The serial passage of Virus myxomatosum through fibroma-recovered domestic rabbits did not alter its pathogenic properties. Fully virulent Virus myxomatosumpersisted in the inoculated testicle of fibromarecovered rabbits for at least 16 days following inoculation. Virus injected into the testicles of myxoma-immune domestic rabbits, on the other hand, was promptly rendered non-demonstrable. The failure of fibroma-recovered domestic rabbits to destroy injected Virus myxomatosumand the absence from their sera of neutralizing antibodies effective against Virus myxomatosum are considered to be evidence against the identity of the fibroma and myxoma viruses. The rapidity with which fibroma-recovered rabbits develop neutralizing antibodies following infection with Virus myxomatosum is considered to be a possible factor in their acquired resistance.
It is believed on the basis of all the evidence that infectious fibroma of rabbits is a definite disease entity and not merely a mild and nonfatal form of infectious myxoma.