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The genetic basis of virulence in a line (YM) of Plasmodium yoelii yoelii was investigated in a cross with a mild line (A/C). The blood forms of the virulent line developed extensively in mature erythrocytes of mice, causing death of the host within 7 days; infections with the mild line were normally restricted to reticulocytes, infected animals recovering after three weeks. Lines YM and A/C differed additionally in enzyme and drug-sensitivity markers. Studies on infections established from each line alone from sporozoite mixtures of the two lines and from the cross between the lines showed that the appearance of virulence had been caused by a genetic change in the parasite, and not by other factors such as a concurrent infection with another organism. An analysis of the characters of 56 clones derived from the cross showed that the virulence character had undergone recombination with the other markers, and appeared to be inherited in Mendelian fashion. Three clones exhibited atypical virulence, although it was not clear whether this had been produced by genetic recombination.

A sudden enhancement in virulence of a mild Plasmodium berghei yoelii 17 x strain resulted in fulminating and fatal infections in CF1 and A/J mice. The virulent strain has maintained its characteristics after ten cyclical transmissions through Anopheles stephensi. The visible expression of virulence of the mutated strain is its ability to cross the blood-brain barrier and cause intravascular sequestration of injected erythrocytes and blockage of brain capillaries. We, therfore, believe that the virulent line of Plasmodium berghei yoelii 17 x could serve as a useful laboratory model for the study of "cerebral malaria."

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