Host-specific cues cause differential attractiveness of Kenyan men to the African malaria vector Anopheles gambiae

BACKGROUND: Many studies have suggested that variability in the attractiveness of humans to host-seeking mosquitoes is caused by differences in the make-up of body emanations, and olfactory signals in particular. Most investigations have either been laboratory-based, utilising odour obtained from sections of the body, or have been done in the field with sampling methods that do not discriminate between visual, physical and chemical cues of the host. Accordingly, evidence for differential attractiveness based on body emanations remains sparse in spite of the far-reaching epidemiological implications of this phenomenon. METHODS: A new three-port olfactometer that accommodates complete human beings as sources of host-seeking stimuli was used to study behavioural responses of Anopheles gambiae Giles sensu stricto (hereafter An. gambiae) under semi-field conditions in western Kenya. Differential attractiveness of nine male Kenyans was assessed by simultaneously exposing the mosquitoes to (a mixture of) total body emanations of 3 people occupying separate tents. Controls (empty tents) were included and the effect of residual odours following tent occupation was also examined. RESULTS: Trap catches increased significantly (P < 0.001) when a tent was occupied. Based on 'competition' experiments, the nine persons were classified into least, medium and most attractive groups. There was no significant interaction between person and trap (P = 0.302) or person and test period (P = 0.223). Presence (P < 0.001) or absence (P = 0.949) of significant differences in the number of mosquitoes caught per trap when tents were simultaneously occupied by one person in each or left empty, respectively, demonstrated that residual odours following tent occupation did not affect behavioural responses of the mosquitoes. CONCLUSION: We provide evidence that in the vicinity of humans, when exposed to a blend of physical and olfactory signals from more than one host, An. gambiae can effectively and consistently express host-selection behaviour that results in non-random biting

An assessment of participatory integrated vector management for malaria control in Kenya

BACKGROUND : The World Health Organization (WHO) recommends integrated vector management (IVM) as a strategy to improve and sustain malaria vector control. However, this approach has not been widely adopted. OBJECTIVES : We comprehensively assessed experiences and findings on IVM in Kenya with a view to sharing lessons that might promote its wider application. METHODS : The assessment used information from a qualitative external evaluation of two malaria IVM projects implemented between 2006 and 2011 and an analysis of their accumulated entomological and malaria case data. The project sites were Malindi and Nyabondo, located in coastal and western Kenya, respectively. The assessment focused on implementation of five key elements of IVM: integration of vector control methods, evidence-based decision making, intersectoral collaboration, advocacy and social mobilization, and capacity building. RESULTS : IVM was more successfully implemented in Malindi than in Nyabondo owing to greater community participation and multistakeholder engagement. There was a significant decline in the proportion of malaria cases among children admitted to Malindi Hospital, from 23.7% in 2006 to 10.47% in 2011 (p < 0.001). However, the projects’ operational research methodology did not allow statistical attribution of the decline in malaria and malaria vectors to specific IVM interventions or other factors. CONCLUSIONS : Sustaining IVM is likely to require strong participation and support from multiple actors, including community-based groups, non-governmental organizations, international and national research institutes, and various government ministries. A cluster-randomized controlled trial would be essential to quantify the effectiveness and impact of specific IVM interventions, alone or in combination.Grants from the Biovision Foundation (Switzerland) through the International Centre of Insect Physiology and Ecology (ICIPE).http://ehp.niehs.nih.govam2016School of Health Systems and Public Health (SHSPH

