Sera from twelve species of native and five species of introduced mammals collected on the south coast of New South Wales between 1982 and 1988 were tested for antibodies to the following arboviruses: Ross River virus (621 animals tested); Barmah Forest virus (371); Gan Gan virus (337); Trubanaman virus (378). Serum neutralising antibodies to Ross River virus were found in bandicoots, wallabies, kangaroos, cattle, goat and horses; to Barmah Forest virus in kangaroo, cattle and horses; to Gan Gan virus in kangaroos, wallabies, rat, cows, horses and sheep; and to Trubanaman virus in kangaroos, wallabies, cows and horses. Titres to Ross River virus in seropositive native animal sera ranged from 32 to 1024 and those in seropositive domesticated animal sera ranged from 8 to 32 768. Prevalence of serum antibodies in macropodids, cattle and horses was: Ross River virus, 68, 19, 62%; Barmah Forest virus, 4, 26, 9%; Gan Gan virus, 44, 13, 13%; Trubanaman virus, 60, 3, 10% respectively. Evidence suggests that: (1) kangaroos and wallabies are major vertebrate hosts for Ross River virus; (2) the role of bandicoots warrants further investigation; (3) horses may be important amplifying hosts of the virus, which causes epidemic polyarthritis in man in Australia.

In 1995, the largest recorded outbreak of human disease resulting from infection with the mosquito transmitted alphavirus Barmah Forest (BF) virus occurred along the south coast of New South Wales, Australia. The virus was first isolated in early January from mosquitoes collected at Batemans Bay and predisposed the recognition of 135 human clinical cases. The cases of BF virus were identified initially from Batemans Bay during late January, and the majority (30%) of all cases came from this town. After 5 wk, all major centers on the south coast had clinical patients. Aedes vigilax (Skuse) were especially abundant at Batemans Bay, with levels up to 8 times greater than normal. This species yielded 111 isolates and appeared to be the major vector of BF virus. Attempts to examine if BF virus was maintained in the field by vertical transmission within Ae. vigilax populations were unsuccessful; no evidence of vertical transmission with BF virus, nor any other arbovirus, was found in > 17,000 adults emerging from field-collected larvae from the region following peak virus activity. In addition to BF virus, other viruses were recovered from field-collected adult mosquitoes, including Ross River (10 isolates), Edge Hill (21), and Stratford (10). Ae. vigilax again yielded the majority of these viral isolates. The BF virus outbreak appeared to be associated with several factors. A lack of recent BF virus activity in the region provided a highly susceptible human population, and unusual weather conditions of above average rainfall coupled with high tides resulted in extraordinarily large populations of Ae. vigilax.

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Summary The prevalence of antibodies in human sera from the south coast of New South Wales to four arboviruses, isolated from mosquitoes collected along the south coast, was determined in an attempt to estimate the importance of these viruses in human infection. Only two viruses, Barmah Forest and Gan Gan, were considered to be of any significance.