Human antibody response to Anopheles gambiae saliva: an immuno-epidemiological biomarker to evaluate the efficacy of insecticide-treated nets in malaria vector control

Abstract

For the fight against malaria, the World Health Organization (WHO) has emphasized the need for indicators to evaluate the efficacy of vector-control strategies. This study investigates a potential immunological marker, based on human antibody responses to Anopheles saliva, as a new indicator to evaluate the efficacy of insecticide-treated nets (ITNs). Parasitological, entomological, and immunological assessments were carried out in children and adults from a malaria-endemic region of Angola before and after the introduction of ITNs. Immunoglobulin G (IgG) levels to An. gambiae saliva were positively associated with the intensity of An. gambiae exposure and malaria infection. A significant decrease in the anti-saliva IgG response was observed after the introduction of ITNs, and this was associated with a drop in parasite load. This study represents the first stage in the development of a new indicator to evaluate the efficacy of malaria vector-control strategies, which could apply in other arthropod vector-borne diseases.

Figure 1.

Evolution of An. gambiae exposure according to the studied period. Exposure to An. gambiae s.s. is presented as the mean of number of An. gambiae collected by trap and by night in six controlled households for each visit in 2005 and 2006. The arrow indicates the installation of ITNs in February 2006.

Figure 2.

Prevalence and intensity of P. falciparum infection before and after the introduction of ITNs. The prevalence (A), arithmetic (B), and geometric (C) mean of infection intensity (parasitaemia) are presented in the whole population (N = 230; solid line) and the “immunological” subsample (N = 109; dotted line), which is described in Materials and Methods. The arrow indicates the installation of ITNs in February 2006.

Figure 3.

IgG antibody response to An. gambiae saliva before and after ITNs use. The median anti-saliva IgG level (solid line) and the intensity of parasite infection (the geometric mean of parasite densities is represented by the dotted line) are presented in the “immunological” subsample (N = 109) before (2005) and after (2006) the installation of ITNs. The arrow indicates the installation of ITNs in February 2006.

Figure 4.

Individual IgG antibody levels to An. gambiae saliva according the season and the use of ITNs. Individual anti-saliva IgG levels for the “immunological” subpopulation (N = 109) before (2005) and after (2006) the installation of ITNs. The arrow indicates the installation of ITNs in February 2006. Bars indicate the median value for each studied month.