Variation in arbovirus infection rates in species of birds sampled in a serological survey during an encephalitis epidemic in the Murray Valley of South-eastern Australia, February 1974.

There was extensive and exuberant breeding of waterbirds before and during an epidemic of arboviral encephalitis in the Murray Valley of south eastern Australia in 1974. As estimated by haemagglutination inhibition tests on 432 bird sera collected between 4th and 13th February, 1974, infection with Murray Valley encephalitis virus, Kunjin virus and possibly other flaviviruses was concentrated in species of the Order Ciconiiformes (55% positive) and Pelecaniformes (41%), compared with only 5% in Anseriformes. Although Sindbis virus infections were also highest in these 2 Orders (56% and 46%, respectively), the incidence of antibodies was spread more uniformly through other Orders than with the flaviviruses: e.g. Anseriformes, 33%, Podicipitiformes, 27%. As viruses were recovered virtually only from Culex annulirostris mosquitoes, the different patterns of infection seem unrelated to host preference by mosquito species or behavioural response to mosquito attack, and suggest a specific relationship between MVE/KUN and Ciconiiformes and Pelecaniformes. The highest infection rate was 22/25 in mature rufous night herons (formerly nankeen night herons) Nycticorax calendonicus, and here tolerance to mosquito attack was probably a contributing factor. Assays of sera from 13 horses indicated high infection rates both alphaviruses and flaviviruses.