Myxomatosis in Australian Rabbits

Introduction: In 1859, twelve pairs of the European rabbit were released on a ranch in Australia. By 1900, the several hundred million rabbits distributed throughout most of the continent were competing efficiently with sheep, on which the Australian economy was based. The myxoma virus, resident among rabbits in South America, caused a cancer among the European rabbits that led to a quick death.

European rabbit:
Biological Control: In 1950, the myxoma virus, spread by mosquitoes, was introduced in one locality in Australia. A myxomatosis epidemic broke out and spread rapidly among the rabbits. The first epidemic killed 99.8 per cent of the individuals in infected rabbit populations. During the following myxomatosis season (coinciding with the presence of mosquitoes), only 90 per cent of the remaining population was killed. During the third outbreak, only 40-60 per cent of rabbits in the disease areas died.

Decline of Virulence: The few rabbits that survived the disease developed immunity by producing antibodies and were unaffected by later outbreaks of the virus. That immunity was conferred to the offspring of immune females through the uterus. Genetically determined immunity to the disease reduced rabbit mortality, and offspring of those surviving parents inherited their parents' resistance. Virus strains with less virulence lengthened the survival time of the rabbits, thus increasing the availability of the infected rabbit to mosquitoes and facilitating dispersal of the virus.

Periodic samples of myxoma virus, collected in the field and tested on European rabbits with no previous exposure to the virus, demonstrated that the virulence of natural myxoma strains decreased over the years following its introduction. In addition, resistance of wild rabbits to the myxoma virus increased steadily from almost total mortality when the virus was first introduced to less than 50% deaths seven years later to a virus strain of moderate virulence. Studies indicated further that whether a rabbit died from myxomatosis depended largely on the inheritance of genetic immunity from its ancestors.

Summary: The myxoma virus caused a massive epidemic among European rabbits that had no previous exposure to it. Both populations experienced genetic changes in their populations while adapting to each other: rabbits that survived had increased resistance and surviving virus strains had decreased virulence.

Adapted and excerpted from: Ricklefs, Robert E. 1979. Ecology. Chiron Press. NY. 2nd Edition. pg 632-633