VIRUSES OF THE MYXOMA‐FIBROMA SUBGROUP OF THE POXVIRUSES.

Abstract

SummaryResults reported in this and the succeeding paper indicate that Brazilian and Californian myoxma viruses, Shope's rabbit fibroma virus, hare fibroma virus, and squirrel fibroma virus form a distinctive subgroup of the poxviruses, which we have designated the myxoma‐fibroma subgroup. Strains of all of these viruses, including three isolates of rabbit fibroma virus from eastern U.S.A. and a large number of isolates of myxoma viruses from California and South America, have been examined for their capacity to produce plaques in monolayers of four types of cell.The myxoma viruses produce plaques in rabbit embryo fibroblasts and in primary and continuous line rabbit kidney cells. Isolates from California produce smaller plaques than those produced by most strains from South America. A plaque type mutant of a virulent Brazilian strain was found to have altered virulence and pathogenicity for laboratory rabbits. All myxoma viruses also produce small clear plaques on chick embryo fibroblasts, if DEAE dextran is included in the overlay medium. Because of the ease of production of the cells this technique is recommended for routine plaque assay of myxoma viruses.Rabbit and squirrel fibroma viruses produce small clear plaques on rabbit embryo fibroblasts and small proliferative foci on rabbit kidney cells, but no plaques on chick embryo fibroblasts. Hare fibroma virus produces a very few, very small plaques on rabbit cells only. All the viruses reactivate heat‐inactivated rabbitpox virus on all types of cells, but with varying efficiencies.Plaque reduction tests and cross‐protection tests in rabbits establish the relationship between the five viruses, but squirrel and hare fibroma viruses produce a relatively low degree of resistance to myxoma virus.