Myxoma virus and Rabbits in Australia
In the 1700’s, European rabbits were introduced to the Australian continent. After introduction, the population of rabbits exploded, leading to devastation of the local ecology. Many different measures were taken in an attempt to control the rabbit population. One measure taken was a form of biological warfare – Australian officials started infecting rabbits with myxoma virus in 1950.
Myxoma virus is a poxvirus that causes myxomatosis in European rabbits – a disease that leads to skin tumors, fevers, fatigue, and death. The release of the virus was initially very successful, reducing the rabbit population from 600 million to 100 million rabbits within a couple of years. At first, the myxoma virus was highly virulent in European rabbits – 99% of rabbits infected with the virus died within two weeks of being infected. However, after that, the number of rabbits that died when infected dropped, and the rabbit population no longer declined.
What happened? First, rabbits that happened to have mutations that increased their resistance to dying from the myxoma virus were more likely to survive and reproduce – thus passing on those resistance genes. Thus, natural selection occurred in the rabbit populations. Second, the virus population also evolved! If a rabbit dies quickly from a virus, the time for virion in that rabbit host to transmit to a new host is limited, especially compared to virion that do not kill their host as quickly. Thus, over time, there was selection for viruses that did not kill their rabbit hosts as rapidly – a less virulent virus.
Coevolution is occurring between the host population (rabbits) and the pathogen population (viruses). Rabbit populations evolved resistance to the virus over time, which in turn led to selection for viruses that were able to suppress the evolved immune response by rabbits. This coevolution between the Myxoma virus and European rabbits has not only been recorded in Australia, but parallel coevolution has been seen in the United Kingdom and France (Alves et al 2019).
The evolution of virulence in the myxoma virus is constrained by a trade-off. Extremely virulent viruses replicate rapidly within the host, but killing the host too quickly can reduce its transmission success. This is largely because the virus is transmitted through insect vectors; the virus has to reach the rabbits’ skin to be picked up through insect bites, but this happens somewhat late into the lifecycle of a myxoma infection. If the rabbit dies before viruses can reach the skin, they will not be able to transmit. However, within a rabbit, rapidly replicating viruses will outcompete those that replicate more slowly. Thus, the selective pressures on virulence face a trade-off - the optimal level of virulence for the virus will balance pressures that favor rapidly replicating viruses that outcompete other viruses within the same host, and pressures that favor viruses that do not replicate so rapidly that they kill their rabbit host before they can transmit.
Both the overpopulation of invasive rabbits and the introduction of Myxoma virus were alterations created by human specific practices. The release of the Myxoma virus occurred without evolutionary foresight, and while it initially resulted in a reduced rabbit population, it has resulted in a coevolutionary arms race between the rabbits and the virus with no end in sight.