CLONAL PHENOMENA 



will be infected with a single infective dose of the mutant 

 form free of the more virulent original type. If one such 

 animal can now serve as a source of infection for a dozen 

 rabbits which are not simultaneously exposed to infection 

 by the virulent type, the new strain will be in a position to 

 compete effectively for survival with the original. 



The features making for survival of virus in the field are as 

 follows : (a) high content of virus in the skin lesions through 

 which the mosquito feeds; {b) prolonged availability of virus 

 in such lesions : that is, if a rabbit survives i o days after skin 

 lesions become infective, it is liable to infect twice as many 

 mosquitoes as would be the case if it died in 5 days. It is 

 reasonable to believe that for (a) it is necessary to have 

 a potent virus and for {b) one that allows the rabbit to survive 

 a relatively long period but not long enough for lesions to 

 become non-infective by the development of immunity. The 

 best compromise under Australian conditions seems to be 

 at the 90 % level of lethality. Incidentally, this is a very 

 attractive solution for the farmer and pastoralist for it would 

 ensure that the rabbit population is kept down to 10 or 20 % 

 of its former level and that myxomatosis remains constantly 

 enzootic so that each spring flush of young rabbits can be 

 decimated in the summer epizootic. 



In 1953 it appeared that in Europe no diminution in the 

 virulence of myxomatosis virus had occurred and the 

 possibility was raised that this strain, much closer to the wild 

 South American virus than the standard Australian 99-5 % 

 strain, might maintain itself at full virulence in Australia. 

 Two introductions were therefore made in areas where events 

 could be closely followed. The results were almost as clear 

 cut as an experiment with bacteria in a test tube. 



Virus was obtained from France and two large batches of 

 artificially injected rabbits were liberated late in November 

 and early in December. Mosquitoes had just become preva- 

 lent and one case of natural myxomatosis had occurred just 

 as the experiment was begun. The results of three months' 



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