Murray Valley encephalitis in Mt Isa, north Queensland. Tropical Public Health Unit Network, Queensland Health.

Tests were carried out to determine the aetiology of the encephalitis, including serological and other tests for the encephalitic flaviviruses. Polymerase chain reaction tests on CSF were negative and attempts at virus isolation were unsuccessful. However, serum collected on day 4 (and again on days 6 and 13) was strongly reactive by enzyme immunoassay for IgM to the encephalitic flavivirus group, but not reactive to any other flaviviruses. Haemagglutination inhibition (HAI) tests on sera showed a rise in titre to Murray Valley encephalitis (MVE) virus from non-reactive to re-active at a dilution of 1:20, between two specimens collected on days 4 and 6 of illness. The reactive sample from day 6 was analysed by ultracentrifugation, fractionation and HAI and reacted specifically and strongly against MVE. The serological results were therefore strongly suggestive of a recent infection with MVE virus.