What caused the decline in virulence?

This decline could result from any combination of increasing host resistance and decreasing viral virulence, and normally we should not know which was operating. But in this case, a carefully controlled set of experiments allowed the two factors to be teased apart.

The decline in virulence of the myxoma virus was demonstrated by infecting standard rabbit strains in the lab with the viruses taken from the wild in successive years. Because the rabbit strain was controlled and constant, any decline in the kill rate must be due to a decline in virulence in the virus. This table shows the results, in Australia and Europe.

In both Australia and Europe the virus started off maximally virulent (killing 100% of infected rabbits); but there was then a rapid increase in the less virulent strains in the viral population - the less virulent strains kill a lower proportion of infected rabbits and take longer to kill them when they do.

Table: Myxoma virus evolved lower virulence over time after its introduction into Australia, France and Britain. Strains of the virus are classified into five virulence grades: I is the most virulent, V is the least. The table shows the percentages of the different strains in rabbit in the wild through time. Modified from Ross (1982).