Estimates of bluetongue virus infection rates in Culicoides midge vectors depend on trap attractant and placement

Bluetongue virus (BTV) is an economically important livestock pathogen, transmitted by Culicoides biting midges. Globally, most Culicoides surveillance for BTV is conducted using UV-light baited suction traps. However, the limitations of such traps (i.e. species and parity biases) have been a concern. Furthermore, a recent study showed that traps baited with CO₂+UV collected midges with lower BTV infection rates than those baited with CO₂ alone, or from sentinel animal collections. We conducted a field study on three dairies in California to test the effect of trap attractant on infection rates in the primary North American BTV vector, C. sonorensis. Insects were collected at three sites (animals, wastewater ponds, fields) on each dairy using suction traps with CO₂, UV, or CO₂+UV over six weeks. Pools of insects were tested for BTV by qRT-PCR.

CO₂-only traps collected a higher proportion of BTV-positive pools, and those insects had significantly higher infection rates than Culicoides in UV traps. UV traps often failed completely to detect active BTV. CO₂ traps also had higher infection rates than CO₂+UV traps, indicating that light repels infected midges. High virus levels in eyes of infected midges suggest impaired vision or light perception. Traps set near ponds showed almost no BTV, regardless of the attractant used. Transmission risk estimates based on light trap collections are likely significantly lower than the actual risk. Disseminated arbovirus infections, including in vectors’ eyes, may cause unanticipated behavioral effects, resulting in a misunderstanding of transmission dynamics.