The differential transmissibility of myxoma virus strains of differing virulence grades by the rabbit flea Spilopsyllus cuniculi (Dale)

Abstract

SUMMARYLaboratory studies showed that few rabbit fleas (SpilopsyllUs cuniculi (Dale)) transmitted myxomatosis after removal from wild rabbits (Oryctolagus cuniculus (L)) that had been infected for fewer than 10-12 days, irrespective of the virulence of the myxoma virus strain involved. Rabbits infected with fully virulent (Grade I) strains died within 10-15 days and few fleas from these hosts became infective; averaging all the samples taken, 12 % of the fleas were infective. Also, few fleas acquired infectivity on individual rabbits which recovered from infection with attenuated strains; the mean was 8 % infective. Rabbits which died between 17 and 44 days after infection had higher proportions of infective fleas at all sampling times; the mean was 42 % infective. Male and female fleas transmitted virus with equal efficiency.For rabbits infected with any of the attenuated virus strains the mean percentage of infective fleas was inversely related to the survival time of the host. Rabbits infected with moderately attenuated strains (Grades IIIA and IIIB) had, on average, the highest proportion of infective fleas; hence such strains have a selective advantage and have become predominant under natural conditions in Britain. The changes that might occur if there is an increase in host resistance to myxomatosis are discussed.

Changes in the virulence of myxoma virus strains in Britain

“…However, in an outbreak caused by a fully virulent strain, spread of the disease would be less effective than with a moderately attenuated strain (Mead-Briggs & Vaughan, 1975) and, although individual infected rabbits would die more quickly, the length of the outbreaks would probably be similar.…”

“…It was shown that the predominance of such strains was due to more effective transmission by mosquitoes in Australia (Fenner, Day & Woodroofe, 1956) and by the rabbit flea in Britain (Mead-Briggs & Vaughan, 1975). Fenner & Ratcliffe (1965) postulated that, in resistant rabbits, strains of higher virulence would be more effectively transmitted and they predicted that the development ofresistance in wild rabbits would lead to an increase in the virulence of field strains of virus.…”

Myxomatosis in farmland rabbit populations in England and Wales

“…Myxomatosis is normally transmitted between host rabbits when virus particles adhere to the piercing mouthparts of a biting insect vector. In Britain the rabbit flea, Spilopsyllus cuniculi, is the most important vector of the disease (1) and its role in transmitting different myxoma virus strains has been investigated (2,3); other blood sucking insects may play a minor role as vectors in some circumstances.…”

Innate resistance to myxomatosis in wild rabbits in England

“…In 1970In , 1974In and 1975 small numbers of rabbits were bred in captivity from adult stock captured in the study area. All rabbits were held in captivity for at least 3 months, to ensure that any myxoma virus antibodies acquired from immune does would have decayed.…”