Serological markers of sand fly exposure to evaluate insecticidal nets against visceral leishmaniasis in India and Nepal: a cluster-randomized trial

Abstract

Background: Visceral leishmaniasis is the world' second largest vector-borne parasitic killer and a neglected tropical disease, prevalent in poor communities. Long-lasting insecticidal nets (LNs) are a low cost proven vector intervention method for malaria control; however, their effectiveness against visceral leishmaniasis (VL) is unknown. This study quantified the effect of LNs on exposure to the sand fly vector of VL in India and Nepal during a two year community intervention trial.

Methods: As part of a paired-cluster randomized controlled clinical trial in VL-endemic regions of India and Nepal we tested the effect of LNs on sand fly biting by measuring the antibody response of subjects to the saliva of Leishmania donovani vector Phlebotomus argentipes and the sympatric (non-vector) Phlebotomus papatasi. Fifteen to 20 individuals above 15 years of age from 26 VL endemic clusters were asked to provide a blood sample at baseline, 12 and 24 months post-intervention.

Results: A total of 305 individuals were included in the study, 68 participants provided two blood samples and 237 gave three samples. A random effect linear regression model showed that cluster-wide distribution of LNs reduced exposure to P. argentipes by 12% at 12 months (effect 0.88; 95% CI 0.83-0.94) and 9% at 24 months (effect 0.91; 95% CI 0.80-1.02) in the intervention group compared to control adjusting for baseline values and pair. Similar results were obtained for P. papatasi.

Conclusions: This trial provides evidence that LNs have a limited effect on sand fly exposure in VL endemic communities in India and Nepal and supports the use of sand fly saliva antibodies as a marker to evaluate vector control interventions.

Figure 1. Study population flow chart.

Number of individuals initially enrolled in the study and number of subjects excluded or lost to follow-up (no blood samples available) per study (intervention and control) group.

Figure 2. Effect of LNs on human exposure to P. argentipes and P. papatasi sand flies.

Individual ELISA Optical Density (OD) per immunological survey (baseline, 12 and 24 months follow-up) for intervention (long-lasting insecticidal nets, LN – black triangles) and control clusters (grey circles), for Phlebotomus argentipes (Panel A) and P. papatasi (Panel B). The geometric means ELISA OD are represented as a solid line for LN and dotted line for control groups. Results represent all the samples available (n = 305). The Mann Whitney t-test was used to compare 12 and 24 month follow-up samples compared to their corresponding baseline values, asterisks denote statistical significance (*, P<0.05; **, P<0.005; *, P<0.005; ns, not significant P>0.05).

Figure 3. Effect of LNs on sand fly exposure, adjusted for non-endemic controls.

Individual ELISA Optical Density (OD) per immunological survey (baseline, 12 and 24 months follow-up) for intervention (long-lasting insecticidal net, LN – black triangles) and control clusters (grey circles), for Phlebotomus argentipes (Panel A) and P. papatasi (Panel B). Individuals with no ELISA results or below the average non-endemic control OD+2×S.D. cut-off values (0.9 for P. argentipes and 1.8 for P. papatasi) at baseline were excluded. The geometric means ELISA OD are represented as a solid line for LN and dotted line for control groups. The Mann Whitney t-test was used to compare 12 and 24 month follow-up samples compared to their corresponding baseline values, asterisks denote statistical significance (*, P<0.05; **, P<0.005; *, P<0.005; ns, not significant P>0.05).