Studies of Anopheles gambiae s.l (Diptera: Culicidae) exhibiting different vectorial capacities in lymphatic filariasis transmission in the Gomoa district, Ghana

Abstract

BackgroundTwo lymphatic filariasis endemic communities Mampong and Hwida in Ghana have been regularly monitored for impact on transmission after annual mass drug administration (MDA) with albendazole and ivermectin. After six MDAs even though the ABR for Mampong was 55883/person/year and that of Hwida was 2494/person/year, they both had ATPs of 15.21 infective larvae/person/year. Interestingly the human microfilaraemia levels had reduced significantly from 14% to 0% at Mampong and 12% to 3% at Hwida. In an attempt to understand this anomaly, we collected mosquitoes over a 5-month period using human landing catches to determine the species composition, the number of cibarial teeth, the lengths and widths of the cibarium and the cibarial dome of the vector populations.ResultsOut of 2553 mosquitoes caught at Mampong, 42.6% were An. gambiae s.l. All 280 identified further by PCR were An. gambiae s.s (275 M and 5 S molecular forms). At Hwida, 112 mosquitoes were obtained; 67 (59.8%) were An. gambiae s.l, comprised of 40 (59.7%) An. melas, 24 (35.8%) An. gambiae s.s (17 and 5 M and S molecular forms respectively) and 3 (4.5%) unidentified. The mean number of teeth for An. melas was 14.1 (median = 14, range = 12-15), An. gambiae s.s., 15.7 (median = 15, range = 13-19) M form 15.5 (median = 15 range = 13-19) and S form 16 (median = 16, range 15-17). The observed differences in teeth numbers were significantly different between An. melas and An. gambiae s.s (p = 0.004), and the M form (p = 0.032) and the S form (p = 0.002).ConclusionsIn this study, An. gambiae s.s was the main vector at Mampong and was found to possess significantly more cibarial teeth than An. melas, the principal vector at Hwida. We postulate that the different impact observed after 6 MDAs may be due to An. gambiae s.s exhibiting 'facilitation' at Mampong and at Hwida An. melas the main vector exhibits 'limitation'. Thus it may be necessary to compliment MDA with vector control to achieve interruption of transmission in areas where An. melas may exhibit limitation.

Dynamics of antigenemia and transmission intensity of Wuchereria bancrofti following cessation of mass drug administration in a formerly highly endemic region of Mali

“…Because limitation of An. gambiae ( sensu stricto ) has been observed in Ghana [31], it should also be assessed in other geographic locations (e.g. Mali) given the possibility of adaptation or specific mutation that can modify mosquito’s transmission pattern [32].…”

Diversity and transmission competence in lymphatic filariasis vectors in West Africa, and the implications for accelerated elimination of Anopheles-transmitted filariasis

“…In contrast, in the Polynesian Islands of Moorea and Maupiti, where the vector was Aedes polynesiensis, over 50 years of MDA using DEC did not eliminate LF [17]. Despite these evidences and the assumption of vector-parasite phenomenon of Facilitation in Anopheles , there is current growing evidence that in certain areas, Anopheles species may be exhibiting the process of Limitation [26,27]. …”

“…melas compared to the M and S forms of the An. gambiae s.s [27]. As such, it is very clear from the above that anopheline LF vectors in West Africa may differ in their capacity to sustain low-level microfilaraemia.…”

Mosquito-Parasite Interactions Can Shape Filariasis Transmission Dynamics and Impact Elimination Programs

“…Other studies have also found sympatric species of the An. gambiae complex and M and S molecular forms [34], [35] to have different competency to transmit W. bancrofti . Further research on the vector competence of primary LF vectors around the world, in the context of a diminishing mf reservoir, is needed in order to maximize the success of the Global Programme to Eliminate Lymphatic Filariasis.…”