F. M. BURNET 



in the failure so far to show any dehydrogenases of the type found with 

 rickettsiae (Moulder and Weiss, 1951). They resemble the poxviruses in the 

 type of hemagglutinin they produce. 



B. Poxvirus 



With vaccinia virus as the type species there can be no doubt that those 

 viruses with infective particles of the same large size and characteristically 

 complex morphology, and showing serological relationship to vaccinia, form 

 a closely knit group. This includes variola vaccinia, mousepox (ectromelia), 

 Jennerian cowpox, and rabbitpox viruses. A second group includes the avian 

 poxviruses, of which classic fowlpox and canary pox have been the forms 

 most studied. Sparrow pox was described by McGaughey and Burnet (1945) 

 and there can be no doubt that there are many more strains of related viruses 

 in wild birds. In Australia, French and Reeves (1954) isolated from mosqui- 

 toes several strains which appear to represent three types of fowlpox-like 

 virus, one being standard virus and the other two presumably enzootic in 

 wild birds. Australian magpies (Gymnorhina) have been found infected in the 

 wild (Fenner, personal communication). As far as they have been studied, 

 the avian pox viruses have no serological relationship to the vaccinia group 

 and produce no hemagglutinin. 



The third group comprises the rabbit myxoma and fibroma viruses, to 

 which can probably be added Kilham's (1955) squirrel fibroma and the disease 

 of deer described by Shope. and perhaps a condition recently observed in 

 West African monkeys (Andrewes, personal communication). Some doubts 

 were raised about the correctness of the inclusion of these viruses in the pox- 

 virus group, but morphologically they are virtually indistinguishable from 

 vaccinia (Fenner, 1953). The acutely fatal character of myxomatosis has its 

 equivalents in malignant smallpox or canary pox, and transfer by mosquitoes 

 is also the most important way in which the avian poxes are spread. 



Molluscum contagiosum still awaits adequate study and is in the group 

 simply by virtue of morphological resemblance. The same holds to varying 

 extent for a variety of other mammalian poxviruses that have been only 

 superficially studied. 



Detailed study within the group has concentrated mainly on vaccinia 

 virus, but it is probable that in all essentials the other types conform. The 

 picture presented is very different from that obtained with the other well 

 studied viruses. In two respects vaccinia virus resembles psittacosis virus. 

 Both contain DNA and produce a nonviral hemagglutinin of similar character 

 with lipid as an important component. RNA is present in viruses of the 

 psittacosis group and has not been reported for vaccinia virus, but further 

 study of this point is probably required. The other feature of interest for any 



