PLASMODIUM-FALCIPARUM MALARIA TRANSMISSION-BLOCKING IMMUNITY UNDER CONDITIONS OF LOW ENDEMICITY AS IN SRI-LANKA

Sera from acute primary Plasmodium falciparum patients in Sri Lanka were tested for the presence of antibodies against gamete antigens and for their functional effects of transmission blocking activity. Comparisons were made with corresponding data from a previous study from sera of patients from Papua New Guinea where malaria is more highly endemic. Although the prevalence of anti-gamate antibodies in the two groups were broadly similar, the prevalence of infectivity suppressive effects in the Sri Lankan sera (56%) was less than in Papua New Guinea sera (75%), suggesting that the generation of functionally effective transmission blocking antibodies requires prolonged exposure to multiple inoculations of malaria. In Papua New Guinea sera there was a good correlation between transmission blocking effects and antibody responses to Pfs 230, a known target of transmission blocking antibodies. Among the Sri Lankan sera no strong correlation was found between transmission blocking effects and the presence of antibodies to gamete surface antigens Pfs 230 nor Pfs 48/45 as detected by immunoprecipitation of radio-iodinated gamate proteins; a strong correlation was however, found between the intensity of response to gamete surface antigens by IFA and transmission blocking effects of these sera. It is possible therefore, that the antigens identified by IFA include non-protein moieties and that these may be the targets of transmission blocking antibodies in sera from acute primary infections of P. falciparum.