Detection of West Nile virus in stable flies (Diptera: Muscidae) parasitizing juvenile American white pelicans.

Stable flies, Stomoxys calcitrans (L.) (Diptera: Muscidae), an economically important pest of livestock and humans, were observed parasitizing prefledged American white pelicans, Pelecanus erythrorhynchos (Pelecaniformes: Pelecanidae), in a pelican breeding colony in northeastern Montana where die-offs attributed to West Nile virus (family Flaviviridae, genus Flavivirus, WNV) have occurred since 2002. Engorged and unengorged flies were collected off nine moribund chicks. Of 29 blood-engorged flies testing positive for vertebrate DNA, all 29 contained pelican DNA. Virus isolation was performed on 60 pools (1,176 flies) of unengorged flies using Vero cell plaque assay. Eighteen pools were positive for WNV for an estimated infection rate of 18.0 per 1,000 flies. Fifty-four percent (36/67) of abdomens from blood-engorged flies tested positive for WNV. Pelican viremia levels from the blood-engorged fly abdomens revealed that at least one of the ill pelicans circulated a viremia capable of infecting Culex mosquito vectors. Stable flies may be involved in WNV transmission within the pelican breeding colony by serving as either a mechanical vector or as a source for oral infection if ingested by predators.