In rural Burkina Faso, the primary malaria vector Anopheles gambiae sensu lato (s.l.) primarily feeds indoors at night. Identification of factors which influence mosquito house entry could lead to ...development of novel malaria vector control interventions. A study was therefore carried out to identify risk factors associated with house entry of An. gambiae s.l. in south-west Burkina Faso, an area of high insecticide resistance.
Mosquitoes were sampled monthly during the malaria transmission season using CDC light traps in 252 houses from 10 villages, each house sleeping at least one child aged five to 15 years old. Potential risk factors for house entry of An. gambiae s.l. were measured, including socio-economic status, caregiver's education and occupation, number of people sleeping in the same part of the house as the child, use of anti-mosquito measures, house construction and fittings, proximity of anopheline aquatic habitats and presence of animals near the house. Mosquito counts were compared using a generalized linear mixed-effect model with negative binomial and log link function, adjusting for repeated collections.
20,929 mosquitoes were caught, of which 16,270 (77.7%) were An. gambiae s.l. Of the 6691 An. gambiae s.l. identified to species, 4101 (61.3%) were An. gambiae sensu stricto and 2590 (38.7%) Anopheles coluzzii. Having a metal-roof on the child's sleeping space (IRR = 0.55, 95% CI 0.32-0.95, p = 0.03) was associated with fewer malaria vectors inside the home.
This study demonstrated that the rate of An. gambiae s.l. was 45% lower in sleeping spaces with a metal roof, compared to those with thatch roofs. Improvements in house construction, including installation of metal roofs, should be considered in endemic areas of Africa to reduce the burden of malaria.
In malaria-endemic countries, large proportions of individuals infected with Plasmodium falciparum are asymptomatic and constitute a reservoir of parasites for infection of newly hatched mosquitoes.
...Two studies were run in parallel in Burkina Faso to evaluate the impact of systematic identification and treatment of asymptomatic carriers of P. falciparum, detected by rapid diagnostic test, on disease transmission and susceptibility to clinical malaria episodes. A clinical study assessed the incidence of symptomatic malaria episodes with a parasite density >5,000/μL after three screening and treatment campaigns ~1 month apart before the rainy season; and an entomological study determined the effect of these campaigns on malaria transmission as measured by entomological inoculation rate.
The intervention arm had lower prevalence of asymptomatic carriers of asexual parasites and lower prevalence of gametocyte carriers during campaigns 2 and 3 as compared to the control arm. During the entire follow-up period, out of 13,767 at-risk subjects, 2,516 subjects (intervention arm 1,332; control arm 1,184) had symptomatic malaria. Kaplan-Meier analysis of the incidence of first symptomatic malaria episode with a parasite density >5,000/μL showed that, in the total population, the two treatment arms were similar until Week 11-12 after campaign 3, corresponding with the beginning of the malaria transmission season, after which the probability of being free of symptomatic malaria was lower in the intervention arm (logrank p < 0.0001). Similar trends were observed in infants and children <5 years and in individuals ≥5 years of age. In infants and children <5 years old who experienced symptomatic malaria episodes, the geometric mean P. falciparum density was lower in the intervention arm than the control arm. This trend was not seen in those individuals aged ≥5 years. Over the year, monthly variation in mosquito density and entomological inoculation rate was comparable in both arms, with September peaks in both indices.
Community screening and targeted treatment of asymptomatic carriers of P. falciparum had no effect on the dynamics of malaria transmission, but seemed to be associated with an increase in the treated community's susceptibility to symptomatic malaria episodes after the screening campaigns had finished. These results highlight the importance of further exploratory studies to better understand the dynamics of disease transmission in the context of malaria elimination.
The M and S molecular forms of Anopheles gambiae are undergoing speciation as they adapt to heterogeneities in the environment, spreading malaria in the process. We hypothesized that their divergence ...despite gene flow is facilitated by reduced recombination at the centromeric (proximal) end of the X chromosome. We sequenced introns from 22 X chromosome genes in M and S from two locations of West Africa where the forms are sympatric. Generally, in both forms nucleotide diversity was high distally, lower proximally, and very low nearest the centromere. Conversely, differentiation between the forms was virtually zero distally and very high proximally. Pairwise comparisons to a close relative, the sibling species Anopheles arabiensis, demonstrated uniformly high divergence regardless of position along the X chromosome, suggesting that this pattern is not purely mechanical. Instead, the pattern observed for M and S suggests the action of divergent natural selection countering gene flow only at the proximal end of the X chromosome, where recombination is reduced. Comparison of sites with fixed differences between M and S to the corresponding sites in A. arabiensis revealed that derived substitutions had been fixed in both forms, further supporting the hypothesis that both have been under selection. These derived substitutions are fixed in the two West African samples and in samples of S from western and coastal Kenya, suggesting that selection occurred before the forms expanded to their current ranges. Our findings are consistent with a role for suppressed genetic recombination in speciation of A. gambiae.
Summary
While symbiosis between bacteria and insects has been thoroughly investigated in the last two decades, investments on the study of yeasts associated with insects have been limited. ...Insect‐associated yeasts are placed on different branches of the phylogenetic tree of fungi, indicating that these associations evolved independently on several occasions. Isolation of yeasts is frequently reported from insect habitats, and in some cases yeasts have been detected in the insect gut and in other organs/tissues. Here we show that the yeast Wickerhamomyces anomalus, previously known as Pichia anomala, is stably associated with the mosquito Anopheles stephensi, a main vector of malaria in Asia. Wickerhamomyces anomalus colonized pre‐adult stages (larvae L1–L4 and pupae) and adults of different sex and age and could be isolated in pure culture. By a combination of transmission electron microscopy and fluorescent in situ hybridization techniques, W. anomalus was shown to localize in the midgut and in both the male and female reproductive systems, suggesting multiple transmission patterns.
Abstract
To determine the relationship between malaria transmission intensity, clinical malaria, immune response, plasmodic index, and to furthermore characterize a malaria vaccine trial site for ...potential malaria vaccines candidate testing, a study was conducted in Tensobtenga and Balonguen, two villages in Burkina Faso characterized by different malaria transmission levels. The study villages are located in a Sudan savanna area. Malaria transmission is seasonal and peaks in September in these villages. Tensobtenga and Balonguen are comparables in all aspects, except the presence of an artificial lake and wetlands in Tensobtenga. The mosquitoes sampling sites were randomly selected, taking into consideration the number of potential breeding sites, and the number of households in each village. Three times a week during 12 mo mosquitoes were collected by the Center for Disease Control and Prevention light traps in sentinel sites. To assess the infectivity the mosquitoes double ELISAs tests were performed on thoraces of female Anopheles gambiae s.l. (Giles) and Anopheles funestus. A total of 54,392 female Anopheles, representing 92.71% of the total mosquitoes, were collected. The peaks of aggressiveness because of either An. gambiae s.l. or An. funestus were observed in September in each of the villages. However, these peaks were lower in Balonguen compared with Tensobtenga. Malaria cumulative aggressiveness and transmission intensity because of both species peaked in September in each of the two villages, with lower values in Balonguen in comparison to Tensobtenga. From February to May, malaria transmission intensity is negligible in Balonguen and <1 bite/person/mo is observed in Tensobtenga. These results have confirmed the marked seasonality of malaria transmission in the study area.
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