A West Nile Virus Outbreak in Eared Grebes and Bald Eagles in Utah: An Unusual Transmission Event in Winter

BACKGROUND: West Nile virus (WNV) is an arbovirus that is typically transmitted by mosquitoes, especially Culex pipiens and C. tarsalis. We describe an unusual die-off of eared grebes and bald eagles due to WNV occurring during late fall and winter months in Utah.

METHODS: Utah Division of Wildlife Resources (UDWR) first became aware of the large die-off of eared grebes (Podiceps nigricollis) around the Great Salt Lake in November 2013.  Beginning in early December, a number of bald eagles (Haliaeetus leucocephalus) were observed displaying neurological symptoms (tremors, limb paralysis, and lethargy) or they were found dead.  Dead or euthanized birds were collected by the UDWR and submitted to the Utah Veterinary Diagnostic Laboratory in Logan, UT, and to the USGS National Wildlife Health Center in Madison, WI, for complete diagnostic analysis. Avian samples from brain, liver and heart were submitted for bacterial culture. Tracheal and cloacal swabs from grebes and eagles were tested for avian influenza, Newcastle Disease Virus and WNV using RT-PCR and/or viral isolation.  We also analyzed human surveillance data for WNV.

RESULTS: An estimated 15,000-20,000 eared grebes died during November 2013-February 2014 in the Great Salt Lake and the seven counties surrounding the lake. Thirty eared grebes collected during November-January were positive for WNV by RT-PCR and/or isolation. Seventy-two dead bald eagles were identified; four are currently in rehabilitation centers. Nine bald eagles examined were confirmed to be infected with WNV.  During this period, no suspect or confirmed human cases were identified with WNV.

CONCLUSIONS: Each year millions of eared grebes traditionally gather on the Great Salt Lake as a staging location for late winter migration. Previous die-offs of grebes have been due to avian cholera.  This year, mortality in grebes was first reported in early November, but could have occurred as early as October.   Unseasonably warm weather may have contributed to late-season mosquito transmission of WNV.  However, persistence of transmission when temperatures were below freezing suggests that some birds may also have become infected through contact with WNV shed orally or cloacally. Morbidity and mortality of bald eagles occurred when temperatures dropped below freezing and mosquito transmission of WNV was not likely, suggesting that they were infected through consumption of infected grebes or their carcasses.  While no human cases were found, this event suggests that changing weather patterns may be contributing to changes in the epidemiology of WNV and may affect future transmission patterns in humans.