Chapter IX 

 Variation in Virulence in Relation to Adaptation to New Hosts 



Frank Fenner and John Cairns 



Department of Microbiology, John Curtin School of Medical Research, 

 Australian National University, Canberra, Australia 



I. Introduction 225 



II. Mechanisms of Virus Variation 227 



A. In Bacterial Viruses 228 



B. In Animal Viruses 228 



1 . Variation by Mutation 229 



2. Host-Induced Variation 229 



3. Genetic Recombination 229 



III. Animal Virus-Host Cell Systems 230 



A. Simple Cell Systems 230 



1. The O-D Change of Influenza Virus 230 



2. Variation of Poliovirus in Tissue Culture 232 



3. Various Examples of Coexistence of Virus and Cell 234 



B. Structurally Complex Systems 234 



1 . Pock variants of the Poxviruses 236 



2. Adaptation of Influenza Virus to Mouse and Hamster Lung 237 



C. Systems Involving Sequence 240 



1. Mousepox 241 



2. Yellow Fever 242 



3. Poliomyelitis 242 



4. Myxomatosis 244 



IV. Summary 246 



References 247 



I. Introduction 



Although there is an increasing trend toward the study of animal viruses 

 and the cells they infect as systems of intrinsic biological interest, the science 

 of animal virology has until recently been developed as a branch of medical 

 science, by men concerned primarily with viruses as agents of disease. The 

 essential prerequisite for the experimental investigation of an animal virus or 

 of a virus disease of man or a domestic animal is the production in some 

 laboratory host of recognizable signs of infection associated specifically with 

 the virus in question. 



Such a result may be observed the first time the virus is inoculated into an 

 experimental host. For example, cowpox virus from natural human or bovine 



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