A pre-intervention study of malaria vector abundance in Rio Muni, Equatorial Guinea: Their role in malaria transmission and the incidence of insecticide resistance alleles

Frances C Ridl¹ , Chris Bass² , Miguel Torrez³ , Dayanandan Govender¹ , Varsha Ramdeen¹ , Lee Yellot³ , Amado Edjang Edu⁴ , Christopher Schwabe⁵ , Peter Mohloai⁶ , Rajendra Maharaj¹ and Immo Kleinschmidt⁷

¹Malaria Research Lead Programme, Medical Research Council, 491 Ridge Road, Durban, South Africa

²Department of Biological Chemistry, Rothamsted Research, Harpenden, AL5 2JQ, UK

³Equatorial Guinea Malaria Control Initiative, Apdo # 606, Bata, Equatorial Guinea

⁴C/O.U.A., Zona Sanitaria s/n, Bata-Litoral, Equatorial Guinea

⁵Medical Care Development International, 8401 Colesville Rd, Silver Spring, Maryland, 20910, USA

⁶One World Development Group International, Punta Europa, Carretera Aeropuerto, Malabo, Bioco Norte, Equatorial Guinea

⁷London School of Hygiene and Tropical Medicine, Keppel St, London, WC1E 7HT, UK

author email corresponding author email

Malaria Journal 2008, 7:194doi:10.1186/1475-2875-7-194


Published:	29 September 2008

Abstract

Background

Following the success of the malaria control intervention on the island of Bioko, malaria control by the use of indoor residual spraying (IRS) and long-lasting insecticide-treated nets (LLITN) was extended to Rio Muni, on the mainland part of Equatorial Guinea. This manuscript reports on the malaria vectors present and the incidence of insecticide resistant alleles prior to the onset of the programme.

Methods

Anopheles mosquitoes were captured daily using window traps at 30 sentinel sites in Rio Muni, from December 2006 to July 2007. The mosquitoes were identified to species and their sporozoite rates, knockdown resistance (kdr) and acetylcholinesterase (AChE) sensitivity measured, to define the role of vector species in malaria transmission and their potential susceptibility to insecticides.

Results

A total of 6,162 Anopheles mosquitoes were collected of which 4,808 were morphologically identified as Anopheles gambiae s.l., 120 Anopheles funestus, 1,069 Anopheles moucheti, and 165 Anopheles nili s.l.. Both M and S molecular forms of Anopheles gambiae s.s. and Anopheles melas were identified. Anopheles ovengensis and Anopheles carnevalei were the only two members of the An. nili group to be identified. Using the species-specific sporozoite rates and the average number of mosquitoes per night, the number of infective mosquitoes per trap per 100 nights for each species complex was calculated as a measure of transmission risk. Both kdr-w and kdr-e alleles were present in the S-form of An. gambiae s.s. (59% and 19% respectively) and at much lower frequencies in the M-form (9.7% and 1.8% respectively). The kdr-w and kdr-e alleles co-occurred in 103 S-form and 1 M-form specimens. No insensitive AChE was detected.

Conclusion

Anopheles gambiae s.s, a member of the Anopheles gambiae complex was shown to be the major vector in Rio Muni with the other three groups playing a relatively minor role in transmission. The demonstration of a high frequency of kdr alleles in mosquito populations before the onset of a malaria control programme shows that continuous entomological surveillance including resistance monitoring will be of critical importance to ensure the chosen insecticide remains effective.