On May 8, 1960, one of the staff of the Parasitology Section of this Institute developed a febrile illness subsequently diagnosed as malaria. Paroxysms of tertian periodicity were produced by a parasite morphologically indistinguishable from Plasmodium vivax. Transfer of the infections from this laboratory worker to a normal rhesus monkey suggested the possibility of accidental transmission to man of one of the strains of Plasmodium cynomolgi under study in the Institute.
Systematic efforts to confirm the above suggestion followed. Human volunteers were bitten by mosquitoes heavily infected with either the bastianellii or M strain of P. cynomolgi. Two of two volunteers inoculated with the bastianellii strain developed patent infections which could be transferred back to the rhesus monkey via injection of human blood. None of the 7 volunteers inoculated with the M strain developed a parasitemia detectable on examination of thick blood films. Blood from one of these volunteers, taken at a time of low-grade fever, did produce an atypical malarial infection in a rhesus monkey. However, typical infections resulted from either blood or sporozoite transfer of the parasites from this monkey to other normal animals.
These developments appear to warrant the conclusions: (1) that the original infection was doubtless caused by accidental transmission of the bastianellii strain of P. cynomolgi; (2) that infection with either the bastianellii or M strain of P. cynomolgi can be transmitted to man; and (3) that transmission of the more recently isolated bastianellii strain can be accomplished more readily than can transmission of the M strain which has been carried in the laboratory by blood to blood transfer for more than 25 years. Stress has been laid on the need for open-minded evaluation of the significance of these findings to malaria eradication and control problems in various parts of the world.