Japanese encephalitis in north Queensland, Australia, 1998

Objective: To describe the circumstances of two cases of Japanese encephali tis (JE) in north Queensland in 1998, including one acquired on the Austral ian mainland. Design: Serological surveillance of sentinel pigs for JE virus activity; se rological surveys of humans and pigs and viral cultures of mosquito collect ions. Setting: Islands in the Torres Strait and communities in the Northern Penin sula Area (NPA) and near the mouth of the Mitchell River in Cape York, Quee nsland, in the 1998 wet season (December 1997-May 1998). Results: Sentinel pigs in the Torres Strait began to seroconvert to JE viru s in February 1998, just before onset of JE in an unvaccinated 12-year-old boy on Badu island. By mid-April, most sentinel pigs had seroconverted. Num erous JE viruses were isolated from Culex annulirostris mosquitoes collecte d on Badu. In early March, a person working at the mouth of the Mitchell Ri ver developed JE. Serological surveys showed recent JE virus infection in 1 3 young pigs on a nearby farm, but not in 488 nearby residents. In NPA comm unities, sentinel pigs seroconverted slowly and JE viruses were isolated fr om three, but none of 604 residents showed evidence of recent infection. Nu cleotide sequencing showed that 1998 JE virus isolates from the Torres Stra it were virtually identical not only to the 1998 isolate from an NPA pig, b ut also to previous (1995) Badu isolates. Conclusions: JE virus activity was more widespread in north Queensland in t he 1998 wet season than in the three previous wet seasons, but ecological c ircumstances leg, less intensive pig husbandry, fewer mosquitoes) appear to have limited transmission on the mainland. Nucleotide sequencing indicated a common source for the 1995 and 1998 JE viruses. Circumstantial evidence suggests that cyclonic winds carried infected mosquitoes from Papua New Gui nea.