Failure of a permethrin treatment regime to protect cattle against bluetongue virus.

Holstein heifers in a confined feedlot setting on a southern California dairy were either sprayed individually along the ventral midline using 0.2% permethrin (250 ml/animal) (two pens) or were not treated (two pens). Treatments (n = 6 dates) were applied every 2 wk during the peak fall bluetongue virus transmission season (22 August-29 October). Animals seronegative for bluetongue virus antibodies at the initial bleeding on 15-18 September (n - 106 in the treatment pens and n = 117 in the control pens) were bled again for testing 2 mo later (12-13 November). Seroconversion rates were not significantly different: 56% for the treated animals and 48% for the controls (P > 0.2). The area has many essentially contiguous, confinement dairies with wastewater ponds that produce large numbers of Culicoides sonorensis Wirth & Jones, the primary bluetongue virus vector. Further, these dairies presumably provided a large reservoir of virus-infected cattle to infect vectors in the immediate area. Under these severe virus challenge conditions, permethrin applied at 2-wk intervals failed to reduce exposure to bluetongue virus.