The effects of sublethal larvicide exposure on Culicoides biting midge vector competence for bluetongue virus

Vectors exhibit innate immune responses to viral infections, determining their susceptibility to pathogens. While immunity is predetermined by genotype, phenotypic immune response is governed by extrinsic factors. Environmental stressors encountered as larvae (e.g. pesticides) weaken arthropods, as resources are shunted towards detoxification and away from immune pathways to survive. Culicoides biting midges develop in habitats such as dairy wastewater produced by farms, which are prone to collect chemical run-offs from pesticides used to treat animals. Pesticide residues in larval habitats may lead to weakened immune response in emerging adults, resulting in higher BTV infection and dissemination rates and putting livestock at increased disease risk. In this study, Culicoides sonorensis larvae were exposed to LC₂₀ and LC₄₀ concentrations of permethrin. Surviving larvae were reared to adulthood and offered sheep blood spiked with BTV-17or uninfected blood three days post-emergence. Engorged females were then held for up to 14 days post infection (dpi). At 0, 1, 24, and 48 hours post infection, and at 7 and 14dpi, 10 females from each treatment group were removed to test for BTV infection and dissemination by qRT-PCR. Dissemination was determined by testing the head and thorax, after removal from the abdomen. Infection was determined by testing the midgut and abdomen. Midges with disseminated infections were assumed to be capable of transmission. The number of infectious midges and respective infection intensity/dissemination rates were compared at each time point among treatments and between treatments to determine whether sublethal permethrin exposures of larvae increased BTV vector competence in adults.