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Yaba monkey tumor

protein' common to the parapoxvirus, orthopoxvirus, leporipoxvirus and avipoxvirus genera has been identified, prepared by alkaline extraction of the virion. Subsequent investigations have shown four cross-reacting polypeptides between orthopoxviruses and leporipoxviruses, and one between capri-poxviruses and parapoxviruses. However, none of these are significant in cross-immunity between the genera.

Within the orthopoxviruses, there is good cross-immunity between all the members, exemplified by the use of vaccinia to protect against smallpox (variola). The antigens which stimulate protective antibodies are present in the surface tubular elements and in the envelope derived from modified cellular membrane that surrounds the naturally released virion. Virions released from disrupted cells lack this external membrane and are reported to be less infectious. The envelope also contains the glycoprotein hemagglutinin, which is common to all orthopoxviruses but absent from the other seven genera. Approximately 80 polypeptides can be identified by one-dimensional polyacrylamide electrophoresis of purified vaccinia virus. Sera prepared against the disrupted virion can be adsorbed with purified heterologous orthopoxviruses to distinguish between the different members of the group using agar gel immu-noprecipitation. Monoclonal antibodies are available to characterize the major antigens.

All members of the capripoxvirus genus also cross-protect, and although differences have been identified between the viral polypeptides of different isolates, these have not yet been used to distinguish between them. As with the orthopoxviruses, polyclonal sera are equally reactive with homologous and heterologous strains of capripoxvirus. Distinction can also be made between enveloped, naturally released virion and unenveloped virion derived from a disrupted cell.

The poxviruses of birds were originally placed together in what was thought to be an antigenicallv closely related group. More recent investigations suggest that at least three immunologically distinct types of avian poxvirus exist, represented by type species: fowl poxvirus, quail poxvirus, and psittacine poxvirus.

The parapoxviruses form a morphologically distinct group, with shared antigens which cross-react in all standard serological tests. However, previous infection gives only limited protection against even homologous challenge, so that the cross-protection studies used to place new strains of poxvirus into any of the other poxvirus genera cannot be applied to novel strains of parapoxvirus.

The poxvirus genome appears to be very stable in the field situation. Isolates of capripoxvirus collected in Kenya over a 30 year period cannot be distinguished by comparing the genome fragments generated by Hind]]] restriction endonuclease digestion. The orthopoxvirus, vaccinia, has also persisted, little changed, in buffaloes as buffalo pox. Major genomic changes may occur when recombination takes place between poxviruses during a dual infection. This has been shown to occur not only in the laboratory between strains of orthopoxvirus and between strains of leporipoxvirus, but also in the field between strains of capripoxvirus. Whether recombination could occur between strains of different poxvirus genera is probably unlikely, but within genera there is potential for the evolution of new combinations with an enlarged host range or increased virulence.

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