Plasmodium falciparum: effect of chloroquine, halofantrine and pyrimethamine on the infectivity of gametocytes for Anopheles stephensi mosquitoes.

The activity of chloroquine, halofantrine and pyrimethamine against the gametocytes and sporogonic stages of Plasmodium falciparum (strain NF54) was tested. Five-day-old gametocytes (stages I and II) from in vitro cultures were exposed to the drugs for 48 hours. The effect of the drugs on gametocyte development was assessed by counting gametocytes on days nine and 15 of culture and determining the infectivity of the drug-treated gametocytes to mosquitoes. Gametocytogenesis was partially inhibited by all three drugs; there were 71% of the number of gametocytes in drug-free control cultures in cultures with 3 x 10(-8) M chloroquine, 51% with 5 x 10(-8) M chloroquine, 78% with 5-7 x 10(-9) M halofantrine, and 48% with 10(-7) M pyrimethamine. Halofantrine- and pyrimethamine-treated gametocytes were found to be more infective to Anopheles stephensi than untreated controls. The three drugs were also administered to the mosquitoes, either in the first bloodmeal, which contained gametocytes from in vitro cultures, or in the second, parasite-free bloodmeal, given four days after infection. The sporontocidal activity of the drugs was evaluated by counting the number of oocysts on the midgut seven or eight days after infection, or the number of sporozoites in the salivary glands 15 days after infection. A sporontocidal effect was observed only when pyrimethamine was administered with the infective bloodmeal. Neither chloroquine nor halofantrine had any marked effect on sporogony at the concentrations tested.