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 we re tested for the presence of antibodies against gamete antigens and f or their functional effects of transmission blocking activity. Compari sons were made with corresponding data from a previous study from sera of patients from Papua New Guinea where malaria is more highly endemi c. Although the prevalence of anti-gamate antibodies in the two groups were broadly similar, the prevalence of infectivity suppressive effec ts in the Sri Lankan sera (56%) was less than in Papua New Guinea sera (75%), suggesting that the generation of functionally effective trans mission blocking antibodies requires prolonged exposure to multiple in oculations of malaria. In Papua New Guinea sera there was a good corre lation between transmission blocking effects and antibody responses to Pfs 230, a known target of transmission blocking antibodies. Among th e Sri Lankan sera no strong correlation was found between transmission blocking effects and the presence of antibodies to gamete surface ant igens Pfs 230 nor Pfs 48/45 as detected by immunoprecipitation of radi o-iodinated gamate proteins; a strong correlation was however, found b etween 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.