Entomological indices of malaria transmission in Chikhwawa district, Southern Malawi

Themba Mzilahowa,Philip J Mccall,Ian M Hastings,Malcolm E Molyneux

doi:10.1186/1475-2875-11-380

Abstract

BackgroundAlthough malaria is highly prevalent throughout Malawi, little is known of its transmission dynamics. This paper describes the seasonal activity of the different vectors, human biting indices, sporozoite rates and the entomological inoculation rate in a low-lying rural area in southern Malawi.MethodsVectors were sampled over 52 weeks from January 2002 to January 2003, by pyrethrum knockdown catch in two villages in Chikhwawa district, in the Lower Shire Valley.ResultsIn total, 7,717 anophelines were collected of which 55.1% were Anopheles gambiae sensu lato and 44.9% were Anopheles funestus. Three members of the An. gambiae complex were identified by PCR: Anopheles arabiensis (75%) was abundant throughout the year, An. gambiae s.s. (25%) was most common during the wet season and Anopheles quadriannulatus occurred at a very low frequency (n=16). An. funestus was found in all samples but was most common during the dry season.Anopheles gambiae s.s. and An. funestus were highly anthropophilic with human blood indices of 99.2% and 96.3%, respectively. Anopheles arabiensis had fed predominantly on humans (85.0%) and less commonly on cattle (10.9%; 1.2% of blood meals were of mixed origin). Plasmodium falciparum (192/3,984) and Plasmodium malariae (1/3,984) sporozoites were detected by PCR in An. arabiensis (3.2%) and An. funestus (4.5%), and in a significantly higher proportion of An. gambiae s.s. (10.6%)(p<0.01). All three vectors were present throughout the year and malaria transmission occurred in every month, although with greatest intensity during the rainy season (January to April). The combined human blood index exceeded 92% and the P. falciparum sporozoite rate was 4.8%, resulting in estimated inoculation rates of 183 infective bites/ person per annum, or an average rate of ~15 infective bites/person/month.ConclusionsThe results demonstrate the importance of An. gambiae s.s., An. arabiensis and An. funestus in driving the high levels of malaria transmission in the south of Malawi. Sustained and high coverage or roll out of current approaches to malaria control (primarily insecticide-treated bed nets and indoor residual house spraying) in the area are likely to reduce the observed high malaria transmission rate and consequently the incidence of human infections, unless impeded by increasing resistance of vectors to insecticides.

Highlights

Malaria is highly prevalent throughout Malawi, little is known of its transmission dynamics
While there has been substantial research in Malawi on the pathology and chemotherapy of malaria, little is known of malaria transmission dynamics and of the vectorial roles of various anopheline species known to occur in Malawi
In 1992, Hawley et al [6] surveyed districts in the southern (Nsanje and Mangochi) and central (Dowa) regions of Malawi over a four month period in dry and wet seasons. They confirmed the presence of An. funestus and A. gambiae s.l. and, for the first time, identified both An. gambiae sensu stricto (s.s.) and Anopheles arabiensis in all three sites

Summary

Introduction

Malaria is highly prevalent throughout Malawi, little is known of its transmission dynamics. Studies on malaria transmission in Malawi began in 1911, when two vectors were identified: An. funestus which was the most common, and An. gambiae s.l. In 1992, Hawley et al [6] surveyed districts in the southern (Nsanje and Mangochi) and central (Dowa) regions of Malawi over a four month period in dry and wet seasons. They confirmed the presence of An. funestus and A. gambiae s.l. and, for the first time, identified both An. gambiae sensu stricto (s.s.) and Anopheles arabiensis in all three sites. The absence of more comprehensive transmission data for Malawi remains an obvious knowledge gap [10], in an era when reducing transmission is increasingly recognized as an important component of malaria control and a necessary step toward eventual elimination of the infection [11]

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