Primary viraemia responses of herons to experimental infection with Murray Valley encephalitis, Kunjin and Japanese encephalitis viruses.

Rufous night herons, Pacific herons, little egrets and intermediate egrets were experimentally infected with Murray Valley encephalitis, Kunjin or Japanese encephalitis viruses. Viraemias of at least one day's duration were detected in all birds except two intermediate egrets inoculated with a very low dose of Kunjin virus and one rufous night heron inoculated with Japanese encephalitis virus. there was usually a viraemia of 3 to 5 days' duration commencing on the first or second day and continuing until day 5 or 6 and rarely until day 7. Maximum titres tended to be higher in young birds, up to 2-5 months of age (10(4)-10(5) mouse LD50/ml), than in older birds more than 8 months of age (10(3)-10(4) mouse LD50/ml). Significant differences in maximum viraemia titres were not observed in the different species or between Murray Valley encephalitis and Kunjin viruses. Japanese encephalitis viraemias were significantly lower, but this was probably due to the high mouse brain passage level of the strain used. The onset of viraemia was earlier in intermediate egrets than in rufous night herons inoculated with similar doses of Murray Valley encephalitis virus, but no difference in the susceptibility to infection was observed. With Kunjin virus there was a significant difference in the susceptibility of intermediate egrets and rufous night herons, with rufous night herons being more susceptible to infection with low doses of virus. This difference in threshold of infection, if it extends to other species with both Kunjin and Murray Valley encephalitis viruses, may, in part, be an explanation for the greater incidence of natural infections observed in rufous night herons compared with other species and orders of water birds.