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.

The research investigates the variations in arbovirus infection rates in different bird species observed during an epidemic of arboviral encephalitis in Australia in 1974. The study found that infection was notably higher in some bird species, suggesting a specific relationship between certain viruses and bird species, despite the viruses being largely carried by a single mosquito species.

Study Background

• The study is set in the context of an arboviral encephalitis epidemic which occurred in the Murray Valley of south eastern Australia in 1974.
• The researchers noted an extensive and exuberant breeding of waterbirds before and during the epidemic, and surveyed these birds for signs of arbovirus infection.

Survey Findings

• The researchers conducted haemagglutination inhibition tests on 432 bird sera collected within a period of 10 days in February 1974.
• They found that among the bird species surveyed, the Order Ciconiiformes and Pelecaniformes showed a higher concentration of infections with Murray Valley encephalitis virus, Kunjin virus and possibly other flaviviruses (55% and 41% positive respectively), compared to only 5% in Anseriformes.
• The researchers also noted a higher infection rate among Sindbis virus infections, however, the presence of Sindbis antibodies was spread more uniformly through other bird Orders than with the flaviviruses.
• The highest infection rate was found in mature rufous night herons (Nycticorax calendonicus), with 22 out of 25 samples testing positive.

Interpretation of Findings

• Despite the viruses in question being largely carried by a single mosquito species, Culex annulirostris, the researchers found that the patterns of infection did not seem related to the preference of the mosquito species for certain hosts or the behavioural response of the birds to mosquito attacks.
• The researchers suggested that there might be a specific relationship between MVE/KUN and the bird Orders Ciconiiformes and Pelecaniformes that explain the disparity in infection rates.
• Whilst the factors leading to this relationship are not definitively identified, the researchers suggested that tolerance to mosquito attack might be a contributing factor, particularly in the case of the rufous night herons.

Addendum

• In addition to the bird species, the researchers also tested sera from 13 horses, indicating a high infection rate for both alphaviruses and flaviviruses in the horses as well.