Suchergebnisse

An ELISA (enzyme-linked immunosorbent assay) for detecting antibodies to myxoma virus was characterised in wild rabbits for use in epidemiological studies. Virus neutralisation assays and virus challenge were used to define sera from rabbits as positive or negative for myxoma-virus antibodies. In a group of naturally infected wild rabbits, antibodies to myxoma virus were readily detectable by ELISA each month for at least 12 months in all rabbits, including those where neutralising antibodies could no longer be detected. Maternally transferred antibodies could be detected in kittens born to immune does for approximately six weeks after birth. IgM antibodies to myxoma virus were detected by ELISA only during the active disease and recovery phase of myxomatosis. The ratio of IgM : IgG at a standard serum dilution provided an index of time since infection and a confirmatory assay for early myxomatosis, because the detection of IgM corresponded approximately with the onset of clinical signs. Rabbit antibodies to the orthopoxvirus, vaccinia, did not cross-react in the ELISA.

Research over the last 15 years has examined whether fertility control can reduce overabundant rabbit populations and whether an effective immunocontraceptive agent can be developed and delivered. The results of this research indicate that for fertility control to have an environmental impact at least 80% of females will need to be infertile and that this infertility will need to be permanent. Epidemiological studies suggest that this level of infertility may be very difficult to obtain with a recombinant myxoma virus because of competition with field strains of virus. Research with laboratory rabbits using recombinant myxoma virus to deliver an immunocontraceptive antigen demonstrated that it was possible to obtain the required level of infertility using rabbit zona pellucida C as an antigen. However, only ~50% of animals remained infertile in the medium term. Further research on delivery vector and antigen selection would be needed to produce a practical immunocontraceptive virus for laboratory testing. Such a virus would then need to be optimised for transmissibility before it would be suitable for field testing.

Release of virulent myxoma virus has been a key component of rabbit-control operations in Queensland, Australia, since the 1960s but its use rests on anecdotal reports. During a routine operation to release virulent myxoma virus we found no evidence to support the continued regular use of the technique in south-west Queensland. Radio-tagged rabbits inoculated with virulent myxoma virus contracted the disease but failed to pass enough virus to other rabbits to spread the disease. Rabbits with clinical signs of myxomatosis that were shot were infected with field strain derived from the original laboratory strain released in 1950 rather than the virulent strain that has been released annually. There was no change in rabbit survival or abundance caused by the release. Nevertheless, the release of virulent virus may be useful against isolated pockets of rabbits mainly because field strains are less likely to be present. Such pockets are more common now that rabbit haemorrhagic disease virus is established in Queensland.

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.