Seasonal fluctuation of Plasmodium falciparum gametocytaemia

Two numerically minor components of Plasmodium falciparum prevalence--gametocytaemia and trophozoite densities greater than 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 greater than 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 semi-immune populations.