An epizootic of myxomatosis in rabbits that occurred during the winter months (March-August) of 1969 is described in detail. Virus of virulence grade 111, the common virulence of field strains of myxoma virus presently in Australia, was isolated during the epizootic. The estimated case mortality was 86% over the 5 months during which the epizootic was active. Only 41228 of the original susceptible and established population completely avoided infection. Transmission through the population was relatively slow and apparently depended on social contact between individual rabbits. The seasonal conditions and the slow progression of the disease through the population make it most unlikely that mosquitoes were involved in transmitting the virus. The origin of the virus that initiated the epizootic is discussed, including the possibility that it was not a reintroduction of myxoma virus to the population. This implies that the virus may have remained latent in the rabbit population for several years.
Results are presented on tests carried out over the past 5 years to measure the antibody titre of various rabbit populations to a soluble antigen of myxoma virus. In an unexpectedly high percentage of immune rabbits the antibody fell below measurable titres, and later rose to measurable titres without the advent of an observable epizootic of myxomatosis. The re-stimulation of the immune response is discussed in terms of re-infection and virus reactivation.
A myxomatosis epizootic, that occurred in February-April 1972 in a rabbit population in which a winter epizootic was observed in 1969, is described. Rabbit fleas had been introduced into the rabbit population in the intervening period. In the 1972 outbreak the case mortality was only 33% compared with 86% in 1969. The difference is thought to be related to the colder conditions during the 1969 epizootic. The morbidity rate could not be accurately measured in 1972 but was a minimum of 65%. It is believed that in the 1972 epizootic the virus was transmitted by rabbit fleas. It appears that any change in the epidemiological pattern of myxomatosis towards summer epizootics in areas in which they have previously occurred in winter will drastically reduce the rabbit mortality currently caused by myxomatosis. The introduction of rabbit fleas may increase the likelihood of epizootics in summer and autumn.
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