Isolation of Ross River virus from mosquitoes and from horses with signs of musculo-skeletal disease

Objective To report clinical and clinicopathological findings in horses naturally infected with Ross River virus (RRV) and identify likely mosquito arbovirus vector species. Procedures Veterinarians submitted serum samples from 750 horses because they suspected Ross River virus (RRV) infection. The samples were tested for the presence of IgM and IgG antibody to RRV and for the presence of virus. Mosquitoes were trapped, differentiated to species level and tested for the presence of RRV by virus isolation. Results RRV was isolated from six species of mosquitoes (Ochlerotatus camptorhyncus, Culex globocoxitus Cx australicus, Cx annulirostris, Cx quinquefasciatus, Anopheles annulipes) and from 13 horses with clinical signs of musculo-skeletal disease. Antibody to RRV was detected in 420 of the 750 serum samples; 307 contained IgG only; 76 contained both IgM and IgG and 37 contained only IgM antibody to RRV. Virus was isolated from horses with IgM antibody only. Conclusions RRV can be isolated from infected horses during the short time period when there is an overlap of clinical signs, positive IgM serology and viraemia. Early spring infections of horses may occur if RRV infected mosquito vectors are present. RRV has not been shown to cause clinical disease in horses. This is the first report of isolation of RRV from Oc camptorhyncus in the Murray region and indicates a potential for infection of humans and animals in autumn as well as in spring.