Genetic structure of anopheles funestus (diptera: culicidae) populations and its association with malaria transmission in some ecological sites in Ghana

Malaria continues to be a major health concern in the tropics. Anopheles funestus Giles and An. gambiae Giles are known major vectors in these areas. However, An. fisnestus just like An. gambiae Giles is a member of a species complex comprising at least nine members, which differ in their distribution, behaviour and vectorial capacity. The present study was aimed at determining the genetic diversity of Anopheles funestus group populations from three ecological sites (sahel savanna, forest, coastal forest) in Ghana and the role the members of the group play in malaria transmission in these areas. Standard WHO procedures involving human landing and indoor resting catches were employed to collect mosquitoes. A total of 5496 An. funestus mosquitoes were collected from the study areas between April 2001 and December 2002. Out of these, Navrongo representing the sahel savanna area yielded 80.0%, followed by 9.2% and 10.8% for the forest and coastal areas respectively. A total of 144 man-nights yielded 1257 An. funestus mosquitoes with the sahel savanna producing 74.7% of all the collections. The degree of man-vector contact of An. funestus group estimated as man biting rates for the three areas were 13.0, 3.3 and 5.5 bites/man/night (B/MIN) for the sahel savanna, forest and coastal forest zones respectively. Moreover, 60% of the species were collected biting indoors than outdoors in all the three ecological areas. An. funestus sensi strictu (s.s.) constituted 50.4% and 38.7% of all the landing catches and indoor resting catches respectively in all the study area whilst An. lessoni made up 3.3% and 7.1% of the catches respectively. There were variations in the biting rates of An. fimestus group during the wet and dry seasons and these were statistically significant. Biting of An. funestus generally increased progressively through the night to peak during 5-6GMT at both indoor and outdoor stations with variations in the biting pattern in each of the three ecological sites. Sporozoite rate of 5.0% (3 1/623) for An. funestus in the sahel savanna area was significantly higher (P<0.05) than in the forest (4.1%, 13/318) and lowland (4.7%, 18/381) areas. All the sporozoite positive samples were An. funestus s.s. None of the An. lessoni was found infective. An. funestus s.s. in all the ecological areas was highly anthropophilic with human blood index (HBI) in the range 80-96%. An. lessoni had mostly fed on animal blood with animal blood index in the range of 83-95%. Annual entomological inoculation rates of An. funestus in the sahel savanna was 255.5 infective bites per man compared to 36.5 and 109.5 in the forest and coastal forest study sites respectively. A significant proportion, 92.7% of An. funestus s.s. were captured during the wet season. Malaria transmission was highest in the sahel savanna than the rest of the ecological zones with An. funestus s.s. being implicated as the vector in all the ecological zones. An. lessoni occurred in all three ecological areas, mostly during the wet season but played no role in malaria transmission, although a proportion of them were caught biting human. The study established the existence of An. lessoni in Ghana and emphasizes the important role of An. funestus s.s. in malaria transmission in all the three ecological areas.

Source: KNUST