Altitude and the risk of bites from mosquitoes infected with malaria and filariasis among the Mianmin people of Papua New Guinea.

The Mianmin are a mobile population occupying a remote lower montane area at 100-1200 m altitude in the north-western interior of Papua New Guinea (PNG). Major medical problems include malaria and bancroftian filariasis. An entomological survey conducted along an altitudinal transect from 170 to 1000m identified Anopheles koliensis as the predominant malaria vector below 650 m, with A. punctulatus dominating at the higher elevations. Proportions of mosquitoes with malaria circumsporozoite antigens diminished with increasing altitude, as did the proportion of mosquitoes infected with stage 3 larvae of Wuchereria bancrofti. These patterns are consistent with increases in the length of the extrinsic incubation period associated with the lower temperatures found at higher altitudes. Inoculation rates varied less regularly with altitude, owing to local variation in biting rates, but were sufficient even at the higher elevations to maintain a high parasite prevalence in the human population. Results support recent suggestions that the 'population-sink' model of the PNG highland fringes needs additionally to consider local variation due to non-altitude-related ecological factors.