Pathogen and Microbiome Institute, Northern Arizona University, Flagstaff, Arizona, United States of America.
Medical Entomology Unit, Institut Pasteur de Madagascar, Antananarivo, Madagascar.
PLoS Negl Trop Dis. 2023 Aug 22;17(8):e0011401. doi: 10.1371/journal.pntd.0011401. eCollection 2023 Aug.
Plague, caused by the bacterium Yersinia pestis, remains an important disease in Madagascar, where the oriental rat flea, Xenopsylla cheopis, is a primary vector. To control fleas, synthetic pyrethroids (SPs) have been used for >20 years, resulting in resistance in many X. cheopis populations. The most common mechanisms of SP resistance are target site mutations in the voltage-gated sodium channel (VGSC) gene.
METHODOLOGY/PRINCIPAL FINDINGS: We obtained 25 collections of X. cheopis from 22 locations across Madagascar and performed phenotypic tests to determine resistance to deltamethrin, permethrin, and/or dichlorodiphenyltrichloroethane (DDT). Most populations were resistant to all these insecticides. We sequenced a 535 bp segment of the VGSC gene and identified two different mutations encoding distinct substitutions at amino acid position 1014, which is associated with knockdown resistance (kdr) to SPs in insects. Kdr mutation L1014F occurred in all 25 collections; a rarer mutation, L1014H, was found in 12 collections. There was a significant positive relationship between the frequency of kdr alleles and the proportion of individuals surviving exposure to deltamethrin. Phylogenetic comparisons of 12 VGSC alleles in Madagascar suggested resistant alleles arose from susceptible lineages at least three times. Because genotype can reasonably predict resistance phenotype, we developed a TaqMan PCR assay for the rapid detection of kdr resistance alleles.
CONCLUSIONS/SIGNIFICANCE: Our study provides new insights into VGSC mutations in Malagasy populations of X. cheopis and is the first to report a positive correlation between VGSC genotypes and SP resistance phenotypes in fleas. Widespread occurrence of these two SP resistance mutations in X. cheopis populations in Madagascar reduces the viability of these insecticides for flea control. However, the TaqMan assay described here facilitates rapid detection of kdr mutations to inform when use of these insecticides is still warranted to reduce transmission of plague.
鼠疫是由鼠疫耶尔森菌引起的一种疾病，在马达加斯加仍然很重要，在那里，东方鼠蚤 Xenopsylla cheopis 是主要的媒介。为了控制跳蚤，已经使用了 20 多年的合成拟除虫菊酯 (SP)，导致许多 X. cheopis 种群产生了抗药性。SP 抗性最常见的机制是电压门控钠离子通道 (VGSC) 基因中的靶位突变。
方法/主要发现：我们从马达加斯加的 22 个地点获得了 25 个 X. cheopis 种群，并进行了表型测试，以确定对溴氰菊酯、氯菊酯和/或滴滴涕的抗性。大多数种群对所有这些杀虫剂都有抗性。我们对 VGSC 基因的 535 bp 片段进行了测序，发现了两个不同的突变，分别在氨基酸位置 1014 编码了不同的取代，这与昆虫对 SP 的击倒抗性 (kdr) 有关。L1014F 突变在所有 25 个种群中都存在；更罕见的突变 L1014H 存在于 12 个种群中。kdr 等位基因的频率与暴露于溴氰菊酯后存活个体的比例之间存在显著的正相关关系。马达加斯加的 12 个 VGSC 等位基因的系统发育比较表明，抗性等位基因至少从三个易感谱系中产生。由于基因型可以合理地预测抗性表型，我们开发了一种 TaqMan PCR 检测方法，用于快速检测 kdr 抗性等位基因。
结论/意义：我们的研究为马达加斯加 X. cheopis 种群的 VGSC 突变提供了新的见解，并且是首次报道在跳蚤中 VGSC 基因型与 SP 抗性表型之间存在正相关关系。这两种 SP 抗性突变在马达加斯加 X. cheopis 种群中的广泛存在降低了这些杀虫剂控制跳蚤的效果。然而，本文描述的 TaqMan 检测方法有助于快速检测 kdr 突变，以告知何时仍需要使用这些杀虫剂来减少鼠疫的传播。
