Plasmodium falciparum malaria disease manifestations in humans and transmission to Anopheles gambiae: a field study in Western Kenya.

Transmission of the malaria parasite Plasmodium is influenced by many different host, vector and parasite factors. Here we conducted a field study at Mbita, an area of endemic malaria in Western Kenya, to test whether parasite transmission to mosquitoes is influenced by the severity of malaria infection in its human host at the time when gametocytes, the transmission forms, are present in the peripheral blood. We examined the infectivity of 81 Plasmodium falciparum gametocyte carriers to mosquitoes. Of these, 21 were patients with fever and other malaria-related symptoms, and 60 were recruited among apparently healthy volunteers. Laboratory-reared Anopheles gambiae s.s. (local strain) were experimentally infected with blood from these gametocyte carriers by membrane-feeding. The severity of the clinical symptoms was greater in febrile patients. These symptomatic patients had higher asexual parasitaemia and lower gametocyte densities (P = 0.05) than healthy volunteers. Ookinete development occurred in only 6 out of the 21 symptomatic patients, of which only 33.3% successfully yielded oocysts. The oocyst prevalence was only 0.6% in the 546 mosquitoes that were fed on blood from this symptomatic group, with mean oocyst intensity of 0.2 (range 0-2) oocysts per mosquito. In contrast, a higher proportion (76.7%) of healthy gametocyte carriers yielded ookinetes, generating an oocyst rate of 12% in the 1332 mosquitoes that fed on them (mean intensity of 6.3, range: 1-105 oocysts per mosquito). Statistical analysis indicated that the increased infectivity of asymptomatic gametocyte carriers was not simply due to their greater gametocyte abundance, but also to the higher level of infectivity of their gametocytes, possibly due to lower parasite mortality within mosquitoes fed on blood from healthy hosts. These results suggest that blood factors and/or conditions correlated with illness reduce P. falciparum gametocyte infectivity.