Assessing the Impact of Insecticide Resistance on Vector Competence: A Review.

The primary strategy to avoid adverse impacts from insect-mediated pathogen transmission is the chemical control of vector populations through insecticides; its continued use has led to insecticide resistance and unknown consequences on vector competence. This review aims to systematically analyze and synthesize the research on the influence of insecticide resistance (IR) on vector competence (VC). Thirty studies met the inclusion criteria. Twenty studies, conducted either in laboratory or field settings, described the influence of phenotypic insecticide resistance and mechanisms on VC in vectors of human pathogens. Seven studies showed the effect of exposure to insecticides on VC in vectors of human pathogens. Three studies reported the influence of phenotypic resistance and mechanisms on VC in crop pests. The evidence shows that IR could enhance, impair, or have no direct effect on VC in either field or laboratory-designed studies. Similar positive and negative trends are found in pest vectors in crops and studies of insecticide exposure and VC. Even though there is evidence that exposure to insecticides and IR can enhance VC, thus increasing the risk of pathogen transmission, more investigations are needed to confirm the observed patterns and what implications these factors could have in vector control programs.

Variation in competence for ZIKV transmission by Aedes aegypti and Aedes albopictus in Mexico.

BACKGROUND: ZIKV is a new addition to the arboviruses circulating in the New World, with more than 1 million cases since its introduction in 2015. A growing number of studies have reported vector competence (VC) of Aedes mosquitoes from several areas of the world for ZIKV transmission. Some studies have used New World mosquitoes from disparate regions and concluded that these have a variable but relatively low competence for the Asian lineage of ZIKV. METHODOLOGY/PRINCIPAL FINDINGS: Ten Aedes aegypti (L) and three Ae. albopictus (Skuse) collections made in 2016 from throughout Mexico were analyzed for ZIKV (PRVABC59-Asian lineage) VC. Mexican Ae. aegypti had high rates of midgut infection (MIR), dissemination (DIR) and salivary gland infection (SGIR) but low to moderate transmission rates (TR). It is unclear whether this low TR was due to heritable salivary gland escape barriers or to underestimating the amount of virus in saliva due to the loss of virus during filtering and random losses on surfaces when working with small volumes. VC varied among collections, geographic regions and whether the collection was made north or south of the Neovolcanic axis (NVA). The four rates were consistently lower in northeastern Mexico, highest in collections along the Pacific coast and intermediate in the Yucatan. All rates were lowest north of the NVA. It was difficult to assess VC in Ae. albopictus because rates varied depending upon the number of generations in the laboratory. CONCLUSIONS/SIGNIFICANCE: Mexican Ae. aegypti and Ae. albopictus are competent vectors of ZIKV. There is however large variance in vector competence among geographic sites and regions. At 14 days post infection, TR varied from 8-51% in Ae. aegypti and from 2-26% in Ae. albopictus.

Vector Competence of American Mosquitoes for Three Strains of Zika Virus.

In 2015, Zika virus (ZIKV; Flaviviridae; Flavivirus) emerged in the Americas, causing millions of infections in dozens of countries. The rapid spread of the virus and the association with disease outcomes such as Guillain-Barré syndrome and microcephaly make understanding transmission dynamics essential. Currently, there are no reports of vector competence (VC) of American mosquitoes for ZIKV isolates from the Americas. Further, it is not clear whether ZIKV strains from other genetic lineages can be transmitted by American Aedes aegypti populations, and whether the scope of the current epidemic is in part facilitated by viral factors such as enhanced replicative fitness or increased vector competence. Therefore, we characterized replication of three ZIKV strains, one from each of the three phylogenetic clades in several cell lines and assessed their abilities to be transmitted by Ae. aegypti mosquitoes. Additionally, laboratory colonies of different Culex spp. were infected with an American outbreak strain of ZIKV to assess VC. Replication rates were variable and depended on virus strain, cell line and MOI. African strains used in this study outcompeted the American strain in vitro in both mammalian and mosquito cell culture. West and East African strains of ZIKV tested here were more efficiently transmitted by Ae. aegypti from Mexico than was the currently circulating American strain of the Asian lineage. Long-established laboratory colonies of Culex mosquitoes were not efficient ZIKV vectors. These data demonstrate the capacity for additional ZIKV strains to infect and replicate in American Aedes mosquitoes and suggest that neither enhanced virus replicative fitness nor virus adaptation to local vector mosquitoes seems likely to explain the extent and intensity of ZIKV transmission in the Americas.