Testing independent and interactive effects of corticosterone and synergized resmethrin on the immune response to West Nile virus in chickens

Public health agencies utilize aerial insecticides to interrupt an active West Nile virus (WNV) transmission cycle, which may expose WNV-infected birds to these agents. Although resmethrin has been considered benign to birds, no studies have evaluated whether the environmentally employed form of resmethrin with PBO synergist (synergized resmethrin (SR)) can suppress avian immunity to WNV infection and enhance a bird's host competence. Recognizing that wild birds confront toxicological stressors in the context of various physiological states, we exposed four groups (n = 9–11) of 9-week-old chickens (Gallus domesticus) to drinking water with either SR (three alternate days at 50 μg/l resmethrin + 150 μg/l piperonyl butoxide), CORT (10 days at 20 mg/l to induce subacute stress), the combination of SR and CORT, or 0.10% ethanol vehicle coincident with WNV infection. Compared to controls, SR treatment did not magnify but extended viremia by 1 day, and depressed IgG; CORT treatment elevated (mean, 4.26 log₁₀ PFU/ml) and extended viremia by 2 days, enhanced IgM and IgG, and increased oral virus. The combination of SR and CORT increased the number of chickens that shed oral virus compared to those treated with CORT alone. None of the chickens developed a readily infectious viremia to mosquitoes (none ≥5 log₁₀ PFU/ml), but viremia in a CORT-exposed chicken was up to 4.95 log₁₀ PFU/ml. Given that SR is utilized during WNV outbreaks, continued work toward a complete risk assessment of the potential immunotoxic effects of SR is warranted. This would include parameterization of SR exposures with immunological consequences in wild birds using both replicating (in the laboratory) and non-replicating (in the field) antigens. As a start, this study indicates that SR can alter some immunological parameters, but with limited consequences to primary WNV infection outcome, and that elevated CORT mildly enhances SRs immunotoxicity in chickens.