Strain Of Myxoma Virus Research Articles

A note on two attenuated strains of myxoma virus isolated in Great Britain.

Two strains of myxoma virus isolated from naturally infected wild rabbits have been described. They showed similar clinical symptoms, the most striking feature being the flat primary lesion. In fifteen rabbits the Brecon strain gave a mean survival time of 26·3 days with a range of 17–47 days, and one recovery. Of 5 rabbits inoculated with the Durham strain three became clinically infected and two remained healthy, the latter were found to be fully susceptible on reinoculation. The three infected rabbits all recovered.The field strains of virus were compared with known myxoma strains—Glenfield, Cornwall, A & H and KM 13. The first three are fully virulent, while KM 13 is attenuated and characteristic of the field strains now being isolated in Australia. The similarity between the clinical symptoms and mean survival time of KM 13 and the Brecon strain was noted.It is suggested that the two field strains described in this paper may be the first indication that the evolution of myxoma virus in this country is similar to the evolution of the virus in Australia.One of us (P.J.C.) is greatly indebted to the Director for allowing this work to be carried out at M.R.E. whilst laboratory accomodation was being prepared at Worplesdon. We are grateful for the help and encouragement of Dr J. C. N. Westwood of M.R.E. To Prof. F. Fenner who gave us the Australian strains of myxoma virus and to Mr J. H. Darbyshire of the Central Veterinary Laboratory who supplied the tissues from which the two attenuated strains were isolated, we extend our thanks.

Studies in the epidemiology of infectious myxomatosis of rabbits. VIII. Further observations on changes in the innate resistance of Australian wild rabbits exposed to myxomatosis.

1. In one Australian study area where the selection pressure of annual epizootics of myxomatosis has been extremely high, the innate resistance of the rabbits was found to have increased to such an extent that only 26% died when inoculated with a strain of myxoma virus which kills 88% of genetically unselected rabbits.2. The level of innate resistance in a rabbit population varies with the total experience of the disease.3. In combined field and laboratory trials it was shown that the mortality rate in the field is influenced by innate resistance at least as much as under the sheltered conditions of an animal house. The effect was apparent with both an attenuated and a highly virulent strain of virus.4. The moderately high ambient temperatures during the field experiments probably contributed to the sparing effect.

Studies in the epidemiology of infectious myxomatosis of rabbits. VII. The virulence of strains of myxoma virus recovered from Australian wild rabbits between 1951 and 1959.

During the 8½ years, February 1951 to July 1959, 672 strains of myxoma virus were recovered from the field in Australia, all but thirteen, which were derived from mosquito pools, being extracted from the tissues of infected wild rabbits. These were tested for virulence by the intradermal inoculation of small groups of rabbits with a small dose of virus. Based primarily on survival times, strains have been allocated to five grades of virulence, ranging from very high to very low. In spite of inadequate sampling during the first 5 years there has clearly been a trend from a predominance of highly virulent strains in the initial epizootics to a mixture of strains of virulence varying from very high (rarely) to very low. For the last 6 years the proportion of strains allotted to the different virulence grades has remained almost constant, just over half the strains tested each year being classified as moderately virulent.

The influence of ambient temperature on the course of myxomatosis in rabbits.

Ambient temperature profoundly affects the mortality rate and symptomatology in rabbits infected with an attenuated strain of myxoma virus. Simulated summer temperatures allowed at least 70% of infected rabbits to recover, whereas only 8% recovered when held at simulated winter temperatures. Similar fluctuating high temperatures did not reduce the lethality of rabbitpox or of a highly virulent strain of myxoma virus.On the chorio-allantoic membrane of developing chicken embryos the proliferation of myxoma virus was almost completely inhibited at 39° C., but the yield of rabbitpox virus was not reduced at this temperature.The serological response and viraemia in rabbits held under different conditions of ambient temperature were investigated. Low temperatures resulted in high titres of circulating virus and soluble antigens, whereas at high temperatures anti-body appeared early. Haemolysin production was not affected by ambient temperature.The widely variable response of rabbits infected with the attenuated strain of myxoma virus when held at mild temperatures indicates that there is a considerable variation in the innate resistance of the host to the invasion of the virus. This resistance is enhanced by high and reduced by low ambient temperatures, so that the host response is relatively uniform under these conditions. The precise nature of this innate resistance has not been elucidated.Now that attenuated strains of virus are common and many rabbits exhibit enhanced genetic resistance, seasonal temperatures will have an important influence on the mortality rate in epizootics of myxomatosis.It is a pleasure to acknowledge the skilled technical assistance of Mr A. Brand.

Studies in the epidemiology of infectious myxomatosis of rabbits. V. Changes in the innate resistance of Australian wild rabbits exposed to myxomatosis.

