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Open Access Research

Malaria transmission after five years of vector control on Bioko Island, Equatorial Guinea

Hans J Overgaard14*, Vamsi P Reddy2, Simon Abaga3, Abrahan Matias4, Michael R Reddy5, Vani Kulkarni2, Christopher Schwabe4, Luis Segura6, Immo Kleinschmidt7 and Michel A Slotman2

Author Affiliations

1 Department of Mathematical Sciences and Technology, Norwegian University of Life Sciences, Ås, Norway

2 Department of Entomology, Texas A&M University, College Station, TX, USA

3 National Malaria Control Program, Ministry of Health, Malabo, Equatorial Guinea

4 Medical Care Development International, Silver Spring, MD, USA

5 Department of Epidemiology and Public Health, Yale University, New Haven, CT, USA

6 Swiss Tropical and Public Health Institute, Basel, Switzerland

7 MRC Tropical Epidemiology Group, Department of Infectious Disease Epidemiology, London School of Hygiene and Tropical Medicine, London, UK

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Parasites & Vectors 2012, 5:253  doi:10.1186/1756-3305-5-253

Published: 12 November 2012

Abstract

Background

Malaria is endemic with year-round transmission on Bioko Island. The Bioko Island Malaria Control Project (BIMCP) started in 2004 with the aim to reduce malaria transmission and to ultimately eliminate malaria. While the project has been successful in reducing overall malaria morbidity and mortality, foci of high malaria transmission still persist on the island. Results from the 2009 entomological collections are reported here.

Methods

Human landing collections (HLC) and light trap collections (LTC) were carried out on Bioko Island, Equatorial Guinea in 2009. The HLCs were performed in three locations every second month and LTCs were carried out in 10 locations every second week. Molecular analyses were performed to identify species, detect sporozoites, and identify potential insecticide resistance alleles.

Results

The entomological inoculation rates (EIR) on Bioko Island ranged from 163 to 840, with the outdoor EIRs reaching > 900 infective mosquito bites per year. All three human landing collection sites on Bioko Island had an annual EIR exceeding the calculated African average of 121 infective bites per year. The highest recorded EIRs were in Punta Europa in northwestern Bioko Island with human biting rates of 92 and 66 mosquito landings per person per night, outdoors and indoors, respectively. Overall, the propensity for mosquito biting on the island was significantly higher outdoors than indoors (p < 0.001). Both Anopheles gambiae s.s. and An. melas were responsible for malaria transmission on the island, but with different geographical distribution patterns. Sporozoite rates were the highest in An. gambiae s.s. populations ranging from 3.1% in Punta Europa and 5.7% in Riaba in the southeast. Only the L1014F (kdr-west) insecticide resistance mutation was detected on the island with frequencies ranging from 22-88% in An. gambiae s.s. No insecticide resistance alleles were detected in the An. melas populations.

Conclusions

In spite of five years of extensive malaria control and a generalized reduction in the force of transmission, parasite prevalence and child mortality, foci of very high transmission persist on Bioko Island, particularly in the northwestern Punta Europa area. This area is favorable for anopheline mosquito breeding; human biting rates are high, and the EIRs are among the highest ever recorded. Both vector species collected in the study have a propensity to bite outdoors more frequently than indoors. Despite current vector control efforts mosquito densities remain high in such foci of high malaria transmission. To further reduce transmission, indoor residual spraying (IRS) needs to be supplemented with additional vector control interventions.

Keywords:
Entomological inoculation rate; Sporozoite; Human biting; Light traps; Anopheles