Artisanal fishing supports breeding of malaria mosquitoes in Western Kenya

Abstract Background Everyday hundreds of people, mainly men, set out to take part in a vibrant artisanal capture fishing (ACF) industry on Lake Victoria. It is not known whether actions of artisanal fishers, in their unrelenting quest for existence, surpass ecosystems’ sustainability thresholds with potentially negative repercussions on human health with respect to malaria transmission potential. This article sought to fill this information gap. Methods This study used an ecosystem approach to find out how ACF processes facilitate the breeding of mosquitoes. The observational study adopted a cross-sectional design and was carried out on Mageta Island situated inside Lake Victoria in western Kenya. Results Of the 87 mosquito larval habitats identified 27 (31%) were created through ACF activities. The ACF-related habitats, hereafter collectively referred to as ‘fishing habitats’, included fishing boats (24), trenches (1) and fish bait mines (2). About half (48%) of Anopheles larvae were recovered from fishing habitats. The mean larval density in the fishing habitats (35.7 ± 1.15) was double that in non-fishing habitats (17.4 ± 0.539). Despite being the most common ‘non-fishing habitat’ type (N = 32), the mean number of Anopheles larvae present in rock pools (30.81 ± 10.54) was significantly less than those found inside fishing boats (N = 24; 40.08 ± 10.16). Overall, man-made habitats and those used to support livelihoods contained significantly more Anopheles larvae. Conclusions These data show that artisanal capture fishing is a key driver of malaria epidemiology on Mageta Island. This suggests that larval source management strategies in the global south should pay attention to the heterogeneity in Anopheles breeding habitats created through livelihood activities

Repellent Activities of Essential Oils of Some Plants Used Traditionally to Control the Brown Ear Tick, Rhipicephalus appendiculatus

Essential oils of eight plants, selected after an ethnobotanical survey conducted in Bukusu community in Bungoma County, western Kenya (Tagetes minuta, Tithonia diversifolia, Juniperus procera, Solanecio mannii, Senna didymobotrya, Lantana camara, Securidaca longepedunculata, and Hoslundia opposita), were initially screened (at two doses) for their repellence against brown ear tick, Rhipicephalus appendiculatus, using a dual-choice climbing assay. The oils of T. minuta and T. diversifolia were then selected for more detailed study. Dose-response evaluations of these oils showed that T. minuta oil was more repellent (RD50 = 0.0021 mg) than that of T. diversifolia (RD50 = 0.263 mg). Gas chromatography-linked mass spectrometric (GC-MS) analyses showed different compositions of the two oils. T. minuta oil is comprised mainly of cis-ocimene (43.78%), dihydrotagetone (16.71%), piperitenone (10.15%), trans-tagetone (8.67%), 3,9-epoxy-p-mentha-1,8(10)diene (6.47%), β-ocimene (3.25%), and cis-tagetone (1.95%), whereas T. diversifolia oil is comprised mainly of α-pinene (63.64%), β-pinene (15.00%), isocaryophyllene (7.62%), nerolidol (3.70%), 1-tridecanol (1.75%), limonene (1.52%), and sabinene (1.00%). The results provide scientific rationale for traditional use of raw products of these plants in controlling livestock ticks by the Bukusu community and lay down some groundwork for exploiting partially refined products such as essential oils of these plants in protecting cattle against infestations with R. appendiculatus

Essential oils of indigenous plants protect livestock from infestations of Rhipicephalus appendiculatus and other tick species in herds grazing in natural pastures in western Kenya

The effects of formulated essential oils of Tagetes minuta and Tithonia diversifolia on Rhipicephalus appendiculatus infesting livestock were evaluated in semi-field experiments. Forty-five zebu cattle naturally infested with ticks were randomly selected from 15 herds, three animals from each. Of the three animals within each herd, one was treated with 1 g of petroleum jelly (control), one with 1 g of essential oil of T. minuta and one with 1 g of essential oil of T. diversifolia on the inner side of ear pinna, the preferred feeding site of R. appendiculatus. Tick infestation on each treated host animal was monitored daily for 18 days by counting the number of ticks attached to the animals. Within 1–4 days post-treatment, the number of ticks on animals treated with essential oils was reduced by more than half the original population. By the 5th day post-treatment, more than 75 and 60% of R. appendiculatus and other tick species, respectively, became dislodged and dropped off. A stronger repellent effect was shown by the essential oil of T. minuta than the essential oil of T. diversifolia. Mean residual protection afforded by T. minuta was 12.5 days and for T. diversifolia 7.9 days. There was no significant difference in the effectiveness of essential oils between male and female R. appendiculatus. Both T. minuta and T. diversifolia essential oils affected several other less dominant but economically important tick species. Results suggest the potential for essential oil formulations in integrated pest management of ticks and associated tick-borne diseases among resource-limited livestock farmers in tropical Africa