Annually for 4 years groups of young Australian wild rabbits have been captured during non-epizootic periods from areas in which myxomatosis has occurred during the previous summer. The intensity of the preceding epizootic was measured by testing a sample of survivors for antibody. The captured young rabbits were raised in the laboratory until they were about 4 months old and then inoculated intradermally with small doses of the slightly attenuated myxoma virus strain Aust/Corowa/12–52/2 (KM 13), all samples of virus being derived from the same batch which has been stored at -70°C.The results show there is a significant negative correlation between the mortality rate and the degree of exposure to myxomatosis of the forbears of the tested animals. Passive and active immunization have been excluded and this result is ascribed to increased genetic resistance.We are greatly indebted to Mr. F. N. Ratcliffe, Officer in Charge of the Wildlife Survey Section of the Commonwealth Scientific and Industrial Organization, and to his officers, especially Messrs W. E. Poole, K. E. Myers, G. Douglas, B. V. Fennessy, H. Frith and K. Keith for their invaluable assistance in collecting rabbits and serum samples.We have had valuable discussions with Dr W. R. Sobey, Dr P. J. Claringbold and Dr G. S. Watson on genetical and statistical aspects of the problems under study.

A comparison of the virulence for European rabbits (Oryctolagus cuniculus) of strains of myxoma virus recovered in the field in Australia, Europe and America.

Strains of myxoma virus associated with a case-mortality rate in wild rabbits of 90—95% can be distinguished from those associated with a case-mortality rate of more than 99% by the use of a standardized virulence test carried out in groups of five laboratory rabbits, and tests carried out on laboratory rabbits are closely correlated with the results obtained with Australian wild rabbits. The test, which attempts to mimic natural mosquito transmission, consists of the intradermal inoculation in one site of about 5 rabbit-infectious doses of the virus, and the subsequent observation of the rate of progress of clinical symptoms and ultimate fate of the rabbit. The mean survival time provides a figure which allows satisfactory classification of all known myxoma virus strains into one of four grades of virulence, very attenuated strains being recognized by their low case-mortality rates.Using this test, 92 strains of myxoma virus have been compared. These comprised 6 laboratory strains, 62 strains recovered from Australian wild rabbits or from wild caught mosquitoes, between February, 1951 and March, 1955, 19 strains recovered from naturally infected rabbits in Europe between October, 1953 and August, 1955, 3 strains from South America, and 2 from California. Eight types have been described and illustrated, differentiation being based on their virulence and the clinical picture of the disease they cause in laboratory rabbits. The differences are summarized in Text-fig. 2. Four of the virus types produce recognizably different pocks on the chorioallantois of developing chick embryos.In Australia the repeatedly introduced standard laboratory strain of virus has been replaced by a moderately virulent strain, which appears to be better adapted for survival than either the highly virulent standard laboratory strain, or less virulent variants of which only a few had been recovered up to 1955. Strains of the type dominant in 1953–55 have appeared independently in many widely separated parts of Australia. Examination of these attenuated variants by pure clone techniques showed that the variability in symptomatology associated with them is not due to a mixture of virus strains, but to the expression of differences in innate resistance of the host animals which are obscured by the overwhelming virulence of the highly virulent strains.Only one introduction of a different strain of virus was made in Europe. All field strains recovered during the first two years after the introduction were of high virulence, but a few attenuated strains appeared in 1955, and they have since become widespread. One of them appears to be a relatively stable mixture of a highly virulent and a greatly attenuated strain.The collection of the large numbers of strains of virus described in this paper has been made possible only by the generous collaboration of many people. In Australia we would like to acknowledge the great help afforded us by Mr F. N. Ratcliffe, officer-in-charge of the Wildlife Survey Section, Commonwealth Scientific and Industrial Research Organization and his officers, especially Messrs J. H. Calaby, G. Douglas, A. L. Dyce, B. V. Fennessy, E. W. Lines, K. Myers, R. Mykytowycz, W. E. Poole and E. Waterhouse. Messrs B. D. Robinson and T. Pearce of the Lands Department of Victoria have also provided us with specimens, as have Mr G. Edgar, Director of the Veterinary Research Station, Glenfield, New South Wales, Dr D. Surrey Dane of the Institute of Medical and Veterinary Science, Adelaide, Mr D. Gooding, Department of Agriculture, Western Australia, and Messrs T. B. Alexander and A. F. Ryan, Department of Agriculture, Tasmania.We are indebted to Dr H. de B. Aragão of the Instituto Oswaldo Cruz, Rio de Janeiro, and Dr B. Szyfres, of the Laboratorio de Biologia Animal, Uruguay, for specimens from South America; Dr D. G. McKercher of the University of California School of Veterinary Medicine, Davis, California, for the Californian strains; Drs P. Lépine and H. Jacotot, of the Institut Pasteur, Paris, for strains from France; and Mr J. R. Hudson, of the Veterinary Laboratory, Weybridge, and Dr C. H. Andrewes of the National Institute for Medical Research, Mill Hill, for material from England.We are grateful to Dr S. Fazekas de St Groth and Dr G. S. Watson of this University for advice on the statistical treatment of the data.Mr V. Paral took the photographs.

Studies in the epidemiology of infectious myxomatosis of rabbits. VI. The experimental introduction of the European strain of myxoma virus into Australian wild rabbit populations.

