Disease Resistance: Rabbits and Myxamatosis

By Branden Holmes (Surroundx) on November 28, 2010

The first fleet to Australia (in 1788) brought with it rabbits to Sydney Cove as a staple. No doubt some of these would have escaped. But the wild population seems to have been kept in-check by the local carnivorous marsupials. However it was a different story on Tasmania. A newspaper article from 1827 states that there were “thousands” on some private properties. Tasmania clearly had an infestation, but it was isolated from the rest of the colonies. The infestation on the mainland is purported to have been caused by an English immigrant to Australia by the name of Thomas Austin. A keen rabbit hunter, he asked his nephew back in England to send him twenty four grey rabbits, five hares, seventy two partridges, and some sparrows. In October of 1859, he released them at his property Barwon Park in Victoria: “the introduction of a few rabbits could do little harm and might provide a touch of home, in addition to a spot of hunting.” But unfortunately the land is so vast that he was barely able to shoot any before their population exploded, helped no doubt by subsequent releases by other colonial settlers. Apart from Mr. Austin and probably a few other hunters the rabbits didn’t really have any predators, apart from birds of prey and the recently introduced dingo (Canis lupus dingo, around 3500-4000 years ago), and consequently they bred like their namesake.

In 1938 the myxamatosis virus was imported into Australia to try to stem this population explosion which was well and truly underway. The virus was subsequently deployed on carefully selected test groups, and the results were very encouraging. After only a few weeks, rabbit populations showed a drastic reduction in numbers. Groups that once had 5,000 members, now had about 50. A percentage of about 1% of the original population had survived, and with one more despatch of myxamatosis they should have all but been eradicated. But this wasn’t the case. When the rabbits were exposed to another dose of the deadly disease rabbits still died but far fewer than had been anticipated-far fewer than had been hoped for. The rabbits had seemingly built up a resistance to the supposedly deadly disease. But how could this be?

The allele’s that coincidentally were resistant to the disease, were already present in the population. From the above comparison, that number would seem to be about 1% of the rabbits who had this resistant combination of alleles, which fits with the probability of such a coincidental event. The variation in the gene pool allowed some of the rabbits to survive. It was purely random which rabbits would possess the combination of resistant alleles. And in this case, there was not enough time to breed a solution, so the necessary variation must necessarily have already been present.

Related articles:
evolution, natural selection, Rabbits

About Branden Holmes
I am an amateur evolutionist interested in the theoretical side of the subject.

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