Myxomatosis: a Search for a Strain of Virus to Immunize a Wild Population of Rabbits, Oryctolagus Cuniculus.

Abstract

FS98, a grade IV strain of myxoma virus, was found to be suitable as an immunising virus for wild rabbits in Australia and suitable for introduction as a biological control agent into a wild rabbit population with the flea Spilopsyllus cuniculi (Dale) as vector. The Nottingham attenuated strain was found not to be a suitable virus for immunising wild rabbits. Seven samples of virus taken from the field in Australia and assessed as grade V virulence in 1965 were more virulent when reassessed in 1976. Possible misclassification by chance was examined by means of randomly selected survival times from actual data and rejected as a complete explanation. Testing with a wide range of different mixtures of a cloned highly virulent and a cloned attenuated strain of virus failed to support, but did not exclude, the idea of changes in virulence grading being caused by differential survival in mixtures during storage. ADDITIONAL ABSTRACT: Strain FS98 of myxoma virus, isolated in Australia in 1967, was suitable for immunizing wild Australian rabbits by means of infected fleas, but the Nottingham attenuated strain was unsuitable. Strain FS98 was of grade IV virulence in the scale of Fenner & Marshall (1957). Changes in virulence during prolonged storage were discussed.