Sudden increase in virulence in a strain of Plasmodium berghei yoelii.

The mild and chronic 17X strain of Plasmodium berghei yoelii showed a sudden increase in virulence following a period of 110 days in the deep freeze. The enhanced virulence was seen in a very high and early parasite peak in the blood and a 100% mortality of all infected mice. The exalted virulence remained unaltered following a number of blood transfers of the strain and after four cyclical transmissions through Anopheles stephensi. Enzyme pattern studies revealed that the virulent strain possessed the enzyme types GPI-1 and 6 PGD-4,-both characteristic for strain 17X of P.b. yoelii. Studies carried out to investigate the possibility of a concomitant viral infection transferred by blood passage, which could have been responsible for the enhanced virulence and mortality in the infected mice, showed no presence of any detectable virus in the blood of the P.b. yoelii infected mice. It is suggested that virulence may provide a marker in genetic work of rodent plasmodia and be included in association with enzyme and drug resistance markers.