Barmah Forest virus serology; implications for diagnosis and public health action.

Barmah Forest virus (BFV) is a commonly occurring arbovirus in Australia. Notifications of Barmah Forest infections diagnosed by a single positive IgM serology test have been increasing in coastal New South Wales north of Newcastle. We report on a 6 month prospective review of all routine notifications of BFV from the Lower Mid North Coast of New South Wales. Sera from 37 consecutive cases were sent for confirmatory testing by ELISA and neutralisation assays and 32 cases were interviewed. On confirmatory testing, 7 patients' sera (19%) was found to contain no BFV antibodies and 6 (16%) had BFV IgG only. Only 4 cases had antibody levels compatible with recent infection. A clinical presentation of fever with either rash or joint pain was associated with confirmation of recent BFV infection. On the basis of these findings, caution is advised in the interpretation of a single positive IgM for Barmah Forest disease and the clinical picture is an important factor in the diagnosis. Serological notifications of BFV alone should not prompt public health action such as public warning and targeted vector control in endemic areas.