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To the editor: Barmah Forest (BF) virus is a mosquito-borne alphavirus, found only in Australia, which causes outbreaks of polyarthritis in humans. The disease is very similar to epidemic polyarthritis caused by infection with Ross River virus, another Australian alphavirus. BF virus was first isolated from mosquitoes in the State of Western Australia in 1989. After this, small clusters of human cases were diagnosed in the arid northern and central regions of Western Australia in 1992, and the first substantial outbreak of human disease due to infection with BF virus (BF virus disease) occurred in the southwestern region of the state during the spring and summer (September-March) of 1993-94 (2). No evidence of BF virus activity had been found in these regions before these events, which suggests that the virus had only recently been introduced to Western Australia. This report describes the timing and distribution of BF virus disease in humans and the isolation of the virus from mosquitoes in Western Australia, which corroborate the view that BF virus is an emerging virus in this state. The ecology of Australian arboviruses that cause human disease, including BF virus, has recently been reviewed (3). BF virus was first isolated from Culex annulirostris mosquitoes collected at the Barmah Forest in northern Victoria (southeastern Australia) in 1974 (4). It was first shown to infect humans in New South Wales (central-eastern and southeastern Australia) in 1986 (5) and was reported as a cause of clinical disease in humans in 1988 (6). The most common clinical features include polyarthritis, arthralgia, myalgia, fever, rash, and lethargy (7); in some cases, symptoms may persist for more than 6 months (2). Although the symptoms are similar to those caused by infection with Ross River virus, there is little cross-reaction between the two viruses in serologic tests (8), and differentiating between infections caused by either is generally not difficult. The first true outbreak of BF virus disease occurred concurrently with an outbreak of Ross River virus infection at Nhulunbuy in the Northern Territory in early 1992 (9). The principal vectors of BF are believed to be mosquitoes, and although the vertebrate hosts of BF virus are not known, serologic surveys in eastern Australia have suggested that marsupials are involved in the natural cycle. BF virus was first detected in Western Australia in 1989. Since then, 73 isolates of the virus have been obtained from mosquitoes trapped in several different regions of Western Australia (Table 1). The first human cases of BF virus disease in Western Australia were reported in 1992, and 67 serologically confirmed cases have now been diagnosed. The locations of towns where human cases have occurred or where mosquitoes that yielded BF virus were collected are shown in Figure 1. Eight isolates of the virus were obtained from five different mosquito species (Table 1) collected at Billiluna, a small, remote aboriginal community in an arid area in the southeastern Kimberley region in April 1989 (10). The infected mosquitoes were collected 3 weeks after heavy local rains. Only moderate wet season rains were recorded in the remainder of the Kimberley region, and no cases of BF virus disease were reported from any region in Western Australia that year. There have been no subsequent isolations of BF virus from mosquitoes collected at Billiluna, despite annual collections in the region. No human cases have been reported from Billiluna. The first cases of BF virus disease in Western Australian were reported almost 3 years later, either individually or in small clusters from towns in the arid East Kimberley, Pilbara, Gascoyne, Murchison and Southeast (Goldfields) regions between April and September (Autumn-Spring) 1992. Most activity was reported from the towns of Exmouth (six cases) and Carnarvon (four cases). All of these cases occurred during or just after much larger outbreaks of disease caused by Ross River virus. This suggested that BF and Ross River viruses may have similar mosquito vectors and require similar environmental conditions for successful transmission. The main environmental factor contributing to the 1992 outbreaks of Ross River virus disease was extremely heavy rain in these normally arid regions during autumn and winter (11). BF virus was isolated from five species of mosquito in the Fortescue region of the Pilbara and from three species in the West Gascoyne, just prior to, and during, these arid-region outbreaks. In coastal regions of the Pilbara, the main vector of BF virus appears to be Aedes vigilax, a salt marsh-breeding species. Large numbers of this species develop after very high tides or heavy rains on salt marshes. It is also the main vector of Ross River virus in these regions (12). Several other temporary freshwater ground pool-breeding species in the subgenus Ochlerotatus, particularly Ae. eidsvoldensis and Ae. EN Marks’ species #85, were found to be infected with the virus in inland areas or coastal areas where such pools develop. These preliminary investiga1 This report is adapted from and expands on a previous bulletin. (1) Dispatches

Emergence of Barmah Forest Virus in Western Australia1