The primary lesions produced in rabbits by the intradermal inoculation of the virulent French strain of myxoma virus were clearly distinguishable from those produced by the standard laboratory strain or by attenuated Australian field strains. Based on this fact a screening test was developed which allowed classification of large numbers of samples of virus into French or attenuated Australian types.The virulent French type of virus was introduced into the wild rabbit population at Lake Urana by the inoculation of 242 rabbits over a period of 6 weeks, the attenuated Australian strain of virus having been recovered from a naturally infected rabbit at the beginning of the series of inoculations. A severe outbreak of myxomatosis occurred in which an estimated 70% of cases were caused by the French strain of virus. In spite of the early predominance of cases due to this introduced strain the majority of samples obtained in the latter half of the outbreak were of the attenuated Australian type, and this was probably the only strain which survived through the following winter and caused an intense epizootic in the spring.A similar picture of early establishment of the artificially introduced virulent French strain, and its subsequent replacement by the naturally occurring attenuated Australian strain, was seen at two other study sites, North Urana Station and Merricumbene.We are greatly indebted to colleagues on the staff of the Wildlife Survey Section at Albury, especially Mr M. P. Hines, for assistance in the field work, and to Mr P. Bentley, the manager of Cocketgedong Station, on which the experimental site is situated. We have profited from numerous discussions with Mr F. N. Ratcliffe, and are grateful to Dr G. S. Watson for assistance in the statistical treatment of the results.

Epidemiological consequences of the mechanical transmission of myxomatosis by mosquitoes.

When examined under similar conditions in the laboratory, the two strains of myxoma virus used to initiate the Australian and European epizootics, and three somewhat attenuated strains recovered in the field in Australia and France, were found to produce lesions containing approximately the same amount of virus, and were transmitted by mosquitoes with approximately equal efficiency. The laboratory variant neuromyxoma multiplied poorly in the skin and was very rarely transmitted by mosquitoes.The survival potential of different virus strains in the field is correlated with the length of time during which rabbits infected with different strains present to biting mosquitoes lesions containing high concentrations of virus. Sickness of the infected rabbit, and extensive viruliferous skin lesions, will increase its suitability as a source of virus for mosquitoes. These considerations explain the dominance of slightly attenuated strains of myxoma virus in the field in Australia.Species of mosquitoes, when tested under standard conditions in the laboratory, differ in their efficiency as vectors of myxomatosis. These differences may be related to differences in the method of feeding, and to structural differences in the mouthparts.Engorgement on an immune rabbit after probing through myxomatous skin lesions had no greater effect on the subsequent ability of mosquitoes to transmit myxomatosis than a similar blood feed on a normal rabbit.Nine serial mosquito-bite passages of the standard laboratory strain of virus, each involving one week in the rabbit and two weeks in the mosquito, had no effect on its pathogenic behaviour.

Occurrence of attenuated strains of myxoma virus in Europe.

SEVERAL slightly attenuated strains of myxoma virus have been recovered from the field in Australia1; indeed, it is now rather uncommon to recover strains having the same high virulence as the material used for initiating the epizootics, and for the subsequent inoculation campaigns. The position in Europe, however, appears to have been different. European investigators2 have been unanimous in their observations that there has been no fall in the virulence of the virus since it was introduced into wild rabbits in France in June 1952. Experiments made in Canberra with several European field-strains have confirmed this view. Very recently, however, we have obtained evidence of attenuation of the virus in the field in Europe.

Passive immunity in myxomatosis of the european rabbit (Oryctolagus cuniculus): the protection conferred on kittens born by immune does

1. The existence of passive immunity to myxomatosis was demonstrated by the inoculation of normal young rabbits with either myxoma-immune serum or saline, and their subsequent inoculation with the standard laboratory strain of myxoma virus. All the passively immunized animals lived longer than the control animals and a few survived.2. Passive immunity could also be demonstrated in the offspring of myxoma-immune mothers. When these were challenged by mosquito bite inoculation with the standard laboratory strain of myxoma virus they either failed to become infected, or survived infection for several days longer than the progeny of normal does. When challenged by the intradermal inoculation of a slightly attenuated strain of myxoma virus 25 % of the progeny of immune does survived the infection, whereas none of the normal kittens survived.3. The survival times of young rabbits in both the normal and passively immunized groups was influenced by their age, very young animals dying several days earlier than rabbits 4 and 6 weeks old.4. The possible epidemiological consequences of passive immunity in the behaviour of myxomatosis in populations of wild rabbits are discussed.

Studies by electron microscopy of thin sections of infectious myxomatosis in rabbits.

Rabbits were inoculated with the C.P.M. strain of myxoma virus and the resulting subcutaneous tumors were fixed, embedded, and sectioned for observation with the electron microscope. Both round cells and the typical stellate myxomatous cells were observed in addition to changes in the collagen pattern at the intercellular spaces. The cytoplasm of the cells showed a great number of bodies of varying size and density, the largest of them having the size and other characteristics of the elementary bodies of the virus. Some of the bodies showed an internal structure, being formed by the tight clumping of small dense particles. Distribution curves of the diameter of the elementary bodies and of the smaller internal particles are presented. The morphological problems involved in the virus-host cell relationship are discussed in the case of the myxoma virus.