From June 1, 1955 to July 24, 1956 Anopheles gambiae were collected in villages in Marshall territory, coastal Liberia, using three collecting techniques: hand-catching mosquitoes on human bait in the open, trapping with human bait, and hand-catching mosquitoes found resting in the houses. All three methods yield valid population samples for estimating population size, and for studying both malaria and filariasis infectivity rates, demonstrating that gambiae feeds readily in the open and does not need to enter houses. The gambiae population is greatest during the rainy season from May to October, reaching peaks at the beginning and at the end of the rains, which are also the two periods of highest transmission. The gambiae population thus varies inversely with the temperature, which is lowest during the rains.
The oöcyst, sporozoite and total infection rates were found to yield the same epidemiologic pattern, the last being the most accurate and the easiest to interpret. The relationship between the sporozoite rate and the oöcyst rate can be used to estimate the average age of wild caught mosquitoes when the timing of the developmental cycle of Plasmodium in the mosquito is also known. The average age thus derived for the mosquitoes studied was 27 days. Species identification of oöcysts was possible in a third of the positive midguts; of these falciparum made up 82.4 per cent, and malariae and ovale each 8.8 per cent. Vivax was not found in wild gambiae. The disposition of the oöcysts in the midgut differed from that described in A. atroparvus and A. stephensi; the density was greatest in the middle third (45.5%), somewhat less in the anterior third (36.2%), and least in the posterior third (18.3%).
The overall rate of infection in A. gambiae with third stage bancroftian filariae (3.6%) was much higher than rates found by previous workers here, but is probably nearer the truth since it is based on many more dissections. Previous reports that A. melas is a more efficient vector of filariae than gambiae were based on small samples and are not convincing. Seasonal variation both in rate and in density of infection was noted, both increasing with the increase in the gambiae population at the beginning of the rainy season, both being lowest during the dry season. Geographically, rates of infection were found to be highest near the tidal rivers. becoming progressively lower inland, away from these rivers, and on higher ground.