Virulence grades of myxoma viruses.

aModified from refs. 1 and 3. Virulence assays were conducted in laboratory rabbits, so that they are independent of the resistance levels of the wild rabbit as it evolved in response to the strong selection imposed by the virus. Virulence grades were assigned based on case fatality rates, average survival time and symptomatology.
bGrade 3 viruses were sometimes divided into 3A (case mortality rate of 90–95%; AST of 17–22 days) and 3B (case fatality rate of 70–90%; AST of 23–29 days) [1].