Ivermectin-treated bird feed to control West Nile virus transmission
Date
2018
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Abstract
West Nile virus is the leading cause of arboviral fever and encephalitis in the United States. The highest WNV disease incidence occurs along the Great Plains region of the United States, as the ecology and land use provide a supportive habitat for the main WNV enzootic and bridge vector of the region, Culex tarsalis. However, due to the lack of dense human population, this area often does not benefit current WNV control measures as applied by conventional mosquito control districts. Based on the ecology of WNV transmission in the Great Plains region, a strategy that targets Cx. tarsalis through its ornithophilic blood feeding behavior could disrupt WNV transmission. Given that the majority of Cx. tarsalis blood meals on the northern Colorado plains may come from doves and passerine species during the WNV transmission season, effective targeting of these or other local preferred hosts with endectocide-treated bird feed could result control of WNV transmission. This study develops and characterizes the effects of IVM-treated bird feed in birds and biting Cx. tarsalis mosquitoes in both a laboratory and field setting. In Chapter 2, the effects of IVM on Cx. tarsalis survival were examined using both in vitro membrane blood meals and direct blood feeding on IVM-treated birds. Chickens and wild Eurasian Collared Doves fed solely on IVM-treated bird feed concentrations up to 200 mg IVM/kg feed exhibited no signs of toxicity, and most Cx. tarsalis that blood fed on these birds died compared to controls. Mosquito survivorship following blood feeding correlated with IVM serum concentrations at the time of blood feeding, which dropped rapidly after the withdrawal of treated feed. These results suggested IVM-treated bird feed should be further explored as a hyper-localized control strategy for WNV transmission. Chapter 3 presents the development of a method to detect and quantify IVM in individual blood meals of Anopheles gambiae and Cx. tarsalis, which will be important in measuring the coverage of this intervention in the field, and accurately assessing IVM's mosquitocidal effects in field situations. This ability to detect IVM in mosquito blood meals was similar between blood fed Cx. tarsalis and An. gambiae, and between sampling times of 0 or 12 hours post blood feed. The quantity of IVM ingested in individual mosquitoes was also compared to the venous serum concentrations of live animals. Chapter 4 presents promising results from two separate pilot field trials of IVM-treated bird feed that were conducted during the summers of 2016 and 2017. Results from 2016 showed that wild birds frequently visit the IVM-treated feeders. In addition, there was an observable trend where "far" traps that are expected to be beyond the zone of control had more WNV-positive pools compared to "near" traps at both ELC and ARDEC South sites. Results from the 2017 study continued to be promising, where birds were again visiting IVM-treated feeders and IVM could be detected in the sera of birds sampled by IVM feeders. There was also a trend of higher VI for the control sites compared to IVM sites for the 2017 season. The efficacy of IVM-treated bird feed was evaluated in two pilot trials where natural WNV transmission cycles occurred in wild birds and Cx. tarsalis, but should be followed up with field seasons with many control and IVM sites to allow for a robust analysis of IVM effects. This study introduces the novel concept of using systemic endectocides for controlling WNV transmission, and this concept could be explored for other arboviruses.
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Subject
West Nile virus
Ivermectin