Host-specificity of myxoma virus: Pathogenesis of South American and North American strains of myxoma virus in two North American lagomorph species

The pathogenesis of South American and North American myxoma viruses was examined in two species of North American lagomorphs, Sylvilagus nuttallii (mountain cottontail) and Sylvilagus audubonii (desert cottontail) both of which have been shown to have the potential to transmit the South American type of myxoma virus. Following infection with the South American strain (Lausanne, Lu), S. nuttallii developed both a local lesion and secondary lesions on the skin. They did not develop the classical myxomatosis seen in European rabbits (Oryctolagus cuniculus). The infection at the inoculation site did not resolve during the 20-day time course of the trial and contained transmissible virus titres at all times. In contrast, S. audubonii infected with Lu had very few signs of disseminated infection and partially controlled virus replication at the inoculation site. The prototype Californian strain of myxoma virus (MSW) was able to replicate at the inoculation site of both species but did not induce clinical signs of a disseminated infection. In S. audubonii, there was a rapid response to MSW characterised by a massive T lymphocyte infiltration of the inoculation site by day 5. MSW did not reach transmissible titres at the inoculation site in either species. This might explain why the Californian myxoma virus has not expanded its host-range in North America.

Virulence and pathogenesis of the MSW and MSD strains of Californian myxoma virus in European rabbits with genetic resistance to myxomatosis compared to rabbits with no genetic resistance

The pathogenesis of two Californian strains of myxoma virus (MSW and MSD) was examined in European rabbits (Oryctolagus cuniculus) that were either susceptible to myxomatosis (laboratory rabbits) or had undergone natural selection for genetic resistance to myxomatosis (Australian wild rabbits). MSW was highly lethal for both types of rabbits with average survival times of 7.3 and 9.4 days, respectively, and 100% mortality. Classical clinical signs of myxomatosis were not present except in one rabbit that survived for 13 days following infection. Previously described clinical signs of trembling and shaking were observed in laboratory but not wild rabbits. Despite the high resistance of wild rabbits to myxomatosis caused by South American strains of myxoma virus, the MSW strain was of such high virulence that it was able to overcome resistance. The acute nature of the infection, relatively low viral titers in the tissues and destruction of lymphoid tissues, suggested that death was probably due to an acute and overwhelming immunopathological response to the virus. No virus was found in the brain. The MSD strain was attenuated compared to previously published descriptions and therefore was only characterized in laboratory rabbits. It is concluded that Californian MSW strain of myxoma virus is at the extreme end of a continuum of myxoma virus virulence but that the basic pathophysiology of the disease induced is not broadly different to other strains of myxoma virus.

Monitoring the spread of myxoma virus in rabbit Oryctolagus cuniculus populations on the southern tablelands of New South Wales, Australia. II. Selection of a strain of virus for release

To be able to study the dynamics of myxoma virus spread following a release in the field, a strain of virus is required that is both highly transmissible and readily differentiated from other field strains. Eight strains of virus of known virulence for laboratory rabbits and with previously mapped and sequenced restriction fragment length polymorphisms, were used to infect groups of seronegative wild rabbits. Based on these trials, and on the nature of the DNA polymorphism, a virus designated Brooklands/2-93 was chosen as a strain suitable for experimental release. These trials confirmed that resistance to myxomatosis within wild rabbit populations continues to be substantial and that some rabbits are highly resistant. These rabbits probably have little role in transmission of virus. Most of the virus strains tested induced very small or invisible primary lesions at the inoculation site. Thus the secondary skin sites such as eyelids, face and ears may be critical for transmission.

Infertility in female rabbits (Oryctolagus cuniculus) alloimmunized with the rabbit zona pellucida protein ZPB either as a purified recombinant protein or expressed by recombinant myxoma virus

Development of immunocontraceptives for wild rabbit populations requires selection of both effective antigens and effective delivery systems. Recombinant rabbit zona pellucida glycoprotein B (ZPB) produced in eukaryotic cells in vitro was an effective antigen and induced sustained infertility in 70% of female rabbits. This required two boosts and serum antibody titers of 12 800 or greater. Antibody titers in females were low after the initial immunization, as might be expected with a self-antigen; however, male rabbits had a strong antibody response, indicating that the protein was immunologically foreign. To develop a delivery system, ZPB was delivered by infection with a recombinant myxoma virus. In contrast to the results with ZPB protein, infection of rabbits induced a similar serum antibody response to ZPB in both sexes. This indicated that presentation of ZPB in the context of a virus infection was able to overcome tolerance in females. However, the antibody titers were lower than 12 800, and only 25% of female rabbits were infertile. This antibody response was boosted by injections of recombinant ZPB protein, after which 80% of female rabbits were infertile. Infertility was associated with antibody binding to zonae and varying degrees of ovarian pathology characterized by follicular degeneration and substantial depletion of primordial follicles. Oocyte and follicular degeneration appeared to be the principal mechanism of infertility and may be primarily induced by antibodies to ZPB.

Serological monitoring of the epidemiology of myxoma virus to assess the effects of imposed fertility control of female rabbits on myxomatosis

The epidemiology of myxomatosis was investigated in 12 populations of wild European rabbits at Wellstead in southern Western Australia, which were part of a larger study of the effects of imposing different levels of female sterility on rabbit population dynamics. Using serology, the aims were firstly to monitor the epidemic pattern of myxomatosis on the study sites between 1993 and 1996, and then to determine whether imposed sterility had any influence on the prevalence of myxomatosis. Two patterns of epidemics were observed during the study. The predominant pattern was a spring–summer outbreak that left approximately 90% of rabbits seropositive for myxoma virus antibodies. The second pattern was a slowly spreading epidemic that lasted from autumn until the following spring. This second type of epidemic may be important in the persistence of myxoma virus. Imposed levels of female sterility as high as 80% for three years had no significant effect on the proportion of animals seropositive for myxoma virus antibodies.