POPULATION GENETICS 



excellent facilities were made available by the authorities 

 concerned for a detailed analysis of the situation by Fenner 

 and his collaborators {ig^ya, b). All the information I shall 

 present is drawn from their work. 



{e) Changes in myxomatosis virulence in the field 



The standard laboratory strain used to initiate the 

 Australian epizootic and all subsequent virus released (with 

 one experimental exception) was obtained from the Rocke- 

 feller Institute about 1930 and had undergone a long series 

 of passages by contact between rabbits before it was effectively 

 liberated in October 1950. It differs somewhat from the 

 virus enzootic in South America by producing less florid 

 lesions, much flatter than those seen when South American 

 strains are inoculated in domestic rabbits. Its lethality is, how- 

 ever, just as high and can be graded as 99*5% or higher. 



Within two years the infection had effectively covered the 

 whole area of Australia which had been occupied by rabbits, 

 that is, all but the tropical and very arid regions. Nowhere 

 were rabbits completely exterminated, and, as far as is 

 known, every considerable region has shown recrudescences 

 of myxomatosis in every subsequent summer. The virus can 

 therefore persist in some way from one summer epizootic 

 to the next. 



What chiefly concerns us is the change in the virulence of 

 the virus. Since the days of Theobald Smith, we have given 

 Hp service to the view that a pathogen 100% fatal to the 

 host would be most unlikely to survive in Nature. Rabies, 

 however, exists to show that there may be exceptions and no 

 one had ever actually followed the changing virulence of 

 a virus spreading naturally. The outstanding aspect of 

 Australian experience was the emergence and persistence as 

 the dominant form of clones of myxomatosis virus of signifi- 

 cantly lower virulence. In practice, changes in virulence 

 can be much more conveniently estimated by the survival 

 times of small groups of rabbits each given a minimal intra- 



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