The author believes that the majority of recently isolated strains have antigen 17 or 13 and are more virulent for laboratory animals and for man than those kept in the laboratory and which have undergone a variable number of transfers. These latter have antigens 1 or 3 and are less productive as regards some or all factors responsible for virulence i. e. infectivity and pathogenicity factors. It would thus seem that antigen loss variation is simultaneously attended by weakening of other biological characteristics. Conclusions from virulence studies will only be valid if experiments have been done on strains which have not yet undergone this antigenic variation.


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