Efficacy of Aquatain, a Monomolecular Film, for the Control of Malaria Vectors in Rice Paddies

Background Rice paddies harbour a large variety of organisms including larvae of malaria mosquitoes. These paddies are challenging for mosquito control because their large size, slurry and vegetation make it difficult to effectively apply a control agent. Aquatain, a monomolecular surface film, can be considered a suitable mosquito control agent for such breeding habitats due to its physical properties. The properties allow Aquatain to self-spread over a water surface and affect multiple stages of the mosquito life cycle. Methodology/Principal Findings A trial based on a pre-test/post-test control group design evaluated the potential of Aquatain as a mosquito control agent at Ahero rice irrigation scheme in Kenya. After Aquatain application at a dose of 2 ml/m2 on rice paddies, early stage anopheline larvae were reduced by 36%, and late stage anopheline larvae by 16%. However, even at a lower dose of 1 ml/m2 there was a 93.2% reduction in emergence of anopheline adults and 69.5% reduction in emergence of culicine adults. No pupation was observed in treated buckets that were part of a field bio-assay carried out parallel to the trial. Aquatain application saved nearly 1.7 L of water in six days from a water surface of 0.2 m2 under field conditions. Aquatain had no negative effect on rice plants as well as on a variety of non-target organisms, except backswimmers. Conclusions/Significance We demonstrated that Aquatain is an effective agent for the control of anopheline and culicine mosquitoes in irrigated rice paddies. The agent reduced densities of aquatic larval stages and, more importantly, strongly impacted the emergence of adult mosquitoes. Aquatain also reduced water loss due to evaporation. No negative impacts were found on either abundance of non-target organisms, or growth and development of rice plants. Aquatain, therefore, appears a suitable mosquito control tool for use in rice agro-ecosystems

Factors affecting fungus-induced larval mortality in Anopheles gambiae and Anopheles stephensi

<p>Abstract</p> <p>Background</p> <p>Entomopathogenic fungi have shown great potential for the control of adult malaria vectors. However, their ability to control aquatic stages of anopheline vectors remains largely unexplored. Therefore, how larval characteristics (<it>Anopheles </it>species, age and larval density), fungus (species and concentration) and environmental effects (exposure duration and food availability) influence larval mortality caused by fungus, was studied.</p> <p>Methods</p> <p>Laboratory bioassays were performed on the larval stages of <it>Anopheles gambiae </it>and <it>Anopheles stephensi </it>with spores of two fungus species, <it>Metarhizium anisopliae </it>and <it>Beauveria bassiana</it>. For various larval and fungal characteristics and environmental effects the time to death was determined and survival curves established. These curves were compared by Kaplan Meier and Cox regression analyses.</p> <p>Results</p> <p><it>Beauveria bassiana </it>and <it>Metarhizium anisopliae </it>caused high mortality of <it>An. gambiae </it>and <it>An. stephensi </it>larvae. However, <it>Beauveria bassiana </it>was less effective (Hazard ratio (HR) <1) compared to <it>Metarhizium anisopliae. Anopheles stephensi </it>and <it>An. gambiae </it>were equally susceptible to each fungus. Older larvae were less likely to die than young larvae (HR < 1). The effect of increase in fungus concentration on larval mortality was influenced by spore clumping. One day exposure to fungal spores was found to be equally effective as seven days exposure. In different exposure time treatments 0 - 4.9% of the total larvae, exposed to fungus, showed infection at either the pupal or adult stage. Mortality rate increased with increasing larval density and amount of available food.</p> <p>Conclusions</p> <p>This study shows that both fungus species have potential to kill mosquitoes in the larval stage, and that mortality rate depends on fungus species itself, larval stage targeted, larval density and amount of nutrients available to the larvae. Increasing the concentration of fungal spores or reducing the exposure time to spores did not show a proportional increase and decrease in mortality rate, respectively, because the spores clumped together. As a result spores did not provide uniform coverage over space and time. It is, therefore, necessary to develop a formulation that allows the spores to spread over the water surface. Apart from formulation appropriate delivery methods are also necessary to avoid exposing non-target organisms to fungus.</p

