CLONAL PHENOMENA 



dermal infection than by infecting sufficiently large numbers 

 to allow a statement about the mortality. However, it has 

 become customary to refer to the standard laboratory strain 

 with an average time from infection to death of 1 1 days as 

 a 99*5 % strain and the dominant Australian field strain with 

 a survival time of about 23 days as a 90 % killer. It should be 

 stressed that these are the findings obtained on laboratory- 

 bred rabbits never exposed to myxomatosis. 



Fig. 2. Changing virulence of myxomatosis virus isolated from successive 

 Australian epizootics (Fenner). The proportion of strains in each category 

 of virulence, as judged by a combination of percentage mortality and 

 length of survival after injection, is shown for each year. 



The accompanying figure gives an indication of the 

 differing virulence of strains of myxomatosis virus isolated 

 in the field in various parts of Australia over the last 5 years. 

 The dominance of 90 % strains over the last 3 years is clear. 

 It is of particular interest that neither the repeated relibera- 

 tion of 99*5 % virus nor the appearance of occasional strains 

 of much less virulent virus has been able to displace the 90 % 

 virus from its dominance. This change to the dominant field 

 virus appears to have occurred independently in several parts 

 of AustraUa, though the swift initial spread of the virus makes 



24 



