Flavivirus isolations from mosquitoes collected from Saibai Island in the Torres Strait, Australia, during an incursion of Japanese encephalitis virus C. A. JOHANSEN 1 , D. J. NISBET 1 ,P.N.FOLEY 2 ,A.F.VANDENHURK 1 , R. A. HALL 1 , J. S. MACKENZIE 1 andS. A. RITCHIE 3 1 Department of Microbiology and Parasitology, School of Molecular and Microbial Sciences, The University of Queensland, St. Lucia, Queensland, Australia, 2 Tropical Public Health Unit, Queensland Health, Aitkenvale, Queensland, Australia and 3 Tropical Public Health Unit, Queensland Health, Cairns, Australia Abstract. Adult mosquitoes (Diptera: Culicidae) were collected in January and February 2000 from Saibai Island in the Torres Strait of northern Australia, and processed for arbovirus isolation during a period of Japanese encephalitis (JE) virus activity on nearby Badu Island. A total of 84 210 mosquitoes were processed for virus isolation, yielding six flavivirus isolates. Viruses obtained were single isolates of JE and Kokobera (KOK) and four of Kunjin (KUN). All virus isolates were from members of the Culex sitiens Weidemann subgroup, which comprised 53.1% of mosquitoes processed. Nucleotide sequencing and phylogenetic analysis of the pre-membrane region of the genome of JE isolate TS5313 indicated that it was closely related to other isolates from a sentinel pig and a pool of Cx. gelidus Theobald from Badu Island during the same period. Also molecular analyses of part of the envelope gene of KUN virus isolates showed that they were closely related to other KUN virus strains from Cape York Peninsula. The results indicate that flaviviruses are dynamic in the area, and suggest patterns of move- ment south from New Guinea and north from the Australian mainland. Key words. Culex gelidus, Culex sitiens, arbovirus vectors, Japanese encephalitis virus, Kokobera virus, Kunjin virus, mosquito, phylogeny, virus movement, Papua New Guinea, Saibai Island, Torres Strait, Australia. Introduction Japanese encephalitis (JE) virus was first recognized in Australia in 1995, when an outbreak was detected on Badu Island and two people died (Hanna etal., 1996). Following the outbreak, sentinel pigs were established on numerous islands in the Torres Strait for surveillance of JE virus activity (Shield etal., 1996). Seroconversions in senti- nel pigs showed that JE virus activity was detected in the Torres Strait during the wet season of each year between 1995 and 1998, inclusive (Shield etal., 1996; Hanna etal., 1999). In two of these years, JE virus activity appeared to be restricted to Saibai Island. The locations of sentinel pigs were reduced to a single herd on Badu Island following widespread activity of JE virus in the Torres Strait and northern Queensland in 1998. Several other flaviviruses are also known to be active in the region. Isolates of JE, Murray Valley encephalitis (MVE) and Sepik (SEP) viruses have been obtained from mosquitoes (Diptera: Culicidae) collected in Western Province of Papua New Guinea (PNG) (Marshall, 1988; Johansen etal., 2000), and flaviviruses including MVE virus, the closely related Kunjin (KUN) virus (lineage I of West Nile virus (Heinz etal., 2000), and Kokobera (KOK) virus have also been active recently in northern Queensland Correspondence: Dr Cheryl Johansen, Department of Micro- biology, School of Biomedical and Chemical Sciences, The University of Western Australia, QEII Medical Centre, Nedlands, WA 6009, Australia. Tel.: þ61 893464656; fax: þ61 893462912; e-mail: cjohanse@cyllene.uwa.edu.au Medical and Veterinary Entomology (2004) 18, 281–287 # 2004 The Royal Entomological Society 281