Comparative studies on sibling species of the Anopheles gambiae Giles complex (Dipt., Culicidae): bionomics and vectorial activity of species A and species B at Segera, Tanzania

Abstract

Collections of Anopheles gambiae Giles complex, A. funestus Giles group and other mosquitoes were made by spray-catch from twelve catching stations indoors and by hand-catch from pit shelters at two catching stations at Segera, Tanzania, between January 1970 and June 1971. Females of A. gambiae were identified cytotaxonomically as sibling species A or B of the complex. In houses during 1970, A. gambiae species B was more numerous at first than A, but A became predominant during the long rains of March-May. In the cool dry weather of June-November both A and B densities declined and the A:B ratio surpassed 50:1. The short rains in December produced a population explosion of species B and less multiplication of species A, the B:A ratio reaching >11:1. In 1971, hot dry weather during January-March caused declines of species A and B with a maximum B: A ratio of 13:1. Long rains, coming in late March, provoked a resurgence of A and a concurrent decline of B, so that the A:B ratio again reached 20:1 in June. Similar cycles of species A and B were observed outdoors, although the relative numbers outdoors/indoors averaged 2·3 times more for species B than for species A. In A. funestus, A. gambiae species A and A. gambiae species B Human Blood Indices were 97·5%, 91·2% and 60·9% indoors and 24%, 2% and 7% outdoors, respectively. Respective malaria sporozoite rates were 1·62%, 4·23% and 0·32% and minimum rates of stage-Ill filarial infection were 0·33%, 0·44% and 0·57%. Sporozoite-positive and sporozoite-negative mosquitoes exhibited similar HBF's in species A and discrepant HBI's in species B. The HBI's were higher in filariapositive A and B females than in filaria-negative females. Gregarines occurred in 1·36% of species A and 0·38% of species B. Trematode cysts were seen in two specimens of species A.Of A and B females 28% and 4%, respectively, had four-banded palps. It is shown mathematically that the discrepant malaria sporozoite rates in species A and B may be explained by extrapolating from the man-biting rate and probable daily survival rate for each species. This implies that no unrecognised factors play a major role in causing the contrasting efficiency of these two sibling species as malaria vectors.

The availability of potential hosts as a determinant of feeding behaviours and malaria transmission by African mosquito populations

“…arabiensis populations in Tanzania and found to vary over quite a wide range (White et al, 1972). The cattle blood indices for An.…”

“…gambiae sensu stricto and An. arabiensis in Segera, Tanzania (White et al, 1972) and for An. gambiae sensu lato in The Gambia .…”

“…Thus, assuming none of the other predictors change, the relative EIR transmitted by any vector species in the presence of cattle (EIR Nc/Nh ) compared to that in the absence of cattle (EIR 0 ) can be calculated as a simple function of their respective human blood index: [14] Thus, the influence of cattle on EIR as a function of Q h , can be predicted using estimates of λ and ρ from Table 2 and assuming that other key predictors of EIR are constant. No major host species other than cattle or humans were identified for any of these vector populations by the preliminary model fitting or, in the case of the Tanzanian sites, by direct blood meal identification (White et al, 1972). We therefore assumed the availability of hosts other than cattle or humans to be negligible (ρ = 0) and predicted the response of EIR to increasing relative cattle abundance using only estimates of λ, and its 95% confidence intervals, as input parameters.…”

“…For the 3 vector populations studied in Tanzania, this relationship was first studied individually for each species and then the impact of cattle abundance on overall EIR was estimated as an average, weighted according to the EIR transmitted by each species. The contribution of each vector species to overall EIR and its corresponding sensitivity was weighted according to reported sporozoite prevalence and relative abundance of these 3 vector species, for this site at the peak of the transmission season in April 1970 (White et al, 1972).…”

“…Although relationships between human blood index and livestock abundance have been reported for Anopheles funestus and important vector species in the An. gambiae complex (White et al, 1972;Highton et al, 1979;Lindsay et al, 1993;Mbogo et al, 1993), these studies have been largely descriptive. Here we describe an availability-based model for mosquito hostseeking behaviour and apply it to the quantitative analysis of host choice by malaria vector populations in African communities.…”