Bulletin of the World Health Organization, 55 (2-3): 403 (1977)

An effective immunization of Aotus trivirgatus monkeys against Plasmodium falciparum: a research note * WASIM AHMAD SIDDIQUI 1

This is the first report of a successful immunization of monkeys against the human malaria parasite, Plasmodium falciparum. Five monkeys (Aotus trivirgatus) were used in this pilot experiment. Two of the monkeys served as controls and the other three were immunized with P. falciparum (FUP, Uganda-Palo Alto strain) antigen. The antigen primarily consisted of mature segmenters containing fully developed individual merozoites. The antigen, emulsified with Freund's complete adjuvant, was administered twice, with an interval of 3 weeks, intramuscularly to each of the three monkeys. Three weeks later, all five monkeys were challenged with P. falciparum (FUP strain) parasites. The course of the infection in the controls and vaccinated monkeys is shown in Fig. 1. Both control monkeys died with a high level of parasitaemia within two weeks of challenge. By contrast, all three immunized monkeys survived. Monkey No. 269 showed solid protection and, in monkey No. 268, the level of parasitaemia remained below 0.1 %. However in monkey No. 270, parasites were first detected on day 11, the peak parasitaemia of 7.0 % was reached on day 17, and after day 30 the parasitaemia dropped below 0.1 %. Although the number of monkeys in this experiment was small, the differences between the course of infection in * This work was partially supported by the United States Agency for International Development. 1 Professor of Tropical Medicine and Medical Microbiology, University of Hawaii School of Medicine, Honolulu, HI 96816, USA.

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Fig. 1. Course of infection of Plasmodium falciparum (FUP strain) in vaccinated and unvaccinated Aotus trivirgatus monkeys.

immunized and in non-immunized monkeys are indeed very significant considering the high virulence of P. falciparum (FUP) for Aotus trivirgatus monkeys. The results reported here are significant with regard to the development of an effective vaccine against human malaria. Of course, the major obstacle to merozoite immunization in man is the difficulty of developing a suitable adjuvant.

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