When examined under similar conditions in the laboratory, the two strains of myxoma virus used to initiate the Australian and European epizootics, and three somewhat attenuated strains recovered in the field in Australia and France, were found to produce lesions containing approximately the same amount of virus, and were transmitted by mosquitoes with approximately equal efficiency. The laboratory variant neuromyxoma multiplied poorly in the skin and was very rarely transmitted by mosquitoes.

The survival potential of different virus strains in the field is correlated with the length of time during which rabbits infected with different strains present to biting mosquitoes lesions containing high concentrations of virus. Sickness of the infected rabbit, and extensive viruliferous skin lesions, will increase its suitability as a source of virus for mosquitoes. These considerations explain the dominance of slightly attenuated strains of myxoma virus in the field in Australia.

Species of mosquitoes, when tested under standard conditions in the laboratory, differ in their efficiency as vectors of myxomatosis. These differences may be related to differences in the method of feeding, and to structural differences in the mouthparts.

Engorgement on an immune rabbit after probing through myxomatous skin lesions had no greater effect on the subsequent ability of mosquitoes to transmit myxomatosis than a similar blood feed on a normal rabbit.

Nine serial mosquito-bite passages of the standard laboratory strain of virus, each involving one week in the rabbit and two weeks in the mosquito, had no effect on its pathogenic behaviour.