An outbreak of Japanese encephalitis in the Torres Strait, Australia, 1995

Objectives: To determine the distribution of virus infection during an outbreak of Japanese encephalitis (JE) in the Torres Strait, and to describe the environmental factors facilitating the outbreak. Design: Human and porcine serological surveys for JE virus activity throughout the Torres Strait, and mosquito and household surveys on the island of Badu. Setting: The island of Badu (where the clinical cases occurred) and the other islands of the Torres Strait, Australia, during April-May 1995. Results: The serological surveys identified recent JE virus infection among residents or domestic pigs on at least nine outer Torres Strait islands. A JE virus, confirmed by nucleotide sequencing, was isolated from two asymptomatic Badu residents. Virus isolations and mosquito surveys implicated Culex annulirostris as the major vector involved in the outbreak. There was prolific Cx. annulirostris breeding in a variety of water bodies close to and within the Badu community. Over half (53%) of the households kept pigs in pens, and many (63%) of the pigpens were situated near standing water; in 56% of these ''wet'' pigpens Cx. annulirostris was breeding. Conclusions: There was evidence of widespread JE virus activity throughout the outer islands of the Torres Strait. We suggest that migratory birds and/or wind-blown mosquitoes could have imported the virus into the Torres Strait from a focus of viral activity, possibly in Papua New Guinea, thereby initiating the outbreak. A combination of environmental factors, with large numbers of domestic pigs in close proximity to human dwellings and mosquito breeding sites, undoubtedly facilitated the outbreak on Badu.