Abstract
BackgroundIvermectin is used extensively globally for treatment of helminthic and ectoparasitic infections in animals and humans. The effect of excreted ivermectin on non-target organisms in aquatic and terrestrial environments has been increasingly reported. Due to its low water solubility and adsorption to sediments, the ivermectin exposure-risk to aquatic organisms dwelling in different strata of water bodies varies. This study assessed the survival of larvae of Anopheles gambiae Giles and Culex quinquefasciatus Say, when exposed to low concentrations of ivermectin under laboratory conditions.MethodsA total of 1800 laboratory reared mosquito larvae of each species were used in the bioassays. Twelve replicates were performed, each testing 6 concentrations of ivermectin (0.0, 0.001, 0.01, 0.1, 1.0 and 10.0 parts per million (ppm)) against third instar larvae of An. gambiae and Cx. quinquefasciatus. Larval mortality was recorded at 24 and 48 h post addition of ivermectin.ResultsSurvival declined markedly with increase in ivermectin concentration in both species. While mean survival of An. gambiae at 24 h of exposure was 99.6 %, 99.2 % and 61.6 % in 0.001, 0.01 and 0.1 ppm of ivermectin, respectively, the mean survival of Cx. quinquefasciatus at the same dosage and time was 89.2 %, 47.2 % and 0.0 %. A similar pattern, but with higher mortality, was observed after 48 h of exposure. Comparison between the species revealed that Cx. quinquefasciatus larvae were significantly more affected by ivermectin than those of An. gambiae, both at 24 and 48 h.ConclusionsLow concentrations of ivermectin in the aquatic environment reduced the survival of larvae of An. gambiae and Cx. quinquefasciatus, with the effect being more marked in the latter species. It is suggested that this difference may be due to the different water strata occupied by the two species, with ivermectin adsorbed in food that sediment being more readily available to the bottom feeding Cx. quinquefasciatus than the surface feeding An. gambiae larvae.
Highlights
Ivermectin is used extensively globally for treatment of helminthic and ectoparasitic infections in animals and humans
Entomological field studies in the programme areas have documented a decline in survival of female Anopheles gambiae that feed on humans shortly after mass treatment with ivermectin [6,7,8]
Mosquito larvae Larvae of An. gambiae sensu stricto and Culex quinquefasciatus mosquitoes were used for the laboratory bioassays
Summary
Ivermectin is used extensively globally for treatment of helminthic and ectoparasitic infections in animals and humans. Ivermectin is an important drug for treatment of many helminthic and ectoparasitic infections in animals and humans globally [1] It has been extensively used in the veterinary field, and its use in humans has recently been scaled up in large programmes to control lymphatic filariasis and onchocerciasis in endemic areas in sub-Saharan Africa [2, 3]. Laboratory studies have confirmed this effect of ivermectin on adult anopheline vector survival, and have demonstrated a reduced fecundity of these vectors following a blood meal from ivermectin treated humans or cattle [9, 10] Due to this effect on important malaria vectors in sub-Saharan Africa, ivermectin has been considered as a potential tool for future malaria control [11, 12]
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