45 out of 124 exposed soldiers fell ill, with O. tsutsugamushi confirmed as the pathogen (attack rate 0.36). There were no further presentations after treatment with a single dose of doxycycline 200 mg at the time of recall. Microbiological testing confirmed the susceptibility of the organism to doxycycline. Scrub typhus remains endemic to an army training area in North Queensland. It is likely that incomplete adherence to the prophylactic regimen, probably at the end of the exercise, contributed to the outbreak. Methods of controlling the risk of infection at the training area were considered, and it was concluded that doxycycline compliance, combined with permethrin-impregnated clothing and DEET personal insect repellent remain safe and effective controls.