Do Ross River and dengue viruses pose a threat to New Zealand?

To determine the prevalence of antibodies to Ross River and dengue viruses in sera from New Zealand residents and travellers and to assess the potential of local mosquitoes to act as vectors of these viruses.

Serum specimens from several population groups were examined by haemagglutination-inhibition and neutralisation tests for antibodies to Ross River and dengue viruses between 1975 and 1993. During this period dengue was active in South East Asia, Australia and the Pacific, and a major epidemic of Ross River infection occurred in the Pacific. Two New Zealand mosquito species were tested for their ability to transmit by bite after they had been fed or injected with these viruses.

Ten percent of 1869 sera from patients suspected of contracting dengue, and 43% of 183 patients suspected of contracting Ross River virus, while overseas, were antibody positive. Many patients showed antibody rises which indicated that they were probably viraemic on entry to this country. Dengue viruses were isolated in Dunedin from two patients with dengue haemorrhagic fever contracted overseas. Antibody studies of persons who had not travelled outside New Zealand provided no evidence of local transmission of these viruses. Two local mosquitoes, Aedes notoscriptus from the Auckland area, and Aedes australis from the Otago area, were able to transmit one or both these viruses under laboratory conditions.

The serological studies showed that both Ross River and dengue viruses have probably been introduced into New Zealand by viraemic travellers on many occasions. Although some local mosquitoes can transmit these viruses in the laboratory, there is no evidence of local spread of virus from these imported cases. Changing environmental conditions such as global warming with concomitant effects on vector distribution, increasingly rapid air travel by viraemic persons and the accidental introduction of new vector mosquitoes, particularly Aedes albopictus, could pose a threat in view of the high percentage of New Zealand residents with no protective antibody.