Horizontal transmission of the symbiont Microsporidia MB in Anopheles arabiensis

&lt;jats:p&gt;The recently discovered &lt;jats:italic&gt;Anopheles&lt;/jats:italic&gt; symbiont, &lt;jats:italic&gt;Microsporidia MB&lt;/jats:italic&gt;, has a strong malaria transmission-blocking phenotype in &lt;jats:italic&gt;Anopheles arabiensis&lt;/jats:italic&gt;, the predominant &lt;jats:italic&gt;Anopheles gambiae&lt;/jats:italic&gt; species complex member in many active transmission areas in eastern Africa. The ability of &lt;jats:italic&gt;Microsporidia MB&lt;/jats:italic&gt; to block &lt;jats:italic&gt;Plasmodium&lt;/jats:italic&gt; transmission together with vertical transmission and avirulence makes it a candidate for the development of a symbiont-based malaria transmission blocking strategy. We investigate the characteristics and efficiencies of &lt;jats:italic&gt;Microsporidia MB&lt;/jats:italic&gt; transmission between &lt;jats:italic&gt;An. arabiensis&lt;/jats:italic&gt; mosquitoes. We show that &lt;jats:italic&gt;Microsporidia MB&lt;/jats:italic&gt; is not transmitted between larvae but is effectively transmitted horizontally between adult mosquitoes. Notably, &lt;jats:italic&gt;Microsporidia MB&lt;/jats:italic&gt; was only found to be transmitted between male and female &lt;jats:italic&gt;An. arabiensis&lt;/jats:italic&gt;, suggesting sexual horizontal transmission. In addition, &lt;jats:italic&gt;Microsporidia MB&lt;/jats:italic&gt; cells were observed infecting the &lt;jats:italic&gt;An. arabiensis&lt;/jats:italic&gt; ejaculatory duct. Female &lt;jats:italic&gt;An. arabiensis&lt;/jats:italic&gt; that acquire &lt;jats:italic&gt;Microsporidia MB&lt;/jats:italic&gt; horizontally are able to transmit the symbiont vertically to their offspring. We also investigate the possibility that &lt;jats:italic&gt;Microsporidia MB&lt;/jats:italic&gt; can infect alternate hosts that live in the same habitats as their &lt;jats:italic&gt;An. arabiensis&lt;/jats:italic&gt; hosts, but find no other non-anopheline hosts. Notably, &lt;jats:italic&gt;Microsporidia MB&lt;/jats:italic&gt; infections were found in another primary malaria African vector, &lt;jats:italic&gt;Anopheles funestus s.s&lt;/jats:italic&gt;. The finding that &lt;jats:italic&gt;Microsporidia MB&lt;/jats:italic&gt; can be transmitted horizontally is relevant for the development of dissemination strategies to control malaria that are based on the targeted release of &lt;jats:italic&gt;Microsporidia MB&lt;/jats:italic&gt; infected &lt;jats:italic&gt;Anopheles&lt;/jats:italic&gt; mosquitoes.&lt;/jats:p&gt

Development of Metarhizium anisopliae and Beauveria bassiana formulations for control of malaria mosquito larvae

<p>Abstract</p> <p>Background</p> <p>The entomopathogenic fungi <it>Metarhizium anisopliae </it>and <it>Beauveria bassiana </it>have demonstrated effectiveness against anopheline larvae in the laboratory. However, utilising these fungi for the control of anopheline larvae under field conditions, relies on development of effective means of application as well as reducing their sensitivity to UV radiation, high temperatures and the inevitable contact with water. This study was conducted to develop formulations that facilitate the application of <it>Metarhizium anisopliae </it>and <it>Beauveria bassiana </it>spores for the control of anopheline larvae, and also improve their persistence under field conditions.</p> <p>Methods</p> <p>Laboratory bioassays were conducted to test the ability of aqueous (0.1% Tween 80), dry (organic and inorganic) and oil (mineral and synthetic) formulations to facilitate the spread of fungal spores over the water surface and improve the efficacy of formulated spores against anopheline larvae as well as improve spore survival after application. Field bioassays were then carried out to test the efficacy of the most promising formulation under field conditions in western Kenya.</p> <p>Results</p> <p>When formulated in a synthetic oil (ShellSol T), fungal spores of both <it>Metarhizium anisopliae </it>and <it>Beauveria bassiana </it>were easy to mix and apply to the water surface. This formulation was more effective against anopheline larvae than 0.1% Tween 80, dry powders or mineral oil formulations. ShellSol T also improved the persistence of fungal spores after application to the water. Under field conditions in Kenya, the percentage pupation of <it>An. gambiae </it>was significantly reduced by 39 - 50% by the ShellSol T-formulated <it>Metarhizium anisopliae </it>and <it>Beauveria bassiana </it>spores as compared to the effects of the application of unformulated spores.</p> <p>Conclusions</p> <p>ShellSol T is an effective carrier for fungal spores when targeting anopheline larvae under both laboratory and field conditions. Entomopathogenic fungi formulated with a suitable carrier are a promising tool for control of larval populations of malaria mosquitoes. Additional studies are required to identify the best delivery method (where, when and how) to make use of the entomopathogenic potential of these fungi against anopheline larvae.</p

