The Aedes problez in Africa, tath particuls reference to Vest Africa by J.Hamon, Medical, I&tonologist, ORSTOM, Frace i INTRODUCTION. -. Investigations carried out since the last 25 years in tropical Africa by several Institutes m d OTgaisations (East African Virus Research Institute, Etntebbe,Uganda R South African Institute for Medicd Research, Johannesburg, South Africa Virus Research Center, Lagos, Nigeria - Pasteur Institutes & and OR9TOM CBRTERX of L9~ka.r~ SkraBgd, and of Yzomd6,Cmeroons - OCCCE, Bob0 Dioulasso, Haute Volta =.or) have shown that arbovimses beloragbgto groups A9 I3 am3 33mymigem ape very widespread, In some areas up to 96% of the Mabitaxits, but hfallrts, posaess ant ibo&dies again& one or several unrelated arbovimea, the highest Fates being generally observed An t.a6oded sa hso Arboviruses, in tropical Africa, asre t nitted by various species belonging to several arthropods groups, but the majority of proven natural md laborato vectors are mosquitos. Kan-biting anophelines are the only kxom..or O nyong nyoag virus, whereas some other arboviruses are transmitted by Culex and Mmsonia. species, but the min roleheens to be pia-y Aeihe~: specieso Orris obsem-bions ~~Cl.1 be restricted. here to the Aedes subgenera which have been 3nplicated in diseases trmmission, G ~ ~ ~ RBGOLOGT A I I OF 7 ABDEX sp. ies E%XPIOAL AFRICA. I Aedes raoaqtzt-koes are clzarac.f;erized by Lheirwdormnts eggs, which can survive through long periods of- drought, md me reac%ivated by their inrtaersion in waterc The hatching of a sane ba2;ch ofeggs cm be scattered on several weeks and, in extrene conclftions, several months and even years. Many types of breeding places are favorable for Aedes development, the nab categories beimgt grow6 pools md marshes, rock holes, crab burrows, plan.% axils, tree holes and domestic containers, LESVZL breeding places as well as &ult behaviour are under the dependmce of the environment and c a differ widely betwem rain-forest, sa-vmnah, and subdesertic conditiona, Crab-burrovs constitute importmt breeding places in coastal areas* They support several species of the sub enw Aedimorphus m i are the only bpeeding places of Ae.(Skx.sea + penbaensis All crab-burrow breeding Aedes bite ma92 in lar e nmbers, by day and by night, in the vicbvof their burro mo Plant axils are mainly used by specialised species of the subgems Stegonyia r&ich are found sometimeb also in tree-holes, They constitute kuportant breeding places izl rainy areas, as the rain-forest, but nay also be importan% in some savannah areas during the rainy season. Large number of such breeding plzces are - due to the humin activity under the Pom of b m a, pine-apple and taro (Colocasia BP,) plantations, and are fwored by&.sirnpsolli, one of the natural vectors of yellow fever ja East Africa, 0. gt. g. T. o. P.2. -\.._._
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, / x "------Seasonal vardations of frequence are under the dependance of rainfall. Species with a very short larval cycle of development appear soon after the first rains whereas species with a lomger larval life are rarely numerous before the middle or the second half of the rainy season. Some species, like Ae.(St;)vittatus, seem very susceptible to parasites and predators md have only one short peak of occurence during the first months of the rainy season; when breedingplaces become established the frequency of predators increase and these Aedes species are unable to maintah high Larval densities, In s a V m and ~ more in subdesertic areas, the number of species is lower than in fokested areas, but the nmber of individuals can be very high during the rainy season, Urbm areas, and to some extent villages, with theib domestic breeding-places offer special opportunities for Aedes development. Where water supply is plentiful the inhabitants do not store usually any water (with the exception of some communities of coastal West Africa) and the only breeding-places are the disused domestic containers filled with rain-water; then seasonal variations of occurencecmd density are rainfall-dependent. But in areas where water is scarce large supplies of water are generally stored all year round, sometimes in open containers which then constitute ideal breeding-places for Aedes aempti. Ae,(St.) aegtypti, urzder all environmental conditions, has its peak of activity by day-time and, irm urban areas, feeds extensively on mant ind-oors;and out of doors. In sparsely inhabited areas very few Aedes species,,das a regular supply of blood. Recent investigations have shown that in West Africa only some Diceromyia (furcffer-taylori group) and Stegomyia (africanus luteocephalus,zmd to a lower extent aempti, vittatus and metallicus) species prefer ",and probably also monkeys, to other vertebrates, The big majority of other Aedes species feed mainly on domestic cattle and wild game aad bite man only by chance, or have entirely unknown/preferences excluding nan. Sut in savam-ah areas many species of Aedes build very large populations duxing the rainy season and even if one percent only of the females bite man, it is sometimes sufficient to constitute clouds of man-biting mosquitoes. ln and around villages, in open areas, the man-biting species have one htense peak of activity at dush, when humans are usually chattering out of doors, which ensures a high degree of contact between mosquitoes and one of their preferred hosts, as almost all african - Aedes are exophageous On the reverse zoophilic species present generally a more or less. i even level of activity all the night long, or have a peak of activity in the middle of the night. I