The ecology of the mosquitos of the Northern Guinea Savannah of Nigeria

Abstract

The ecology of the adult mosquitos of the Northern Guinea Savannah was studied from 1960 to 1962 in the Kaduna area mainly in two Hausa villages, Anguwun Rimi and Kangimi, and in nearby Fulani settlements. A description of the area and various types of village huts is given together with an account of the climate and meteorological data. Emphasis was on the Anophelines and their relation to malaria but at the same time information on the Culicines is presented.Biting catches, in which mosquito scouts acting as human bait collected adults in test-tubes, were performed for over a year in A. Rimi. A total of 38 species was caught in these collections, but many appeared only infrequently. By far the commonest mosquito biting man was Anopheles gambiae Giles which had its peak density towards the end of the rains, in September, and although its population was greatly reduced during the dry season it however remained the principal anthropophilous species. In the area in which the catches were performed A. nili (Theo.) was the second commonest mosquito, and with the exception of March it was more abundant than A. funestus Giles. In other areas it was not so common, and in Kangimi A. funestus was almost as abundant during the rains as A. gambiae, and at the beginning of the dry season it became the main Anopheline. Most of the other Anophelines that were taken at bait were only caught in any numbers during the latter part of the wet season; for the remainder of the year they were virtually absent. Mansonia uniformis (Theo.) was abundant in October, but for the rest of the year it was rarely taken; M. africana (Theo.) was found to be uncommon in the area. Due to the close proximity of Kaduna to A. Rimi, Culex pipiens fatigans Weid., a species now associated with urbanisation and not with villages, was common. Both bait catches and pyrethrum spray-sheet collections in houses showed that it greatly increased in numbers at the beginning of the rains, but that this high level of density was not maintained, and during the greater part of the wet season the species was less common than in the early rains. Aedes aegypti (L.) sens. lat. was rare both in houses and in bait catches; the two principal Aedes species were Ae. vittatus (Big.) and Ae. furcifer (Edw.); the latter species is confined to the Savannah zones. The biting cycles of the three main Anophelines were characterised by maximum feeding occurring after midnight. In both A. gambiae and A. nili there were two main peaks, between 2400 and 0100 and between 0300 and 0400 hr.; in A. funestus, maximum biting occurred between 2400 and 0200 and between 0300 and 0400 hr.By the use of mosquito nets baited with humans, both inside huts and out of doors, the relative attractiveness, to the anthropophilous Anophelines, of man in the two situations was assessed. It was found that A. nili was equally attracted to man in both situations, and although slightly greater numbers of A. gambiae and A. funestus were found biting out of doors, for practical purposes man is equally attractive in both places. Certain species, notably A. coustani Lav. sens, str. and A. hancocki var. brohieri Edw., were rare or absent in the inside and outside bait-net collections, whereas they were readily taken in both situations in test-tube catches performed in the same locality on the same nights. In fact, more specimens of A. coustani sens. str. were taken in the outside test-tube catches than of either A. funestus or A. nili, and A. hancocki var.brohieri was commoner than A. funestus.Catches of adults from mud walls and thatch roofing of typical round village huts showed that there was generally no appreciable difference in the attractiveness of these two surfaces to either A. gambiae or A. funestus, but as the thatch only comprised about 34–44 per cent, of the internal surface area of the huts the absolute number resting on the walls will be greater than on the roof.Spray-sheet collections showed that besides large numbers of A. gambiae and A. funestus resting; during the day-time in normal mud-walled living huts of the Hausa, high densities of these species were found in the Fulani huts, which do not incorporate any mud in their design, and moderate numbers were also caught in Hausa villages in the zaures (huts resembling living ones, but without doors, furniture or fires and not normally slept in at night). Collections from outdoor resting places demonstrated that considerable day-time resting occurred in uninhabited huts, under eaves of huts, both inside and under eaves of goat stables, in kitchens, amongst the tightly woven grass mats termed ‘ zana ’ matting, in rodent holes, tree holes, pots, near wells, amongst grass and vegetation and underneath and under the eaves of grain bins. Surprisingly few examples of A. nili were found in any of the outside resting shelters examined, despite the fact that both exit traps and spray-sheet collections showed that it bit freely in huts but rarely stayed in them. The large number of A. gambiae and A. funestus found out of doors, coupled with the exodus of blood-fed individuals from huts, showed that an appreciable amount of exophily occurred in both species. Entrance traps fitted to village huts caught, in addition to unfed, both blood-fed and gravid (taken to include half-gravid and full-gravid) females of A. gambiae and A. funestus, suggesting that a certain amount of exophagy and re-entry of gravid individuals was occurring. Numerous precipitin tests were made on blood-meals obtained from A. gambiae, A. funestus and A. nili. and a few also from most of the other species. Results showed that the three principal Anophelines were strongly anthropophilous but the exophilous population of A. funestus and A. gambiae showed some deviation to other vertebrates. Most of the other Anophelines, e.g., A. rufipes (Gough) sens, lat.,A. hancocki var. brohieri, A. flavicosta Edw. and A. longipalpis var.domicolus Edw., found resting in goat shelters and other outdoor sites had fed mainly on other vertebrates.On account of the already available information on the infectivity of both A. gambiae and A. funestus in Northern Nigeria, dissections for estimation of sporozoite rates were only carried out on their exophilous populations, and in both species these were shown to contribute to malaria transmission. Attention was concentrated on dissecting the other Anophelines, special attention being paid to A. nili. Only two species were found with infected salivary glands. A. flavicosta had a crude sporozoite rate of 2·74 per cent, of the 73 dissected, and from 998 examples of A. nili a crude sporozoite rate of 0·80 per cent, was obtained. It was thought that as A. flavicosta was mainly zoophilous it was possible that the sporozoites in the glands were of malaria not of human origin. It appeared that where A. nili was common it could become a secondary vector, and on account of its marked exophilic nature it might prove difficult to interrupt transmisssion by this species.The relatively recent increase in towns of C. p. fatigans and the almost virtual disappearance of C. nebulosus Theo., formerly the predominant Culicine, and the possibility that this change might have been initiated by the practice in recent years of using DDT larvicidal formulations and not oil, is discussed.