SEASONAL FLUCTUATION OF PLASMODIUM-FALCIPARUM GAMETOCYTAEMIA

Two numerically minor components of Plasmodium falciparum prevalence-gametocytaemia and trophozoite densities >99/500 white blood cells-displayed an annual cycle that reflected the seasonal abundance of infective Anopheles dirus at a hyperendemic focus in Thailand, even though the gross monthly prevalence for combined ages remained stable. Gametocyte prevalence rose more than 300% within 30 d after the capture of the dry season's first infective mosquito, remained at about 8% until the beginning of the monsoon 7 months later, then fell within 60 d to about 2%. The number of cases with a high density of trophozoites behaved similarly. These periodic fluctuations represented changes in incidence, at least half of which appeared to be due to superinfection. Almost 49% of all gametocyte carriers were older than 14 years, but nearly all gametocyte densities >20/500 white blood cells were in children. These observations, as well as the calculated efficiency of human infectivity, imply that superinfection of adults may contribute significantly to transmission in semiimmune populations. © 1990 OUP Publishing Ltd.