Dry season refugia of malaria-transmitting mosquitoes in a dry savannah zone of east Africa.

Abstract

Dry season survival of Anopheles funestus, Anopheles gambiae and Anopheles arabiensis in the Kilombero valley a dry savannah zone of east Africa, was investigated with over 400 collections from 23 areas, covering 300 sq km of the valley. Anopheles gambiae was found only in association with humans, in forested areas of high annual rainfall, while An. funestus occurred at high densities at the valley edge where large non-moving bodies of water remained. A large population of An. arabiensis was present along the river system throughout the middle of the valley, and mosquitoes probably derived from this population were occasionally caught in villages bordering the valley. No evidence was obtained of aestivation in any mosquito species. Anopheles gambiae was the most long lived, 6.3% compared to 2.0% of the An. arabiensis and 4% of the An. funestus surviving for four or more gonotrophic cycles, the approximate duration of the extrinsic cycle of most malaria parasites. Oocysts of malaria parasites were found in 5.4% of An. funestus and 2.3% of An. arabiensis from villages. Oocyst rates in An. funestus differed significantly between areas but not between houses within areas. Anopheles funestus is the most important dry season malaria vector in the valley, and remains in foci closely associated with groups of houses. All three species survive at high densities but as otherwise hidden refugia populations.

Revisiting the role of introgression vs shared ancestral polymorphisms as key processes shaping genetic diversity in the recently separated sibling species of the Anopheles gambiae complex

“…MJ Donnelly et al (Charlwood et al, 2000;Spiers et al, 2002) and differing species' density is thought to be one of the major determinants of introgression (Avise and Saunders, 1984). Whether introgression is frequent enough to play a role in the differentiation within A. gambiae S form that is observed either side of the Rift Valley remains to be investigated.…”

Anopheline Mosquitoes and the Malaria Scourge

“…In the field only a small percentage of females of An. gambiae survive for more than three or four gonotrophic cycles [36]. Although a small percentage was found to survive for over ten cycles [37].…”

Rapid assessment of malaria risk using entomological techniques: taking an epidemiological snapshot

“…This can be determined by dissection of unfed mosquitoes (Figure 2) for the presence of a mating plug, the ovarian stage and parity status. Plugs are found only in young females and suggest localized breeding (Gillies 1956;Charlwood, Vij and Billingsley 2000) and young females with early stage ovarioles also indicate breeding taking place. The presence of ovariolar sacs in parous females indicates that the mosquitoes returned to feed shortly Proportion hosts Plasmodium-positive Proportion mosquitoes Plasmodium-positive Figure 7.…”

“…Alternatives to parous-rate estimations (e.g. time series analysis) all require longitudinal collections, placing them outside the RA remit, and have generally proven inapplicable to mosquito studies (Charlwood 1986;Charlwood, Vij and Billingsley 2000).…”

“…Combined parous and oocyst rates provide a suitable estimate of the proportion of feeds infectious to the mosquito (Charlwood, Vij and Billingsley 2000); by further including sporozoite rate, mosquito survival can also be estimated (Charlwood et al 1995a;Beier and Koros 1991). Estimates of sporozoite rates are a needed for EIR determination and are therefore likely to be included in any RA protocol.…”