Evolutionary changes in myxoma virus in Britain: An examination of 222 in naturally occurring strains obtained from 80 counties during the period October–November 1962

Abstract

Samples of lesions containing viable myxoma virus were collected from 222 infected wild rabbits captured in 80 counties of Britain during October–November 1962. They were dispatched by air to Australia and passaged once in domestic rabbits before being tested for their virulence by the intradermal inoculation of groups of six laboratory rabbits with small doses of virus.The results showed that a wide range of viruses of differing properties now coexist in Britain. Their virulence ranges from very high (99 % lethal) to low (about 50 %), but the majority of strains fall into the grade III virulence group, with estimated case mortality rates varying between 70 and 95%. The viruses also produce disease of varying symptomatology, skin lesions being very prominent, raised or flat. There was no obvious association between lesion type and virulence.Tests made on the wild rabbits from which the samples were obtained, and on Inoculated domestic rabbits, showed that virulence could not be predicted by gel-diffusion tests.

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

“…Britain (Fenner & Chapple, 1965;Fenner & Ratcliffe, 1965;Marshall & Fenner, 1960). Experiments by Fenner, Day & Woodroofe (1952) indicated that transmission of myxoma virus by mosquitoes, the principal vectors of myxomatosis in Australia, was an example of mechanical transmission.…”

Innate resistance to myxomatosis in wild rabbits in England

“…The development of resistance to myxomatosis in British wild rabbits is not unexpected, because of the earlier Australian results, and it is an example of one side of the natural accommodation of a virus-host relationship; the other side being the attenuation of the virus (Marshall & Fenner, 1960;Fenner & Chapple, 1965). How quickly and how far this accommodation is likely to go are questions which cannot yet be answered, but our knowledge of the relationship between myxoma virus and Oryctolagus over the last 25 years indicate that the process will probably be fairly slow.…”

“…In Australia and in Great Britain, moderately attenuated strains of virus quickly became the most common strains found in the field (Marshall & Fenner, 1960;Fenner & Chapple, 1965). It is believed that this state of affairs has continued and that further attenuation has not occurred, because strains of moderate virulence are more efficiently transmitted than strains of higher or lower virulence (Fenner, Day & Woodroofe, 1956;Mead-Briggs & Vaughan, 1975).…”

“…In European rabbits (Oryctolagus) it produces the disease known as myxomatosis and when first introduced to wild European rabbits in Australia and in Great Britain it proved virtually 100 % lethal. Since the first introduction there have been clear indications of an accommodation in the virus-host relationship; the appearance of attenuated types of myxoma virus and the establishment of moderately attenuated types as the most common viruses found in wild rabbit populations (Marshall & Fenner, 1960;Fenner & Chapple, 1965). There is evidence also, from Australia, that several populations of wild rabbits have developed a considerable degree of inherited resistance to myxomatosis (Marshall & Fenner, 1958;Marshall & Douglas, 1961;Douglas & Tighe, 1965).…”

Myxomatosis. The effect of age upon survival of wild and domestic rabbits (Oryctolagus cuniculus) with a degree of genetic resistance and unselected domestic rabbits infected with myxoma virus

“…The consequences of the immune status of the sire have not yet been determined and in this regard it should be noted that the wild rabbits from Urana in the above study were all sired by bucks which had never been infected, whereas all the selected domestic rabbits had sires which had recovered from myxomatosis. Before the epidemiological consequences of the age effect can be evaluated it will be necessary to show that it occurs where more attenuated virus strains are used, since infection in the field at present is largely by strains of Grade III virulence (Fenner & Chapple, 1965). However, since selection in wild rabbits with attenuated strains of virus and in domestic rabbits with more virulent strains have both resulted in a similar age effect to SS it seems likely that this age effect will be expressed for virus strains other than SS.…”