Unexpectedly high Plasmodium sporozoite rate associated with low human blood index in Anopheles coluzzii from a LLIN-protected village in Burkina Faso

Abstract

Despite the effectiveness of mass distribution of long-lasting insecticidal nets (LLINs) in reducing malaria transmission in Africa, in hyperendemic areas such as Burkina Faso the burden of malaria remains high. We here report the results of a 4-month survey on the feeding habits and Plasmodium infection in malaria vectors from a village in Burkina Faso one year following a national LLIN distribution programme. Low values of human blood index (HBI) observed in the major malaria vectors in the area (Anopheles coluzzii: N = 263, 20.1%; An. arabiensis: 5.8%, N = 103) are consistent with the hypothesis that LLINs reduced the availability of human hosts to mosquitoes. A regression meta-analysis of data from a systematic review of published studies reporting HBI and sporozoite rates (SR) for An. gambiae complex revealed that the observed SR values (An. coluzzii: 7.6%, N = 503; An. arabiensis: 5.3%, N = 225) are out of the ranges expected based on the low HBI observed. We hypothesize that a small fraction of inhabitants unprotected by bednets acts as a “core group” repeatedly exposed to mosquito bites, representing the major Plasmodium reservoir for the vectors, able to maintain a high risk of transmission even in a village protected by LLINs.

Behavioural plasticity of Anopheles coluzzii and Anopheles arabiensis undermines LLIN community protective effect in a Sudanese-savannah village in Burkina Faso

“…Nevertheless, results suggest that LLINs have elicited a "behavioural resilience" (sensu Govella et al, [102]) in An. coluzzii, leading to the biting rhythms observed in the present study [72,75,84], as well as to a higher zoophagy, as detected in 2011 [24]. Indeed, An.…”

“…gambiae s.l. in Goden (1.8<SR<12.1[43])and in the same geographical area before LLIN implementation (0.5<HBR<26.3[64,65]; 3<SR<10[64][65][66] in 6 villages in a radius of 40 km from Ouagadougou) and comparable to SR in Goden in 2011 (6.9%,[24]) and 2012 (6.6%,[25]). Notably, even though SRs were assessed by different approaches in different studies, results by recently developed rDNA-based TaqMan assays [33,67] do not significantly differ to those obtained by traditionally used CSP-ELISA[67].Data also shed some light on other entomological factors, which could have reduced LLINeffectiveness after 5-year implementation (i.e.…”

High Plasmodium infection intensity in naturally infected malaria vectors in Africa

“…In Burkina Faso there were on average over 10 oocysts per mosquito during the rainy season seven days after collection, which is substantially more than previous work reported (less than 5 oocysts/infected wild mosquito(24)). Estimates of sporozoite rates at day 0 (predicted using the mathematical model) were high, though were similar to another recent study from Burkina Faso(28). Observed oocyst loads were only slightly lower in Cameroon than in Burkina Faso, and consistent with a previous Cameroon study(26).…”

Evidence for natural hybridisation and novel Wolbachia strain superinfections in the Anopheles gambiae complex from Guinea

“…In this study, we screened for P. falciparum infection and found very low prevalence rates (<1%) (data not shown) preventing any statistical analysis on Wolbachia - Plasmodium interactions. This study and previous studies measuring a direct impact on Plasmodium infection in wild populations are dependent on parasite infection rates which can be low even in malaria-endemic areas [24] and particularly for the infective sporozoite stage [54]. Low pathogen prevalence rates are also limiting factors in assessing the effect of natural strains of Wolbachia on arboviruses in wild mosquito populations [55].…”