Characterization of malaria transmission by vector populations for improved interventions during the dry season in the Kpone-on-Sea area of coastal Ghana

BACKGROUND: Malaria is a major public health problem in Ghana. We present a site-specific entomological study of malaria vectors and transmission indices as part of an effort to develop a site for the testing of improved control strategies including possible vaccine trials. METHODS: Pyrethrum spray catches (PSC), and indoor and outdoor human landing collections of adult female anopheline mosquitoes were carried out over a six-month period (November 2005 - April 2006) at Kpone-on-Sea, a fishing village in southern Ghana. These were morphologically identified to species level and sibling species of the Anopheles gambiae complex further characterized by the polymerase chain reaction (PCR). Enzyme-linked immunosorbent assay was used to detect Plasmodium falciparum mosquito infectivity and host blood meal sources. Parity rate was examined based on dilatation of ovarian tracheoles following dissection. RESULTS: Of the 1233 Anopheles mosquitoes collected, An. gambiae s.l. was predominant (99.5%), followed by An. funestus (0.4%) and An. pharoensis (0.1%). All An. gambiae s.l. examined (480) were identified as An. gambiae s.s. with a majority of M molecular form (98.2%) and only 1.8% S form with no record of M/S hybrid. A significantly higher proportion of anophelines were observed outdoors relative to indoors (χ(2) = 159.34, df = 1, p < 0.0000). Only An. gambiae M molecular form contributed to transmission with a high degree of anthropophily, parity rate and an estimated entomological inoculation rate (EIR) of 62.1 infective bites/person/year. The Majority of the infective bites occurred outdoors after 09.00 pm reaching peaks between 12.00-01.00 am and 03.00-04.00 am. CONCLUSION: Anopheles gambiae M molecular form is responsible for maintaining the status quo of malaria in the surveyed site during the study period. The findings provide a baseline for evidence-based planning and implementation of improved malaria interventions. The plasticity observed in biting patterns especially the combined outdoor and early biting behavior of the vector may undermine the success of insecticide-based strategies using insecticide treated nets (ITN) and indoor residual spray (IRS). As such, novel or improved vector interventions should be informed by the local malaria epidemiology data as it relates to vector behavior.