Ross River virus exported from Australia to the South Pacific in the late 1970s appears to have become endemic there despite the lack of known reservoir hosts.
An epidemic of Ross River virus in 1979-80 affected about 500,000 people in American Samoa, Fiji, the Cook Islands and New Caledonia.
Given the island nations do not have marsupials – the only known reservoir hosts of Ross River virus – it was believed that local virus transmission had ceased.
However the findings from a serological study in the islands of American Samoa indicate the potential for Ross River virus to become established in a much wider area than currently recognised.
Dr Colleen Lau, NHMRC Research fellow in the Research School of Population Health at the Australian National University, told the limbic it now appears non-marsupial hosts such as rodents, pigs, dogs or bats may be competent reservoir hosts.
“One we know which hosts are involved we can better define the geographic regions at risk. However all of these animals are common and open up the number of places where the virus can be sustained.”
The study found serological evidence of Ross River virus infection in 74% of 200 human serum samples collected in 2010.
Consistent with exposure patterns to mosquito-borne infections, evidence of a serological response to Ross River virus was more common in outdoor workers and people who participate in activities such as hiking and gardening.
Dr Lau said seroprevalence was highest in people born before or during the 1979-80 epidemic but remained as high as 63% in people born after 1980 who had lived their entire lives in American Samoa.
“So many people are immune there that the chance of a local outbreak is low but travellers from these islands in the early phase of the illness with high viraemia, are capable of transmitting the infection to populations who are immunologically naive,” she said.
A number of cases of Ross River virus in travellers returning from the Pacific islands have already been reported in countries including New Zealand, which has no local mosquito-borne diseases.
Dr Lau said field-based studies are required to isolate the virus in both mosquitoes and the likely reservoir hosts.