Quantifying the intensity of permethrin insecticide resistance in Anopheles mosquitoes in western Kenya

Abstract Background The development and spread of resistance among local vectors to the major classes of insecticides used in Long-Lasting Insecticidal Nets (LLINs) and Indoor Residual Spraying (IRS) poses a major challenge to malaria vector control programs worldwide. The main methods of evaluating insecticide resistance in malaria vectors are the WHO tube bioassay and CDC bottle assays, with their weakness being determination of resistance at a fixed dose for variable populations. The CDC bottle assay using different insecticide dosages has proved applicable in ascertaining the intensity of resistance. Methods We determined the status and intensity of permethrin resistance and investigated the efficacy of commonly used LLINs (PermaNet® 2.0, PermaNet® 3.0 and Olyset®) against 3–5 day-old adult female Anopheles mosquitoes from four sub-counties; Teso, Bondo, Rachuonyo and Nyando in western Kenya. Knockdown was assessed to 4 doses of permethrin; 1× (21.5 μg/ml), 2× (43 μg/ml), 5× (107.5 μg/ml) and 10× (215 μg/ml) using CDC bottle assays. Results Mortality for 0.75% permethrin ranged from 23.5% to 96.1% in the WHO tube assay. Intensity of permethrin resistance was highest in Barkanyango Bondo, with 84% knockdown at the 30 min diagnostic time when exposed to the 10× dose. When exposed to the LLINs, mortality ranged between— 0–39% for Olyset®, 12–88% for PermaNet® 2.0 and 26–89% for PermaNet® 3.0. The efficacy of nets was reduced in Bondo and Teso. Results from this study show that there was confirmed resistance in all the sites; however, intensity assays were able to differentiate Bondo and Teso as the sites with the highest levels of resistance, which coincidentally were the two sub-counties with reduced net efficacy. Conclusions There was a reduced efficacy of nets in areas with high resistance portraying that at certain intensities of resistance, vector control using LLINs may be compromised. It is necessary to incorporate intensity assays in order to determine the extent of threat that resistance poses to malaria control

Field Evaluation of a Push-Pull System to Reduce Malaria Transmission

<div><p>Malaria continues to place a disease burden on millions of people throughout the tropics, especially in sub-Saharan Africa. Although efforts to control mosquito populations and reduce human-vector contact, such as long-lasting insecticidal nets and indoor residual spraying, have led to significant decreases in malaria incidence, further progress is now threatened by the widespread development of physiological and behavioural insecticide-resistance as well as changes in the composition of vector populations. A mosquito-directed push-pull system based on the simultaneous use of attractive and repellent volatiles offers a complementary tool to existing vector-control methods. In this study, the combination of a trap baited with a five-compound attractant and a strip of net-fabric impregnated with micro-encapsulated repellent and placed in the eaves of houses, was tested in a malaria-endemic village in western Kenya. Using the repellent delta-undecalactone, mosquito house entry was reduced by more than 50%, while the traps caught high numbers of outdoor flying mosquitoes. Model simulations predict that, assuming area-wide coverage, the addition of such a push-pull system to existing prevention efforts will result in up to 20-fold reductions in the entomological inoculation rate. Reductions of such magnitude are also predicted when mosquitoes exhibit a high resistance against insecticides. We conclude that a push-pull system based on non-toxic volatiles provides an important addition to existing strategies for malaria prevention.</p></div

Mean number of mosquitoes caught inside the houses.

<p>Error bars indicate standard error of the mean (SEM), n = 8 for the baseline data (n = 7 for house 3) and n = 25 for the intervention data. Asterisks indicate a significant difference-in-differences between the control and the intervention: * p < 0.05; ** p < 0.01; *** p < 0.001.</p

Scanning electron microscope (SEM) image of a cotton net fabric containing microcapsules.

<p>Scanning electron microscope (SEM) image of a cotton net fabric containing microcapsules.</p