Microsporidia: a promising vector control tool for residual malaria transmission

Long-lasting insecticidal nets (LLINs) and indoor residual spraying (IRS) have resulted in a major decrease in malaria transmission. However, it has become apparent that malaria can be effectively transmitted despite high coverage of LLINs/IRS. Residual transmission can occur due to Plasmodium-carrying Anopheles mosquitoes that are insecticide resistant and have feeding and resting behavior that reduces their chance of encountering the currently deployed indoor malaria control tools. Residual malaria transmission is likely to be the most significant hurdle to achieving the goal of malaria eradication and research and development towards new tools and strategies that can control residual malaria transmission is therefore critical. One of the most promising strategies involves biological agents that are part of the mosquito microbiome and influence the ability of Anopheles to transmit Plasmodium. These differ from biological agents previously used for vector control in that their primary effect is on vectoral capacity rather than the longevity and fitness of Anopheles (which may or may not be affected). An example of this type of biological agent is Microsporidia MB, which was identified in field collected Anopheles arabiensis and caused complete inhibition of Plasmodium falciparum transmission without effecting the longevity and fitness of the host. Microsporidia MB belongs to a unique group of rapidly adapting and evolving intracellular parasites and symbionts called microsporidia. In this review we discuss the general biology of microsporidians and the inherent characteristics that make some of them particularly suitable for malaria control. We then discuss the research priorities for developing a transmission blocking strategy for the currently leading microsporidian candidate Microsporidia MB for malaria control

Evaluation of the solar-powered Silver Bullet 2.1 (Lumin 8) light trap for sampling malaria vectors in western Kenya

Abstract Background Centers for Disease Control and Prevention (CDC) light traps are widely used for sampling mosquitoes. However, this trap, manufactured in the USA, poses challenges for use in sub-Saharan Africa due to procurement costs and shipping time. Traps that are equally efficient than the CDC light trap, but which are amenable for use in remote African settings and made in Africa, are desirable to improve local vector surveillance. This study evaluated a novel solar-powered light trap made in South Africa (Silver Bullet trap; SB), for its efficiency in malaria vector sampling in western Kenya. Methods Large cage (173.7 m3) experiments and field evaluations were conducted to compare the CDC-incandescent light trap (CDC-iLT), CDC-UV fluorescent tube light trap (CDC-UV), SB with white diodes (SB-White) and SB with UV diodes (SB-UV) for sampling Anopheles mosquitoes. Field assessments were done indoors and outdoors following a Latin square design. The wavelengths and absolute spectral irradiance of traps were compared using spectrometry. Results The odds of catching a released Anopheles in the large cage experiments with the SB-UV under ambient conditions in the presence of a CDC-iLT in the same system was three times higher than what would have been expected when the two traps were equally attractive (odds ratio (OR) 3.2, 95% confidence interval CI 2.8–3.7, P < 0.01)). However, when the white light diode was used in the SB trap, it could not compete with the CDC-iLT (OR 0.56, 95% CI 0.48–0.66, p < 0.01) when the two traps were provided as choices in a closed system. In the field, the CDC and Silver Bullet traps were equally effective in mosquito sampling. Irrespective of manufacturer, traps emitting UV light performed better than white or incandescent light for indoor sampling, collecting two times more Anopheles funestus sensu lato (s.l.) (RR 2.5; 95% CI 1.7–3.8) and Anopheles gambiae s.l. (RR 2.5; 95% 1.7–3.6). Outdoor collections were lower than indoor collections and similar for all light sources and traps. Conclusions The solar-powered SB trap compared well with the CDC trap in the field and presents a promising new surveillance device especially when charging on mains electricity is challenging in remote settings

Efficacy of <i>Metarhizium anisopliae</i>, Isolate ICIPE 7, against <i>Anopheles arabiensis</i>, <i>Glossina fuscipes</i>, and <i>Rhipicephalus</i> spp.

Arthropod vectors are responsible for a multitude of human and animal diseases affecting poor communities in sub-Saharan Africa. Their control still relies on chemical agents, despite growing evidence of insecticide resistance and environmental health concerns. Biorational agents, such as the entomopathogenic fungus Metarhizium anisopliae, might be an alternative for vector control. Recently, the M. anisopliae isolate ICIPE 7 has been developed into a commercial product in Kenya for control of ticks on cattle. We were interested in assessing the potential of controlling not only ticks but also disease-transmitting mosquitoes and tsetse flies using cattle as blood hosts, with the aim of developing a product for integrated vector management. Laboratory bioassays were carried out with M. anisopliae, isolate ICIPE 7 and isolate ICIPE 30, to compare efficacy against laboratory-reared Anopheles arabiensis. ICIPE 7 was further tested against wild Glossina fuscipes and Rhipicephalus spp. Dose–response tests were implemented, period of mosquito exposure was evaluated for effects on time to death, and the number of spores attached to exposed vectors was assessed. Exposure to 109 spores/mL of ICIPE 7 for 10 min resulted in a similar mortality of An. arabiensis as exposure to ICIPE 30, albeit at a slower rate (12 vs. 8 days). The same ICIPE 7 concentration also resulted in mortalities of tsetse flies (LT50: 16 days), tick nymphs (LT50: 11 days), and adult ticks (LT50: 20 days). Mosquito mortality was dose-dependent, with decreasing LT50 of 8 days at a concentration of 106 spores/mL to 6 days at 1010 spores/mL. Exposure period did not modulate the outcome, 1 min of exposure still resulted in mortality, and spore attachment to vectors was dose-dependent. The laboratory bioassays confirmed that ICIPE 7 has the potential to infect and cause mortality to the three exposed arthropods, though at slower rate, thus requiring further validation under field conditions