Myxoma virus was introduced into the European rabbit population of Australia in 1950. Although the virus was initially highly lethal in rabbits, there was rapid selection for less virulent strains of virus and innately resistant rabbits. To investigate the basis of resistance to myxoma virus, we have compared the pathogenesis of the virulent strain of myxoma virus originally released into Australia and an attenuated, naturally derived field strain of myxoma virus. This was done in laboratory rabbits, which have not been selected for resistance, and in wild rabbits that have developed significant resistance. Wild rabbits were able to recover from infection with virus that was always lethal in laboratory rabbits. Laboratory rabbits were able to control and recover from infection with attenuated virus. This virus caused a trivial disease in wild rabbits. There was little difference between laboratory and wild rabbits in titers of either virulent or attenuated virus in the skin at the inoculation site. However, resistant wild rabbits had a 10- to 100-fold lower titer of virulent virus within the lymph node draining the inoculation site and controlled virus replication in tissues distal to the draining lymph node. Replication of virus in lymphocytes or fibroblasts cultured from wild and laboratory rabbits demonstrated that resistance was not due to altered cellular permissivity for replication. Neutralizing antibodies were present in both susceptible and resistant rabbits, suggesting that these have no significant role in resistance. We hypothesise that resistance is due to an enhanced innate immune response that allows the rabbit to mount an effective cellular immune response. (C) 2000 Academic Press.
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