It has been possible to achieve active immunity against Plasmodium berghei malaria in mice without a primary patent infection. This was accomplished by vaccination with a strain of P. berghei which had lost its infectivity for mice, though not for rats, following a number of passages in tissue culture containing the serum of young male hamsters. The immunity is of the sterile type, and not due to premunition. It was shown that latency in mice denotes a partial breakdown or imperfect development of immunity rather than degree of protection. Immunity takes from 2–3 months to develop fully, and during the pre-immune period mice are particularly sensitive to reinfection. Immunity, once established, persists at a high level for many months. The modified strain is believed to present more antigenic groupings of the type which can effectively stimulate protective antibody production. It appears that trophozoites capable of stimulating immune responses are screened out of the circulation. The trophozoites, which are not screened out and appear in the peripheral blood stream, are not believed to contribute greatly to the stimulation of immunity, even though immunity appears directed against erythrocytic forms. The modified strain reverts to its original character during routine blood passage every 2 weeks in rats over a period of 46 weeks. Different characteristics reverted at different rates. It is concluded that invasiveness, virulence, and ability to stimulate an effective immune response are separate and independent characteristics of the parasite.
Assistant Professor of Biology, Department of Biology.
Professor of Parasitology, Department of Biology and Department of Medical Microbiology.