Malaria infection and disease in an area with pyrethroid-resistant vectors in southern Benin

Background: This study aimed to investigate baseline data on malaria before the evaluation of new vector control strategies in an area of pyrethroid-resistance of vectors. The burden of malaria was estimated in terms of infection (prevalence and parasite density) and of clinical episodes. Methods: Between December 2007 and December 2008 in the health district of Ouidah - Kpomasse - Tori Bossito (southern Benin), a descriptive epidemiological survey of malaria was conducted. From 28 selected villages, seven were randomized from which a total of 440 children aged 0 to 5 years were randomly selected. Clinical and parasitological information was obtained by active case detection of malaria episodes carried out during eight periods of six consecutive days scheduled at six weekly intervals and by cross-sectional surveys of asymptomatic infection. Entomological information was also collected. The ownership, the use and the correct use of long-lasting insecticide-treated nets (LLINs) were checked over weekly-survey by unannounced visits at home in the late evening. Results: Mean parasite density in asymptomatic children was 586 P. falciparum asexual forms per mu L of blood (95% CI 504-680). Pyrogenic parasite cut-off was estimated 2,000 P. falciparum asexual blood forms per mu L. The clinical incidence of malaria was 1.5 episodes per child per year (95% CI 1.2-1.9). Parasitological and clinical variables did not vary with season. Anopheles gambiae s. l. was the principal vector closely followed by Anopheles funestus. Entomological inoculation rate was 5.3 (95% CI 1.1-25.9) infective bites per human per year. Frequency of the L1014F kdr (West) allele was around 50%. Annual prevalence rate of Plasmodium falciparum asymptomatic infection was 21.8% (95% CI 19.1-24.4) and increased according to age. Mean rates of ownership and use of LLINs were 92% and 70% respectively. The only correct use of LLINs (63%) conferred 26% individual protection against only infection (OR = 0.74 (95% IC 0.62-0.87), p = 0.005). Conclusion: The health district of Ouidah-Kpomasse-Tori Bossito is a mesoendemic area with a moderate level of pyrethroid-resistance of vectors. The used LLINs rate was high and only the correct use of LLINs was found to reduce malaria infection without influencing malaria morbidity