Age-dependence of susceptibility to single and repeated deltamethrin exposure in pyrethroid-resistant Aedes aegypti strains

Ashwaq M. Al Nazawi , David Weetman
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Abstract

Monitoring insecticide resistance is crucial in disease-transmitting mosquitoes to allow assessment of viable candidate insecticides to use for control and to provide indication of changes in resistance. Insecticide resistance bioassays are typically performed on young female mosquitoes, yet disease is transmitted by older females, which may also have encountered insecticide multiple times during their adult life. If insecticide mortality rates increase with age directly, or indirectly via cumulative toxicity from repeated exposure, the strategy of testing young mosquitoes as the least susceptible cohort would be supported. We tested three hypotheses via examination of how age and cumulative exposure impact mortality rates to the pyrethroid deltamethrin in strains of Aedes aegypti from Jeddah, Saudi Arabia and the Cayman Islands, which show differences in resistance mechanisms. Females of different ages (5, 7, 10 and 14 days-old) were exposed using WHO tube assays to either a single dose of insecticide, or in a second experiment females (initially 5 days-old) were exposed daily over 10 days. Age only increased mortality in the Jeddah strain at 14 days-old and had no impact on the Cayman strain. This is consistent with greater impact linked to metabolic resistance in the Jeddah strain, though results from qPCR of four candidate genes, failed to provide evidence for a candidate underpinning an age-dependent change in resistance. With repeated exposure, mortality rates of surviving females decreased to very low levels, suggesting that surviving older cohorts of females may exhibit substantially lower susceptibility than young females in single exposure assays. Our results indicate that testing young females with a single insecticide exposure should capture minimum susceptibility for the majority of the population, but a small fraction of older females may prove particularly unresponsive to pyrethroid-based control measures.

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拟除虫菊酯抗性埃及伊蚊菌株对单次和多次溴氰菊酯暴露敏感性的年龄依赖性
监测杀虫剂耐药性对于传播疾病的蚊子至关重要,以便评估可用于控制的可行候选杀虫剂,并提供耐药性变化的迹象。杀虫剂抗性生物测定通常在年轻的雌性蚊子身上进行,但疾病是由年长的雌性蚊子传播的,它们在成年后也可能多次遇到杀虫剂。如果杀虫剂死亡率直接或通过反复暴露的累积毒性间接随着年龄的增长而增加,那么将年轻蚊子作为最不易感群体进行测试的策略将得到支持。我们通过检验吉达、沙特阿拉伯和开曼群岛埃及伊蚊菌株的年龄和累积暴露量如何影响拟除虫菊酯-溴氰菊酯的死亡率,检验了三种假设,这三种假设显示出耐药性机制的差异。使用世界卫生组织试管测定法将不同年龄(5、7、10和14天大)的雌性暴露于单剂量杀虫剂,或者在第二个实验中,雌性(最初为5天大)每天暴露10天以上。年龄只会增加吉达菌株14天大时的死亡率,对开曼菌株没有影响。这与吉达菌株中与代谢抗性相关的更大影响一致,尽管四个候选基因的qPCR结果未能为支持抗性随年龄变化的候选基因提供证据。随着重复暴露,存活女性的死亡率降至非常低的水平,这表明在单次暴露试验中,存活的老年女性队列可能表现出比年轻女性低得多的易感性。我们的研究结果表明,对接触过单一杀虫剂的年轻雌性进行测试,应该能捕捉到大多数人群的最低易感性,但一小部分老年雌性可能对基于拟除虫菊酯类的控制措施特别没有反应。
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