Background: Most malaria vector control programmes rely on indoor residual spraying of insecticides and insecticide-treated bed nets. This is efective against vector species that feed indoors at night and rest inside the house afterwards. In Central America, malaria vectors have diferent behaviours and are typically exophagic (i.e., bite outdoors), exophilic (i.e., remain outdoors after feeding), and zoophagic (i.e., as likely to feed on non-humans as humans). Thus, malaria elimination in Central America may require additional tactics. This pilot study investigated whether commercially-available products used to treat livestock for ticks could also be used to kill and/or sterilize zoophagic malaria vectors that feed on treated cattle in Belize.
Methods: Cattle were treated with either a pour-on formulation of 1% fpronil (3 heifers) or injection of 1% ivemectin (1 heifer). Control heifers (n=2) were left untreated. Field-collected Anopheles albimanus contained in screen-top cages were strapped onto cattle at 2, 5, 7, and 14 days after treatment. Mosquito mortality was monitored once a day for 4 successive days. Surviving mosquitoes were dissected to assess blood meal digestion and ovarian development.
Results: A total of 1078 female An. albimanus mosquitoes were fed and monitored for mortality. Both fpronil and ivermectin signifcantly reduced survivorship of An. albimanus for up to 7 days after treatment. By 14 days, efcacy had declined. The ivermectin treatment completely lost its efectiveness and even though the fpronil-treated heifers were still killing signifcantly more mosquitoes than the untreated heifers, the amount of mosquito killing had diminished greatly. Both treatments signifcantly reduced ovary development in mosquitoes fed on treated cattle for the duration of the 2-week trial.
Conclusions: Treatment of cattle in northern Belize with topical fpronil and injectable ivermectin had signifcant lethal and sublethal efects on wild An. albimanus females. These results suggest that eforts towards eliminating residual transmission of malaria by zoophagic vectors in Central America may beneft by the judicious, targeted treatment of livestock with mosquitocidal compounds, such as fpronil or ivermectin.
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Staci M. Dreyer, Donovan Leiva, Marla Magana, et al.. "Fipronil and Ivermectin Treatment of Cattle Reduced the Survival and Ovarian Development of Field-Collection Anopheles Albimanus in a Pilot Trial Conducted in Northern Belize" (2019). Biology Faculty Publications. 13.