Malaria vectors in Angola: distribution of species and molecular forms of the Anopheles gambiae complex, their pyrethroid insecticide knockdown resistance (kdr) status and Plasmodium falciparum sporozoite rates

Abstract

Background: Malaria is by far the greatest cause of morbidity and mortality in Angola, being responsible for 50% of all outpatient attendance and around 22% of all hospital deaths, yet by 2003 only 2% of under-5s used insecticide-treated nets. Entomological studies are an essential foundation for rational malaria control using insecticide-treated nets and indoor residual spraying, but there have been no published studies of malaria vectors in Angola over the 27 years of the civil war, to its end in 2002. This paper describes studies arising from a WHO-sponsored visit in support of the National Malaria Control Programme.

Methods: During April 2001, mosquitoes were sampled by indoor pyrethrum spray collection from four sites in the semi-arid coastal provinces of Luanda and Benguela and two sites in Huambo province, in the humid tropical highlands. Collections took place towards the end of the rainy season and were used to determine the Anopheles species present, their sporozoite rates and the frequency of a kdr allele conferring resistance to pyrethroid insecticides.

Results: A PCR test for the Anopheles gambiae complex showed a preponderance of An. gambiae, with indoor resting densities ranging from 0.9 to 23.5 per house. Of 403 An. gambiae identified to molecular form, 93.5% were M-form and 6.5% S-form. M and S were sympatric at 4 sites but no M/S hybrids were detected. The highest proportion of S-form (20%) was in samples from Huambo, in the humid highlands. Anopheles funestus was found at one site near Luanda. The sporozoite rate of mosquitoes, determined by an ELISA test, was 1.9% for An. gambiae (n = 580) and 0.7% for An. funestus (n = 140). Of 218 An. gambiae (195 M-form and 23 S-form) genotyped for the West African kdr-resistance allele, all were homozygous susceptible.

Conclusion: An. gambiae M-form is the most important and widespread malaria vector in the areas studied but more extensive studies of malaria vectors are required to support the malaria control programme in Angola. These should include standard insecticide resistance biossays and molecular assays that can detect both metabolic resistance and target site insensitivity.

Figure 1

Map showing the localities in Angola where anopheline mosquitoes were collected: Luanda (Samba and Cacuaco), Lobito (São Pedro), Benguela (Cawanga and Bela Vista) and Huambo (São Jose and Cazenga).