ANIMAL VIRUSES: A COMPARATIVE SURVEY 9 



The resemblance to the life cycle of yellow fever may be only superficial, but 

 it at least underlines the ecological similarity of the two sorts of agents. 

 Most of the arbor group are mosquito-borne viruses of characteristically 

 tropical habitat with an extraordinary catholicity of host cells in which 

 multiplication can occur. Apparently almost any mammal or bird can be 

 infected, usually without symptoms, and a wide range of Culicine mosquitoes 

 will allow multiplication in the alternate phase. 



There is still scope for further work on the physical nature of the infective 

 virus particles of this group. Though one can recognize the great technical 

 difficulties involved, it would be of the greatest interest to know whether the 

 infective form in the mosquito salivary gland has the same physical form as 

 that produced in mouse brain or chick brain or chick embryo tissues. The 

 only features especially characteristic are the presence of RNA only, the high 

 content of lipid, presumably on the surface, and the presence of not very 

 robust hemagglutinating power. 



The small intestinal viruses, as observed in man, have as common ecolo- 

 gical features a concentration of spread in the warm season of the year, 

 transfer by ingestion of fecally contaminated material, and the development 

 of substantial but rather highly specific immunity following infection. These 

 three features are responsible for almost all the well-known aspects of polio- 

 myelitis as a disease. Broadly speaking, the capacity of some of these strains 

 and not others to invade central nervous system, muscle, or other organs in 

 a proportion of individual hosts is a biological accident of only medical signi- 

 ficance. The existence of herpangina in man provides a link to justify the 

 extension of the ecological niche to the whole gastrointestinal tract and so 

 allow the inclusion of foot-and-mouth disease viruses in the enterovirus group. 



These are the simplest of all viruses — uniform spheres which pack smoothly 

 into crystal lattices containing no other significant components than 

 protein and RNA (Schwerdt, 1957). Their immunological behavior suggests 

 that the surface of the particle is of protein. All that have been examined 

 seem also capable of producing in infected cells material of smaller particle 

 size than the virus, with immunological specificity related to virus and not to 

 the host. There are hints from genetic work (Sprunt et al., 1955) that pheno- 

 typic mixtures containing two distinct antigens can be produced in mixed 

 cultures. 



It is probably too early to make more than tentative suggestions about 

 either the ecological or physical characters of the adenoviruses. In man they 

 are parasites of mucous membranes which seem to have a special predilection 

 to produce proliferation of lymphoid cells in submucosal situations and, at 

 least under some conditions, to persist for long periods in this situation. There 

 is as yet no adequate ground to discuss the relation of the intracellular 

 crystalline structures to the infective particles. 



