ANIMAL VIRUSES: A COMPARATIVE SURVEY 3 



in the discussion of general aspects of structure and functional activity; and, 

 the second, that for various reasons these were the three virus types which 

 had been most extensively investigated in the laboratory. 



This is clearly an arbitrary arrangement and where information available 

 is relevant other types are freely discussed. An ideal presentation would 

 probably have to provide at least three additional prototype viruses 

 representing, respectively, the psittacosis group, the adenoviruses, and the 

 arthropod-borne viruses (arborviruses), typified by yellow fever and western 

 equine encephalitis viruses. 



II. A Basis for the Classification of Animal Viruses 



Within the last ten years a beginning has been made in the classification 

 of the better-known animal viruses. Definitive accounts of the classification 

 accepted at the Sixth International Congress of Microbiology, Rome, 1953, 

 for several of the groups, have now appeared (Andrewes et al., 1955; von 

 Magnus et al., 1955; Fenner and Burnet, 1957). 



Since the Rome discussions, a number of new points that seem relevant 

 have arisen and it may be of value to make some comments in regard to 

 each of the groups which have been accepted as appropriate for naming, 

 either at the Congress or by unofficial action since. 



A. Psittacosis Group 



This group was deliberately excluded from the discussion of virus classifi- 

 cation at the Rome meeting. It has, however, some marked resemblance to 

 the poxvirus group and has been regarded by the editors as falling within the 

 scope of this work. 



There is no uniformity of opinion as to the number of species contained in 

 the group or whether the name Miyagaivanella used in Bergey is a valid one 

 (Meyer, 1953; Rake, 1953). 



Psittacosis virus proper is typical of a widespread group of avian pathogens 

 and there is an almost equally wide range of forms parasitic in mammalian 

 tissues, of which the virus of lymphogranuloma venereum has been most 

 extensively studied. 



They all show a complex intracellular development, producing in early 

 stages forms much larger than the elementary bodies, which represent the 

 final infective product liberated from the cell. These elementary bodies, when 

 prepared for electron microscopy, show a central "nucleus" surrounded by a 

 loose-fitting, wrinkled skin. The central region is Feulgen-positive and both 

 DNA and RNA have been shown to be present in purified virus particles 

 (Zahler and Moulder, 1953). 



Psittacosis group viruses approach the rickettsiae in being sensitive to the 

 action of tetracycline antibiotics and in their staining reactions, but differ 



