Stable Flies Suspect in Pelican West Nile Deaths

Posted Sep 28, 2007 by Bob Ewing
An entomologist from Montana State University is considering moving his study of West Nile virus from mosquitoes to stable flies. his recent fieldwork is providing support for this new approach.
stable flies
stable flies
Veterinary entomologist Greg Johnson of Montana State University was looking at the possibility that lice were transmitting West Nile virus to pelicans. His focus has switched to stable flies as being the guilty party in the deaths West Nile deaths of 800 to 1,000 pelican chicks in 2003, averaged 400 in each of the next three summers and more than 600 this year
Johnson became suspicious in 2006 when he only collected a few mosquitoes but the pelicans continued to die at a high rate. Johnson found out that the Culex tarsalis mosquito is the primary carrier of West Nile virus in Montana and that the Medicine Lake refuge was one of the hot spots for the virus.
The lice were implicated because they were found crawling over many of the dead pelicans but were later cleared when the results of tests on both pelicans and lice came back from the Center for Disease Control and Prevention in Fort Collins, Colo. The lice did not have West Nile even though they came from pelicans that tested positive for the disease.
Johnson found a bird that had blood on its beak and some flies having the blood for lunch.
"This (the stable fly scene) was very unusual because stable flies are only reported to feed on domestic livestock, humans and companion animals," Johnson said. "There are no reports in the literature of stable flies feeding on domestic or wild birds."
Johnson continued his observations of stable flies and collected about 1,300 flies, which he then divided into 60 groups. Eighteen of those groups tested positive for West Nile virus.
"This is the first report of stable flies feeding on wild birds, or pelicans for that matter, and the first report of stable flies infected with West Nile virus," Johnson said. "These results suggest that stable flies might be involved in amplification and/or transmission of West Nile virus at the pelican colony and possibly could serve as a vector of West Nile virus to other pelicans."
Johnson may have to modify his study methods as they currently focus on mosquitoes, as for the lice: "I don't think they are playing a primary role in West Nile transmission because they don't have to have blood for egg development, energy and survival," Johnson said.
"Rather, they feed on epidermal or skin cells which create wounds, causing blood to exude and then they feed on the blood. The wounds they cause may provide entry sites for West Nile virus, and the young pelicans can get infected that way."