West nile virus transmission in winter: the 2013 great salt lake bald eagle and eared grebes mortality event

Abstract

West Nile Virus (WNV) infection has been reported in over 300 species of birds and mammals. Raptors such as eagles, hawks and falcons are remarkably susceptible, but reports of WNV infection in Bald Eagles (Haliaeetus leucocephalus) are rare and reports of WNV infection in grebes (Podicipediformes) even rarer. We report an unusually large wild bird mortality event involving between 15,000-20,000 Eared Grebes (Podiceps nigricollis) and over 40 Bald Eagles around the Great Salt Lake, Utah, in November-December 2013. Mortality in grebes was first reported in early November during a period when the area was unseasonably warm and the grebes were beginning to gather and stage prior to migration. Ten out of ten Eared Grebes collected during this period were WNV RT-PCR and/or isolation positive. This is the first report of WNV infection in Eared Grebes and the associated mortality event is matched in scale only by the combined outbreaks in American White Pelican (Pelecanus erythrorhynchos) colonies in the north central states in 2002-2003. We cannot be sure that all of the grebes were infected by mosquito transmission; some may have become infected through contact with WNV shed orally or cloacally from other infected grebes. Beginning in early December, Bald Eagles in the Great Salt Lake area were observed to display neurological signs such as body tremors, limb paralysis and lethargy. At least 43 Bald Eagles had died by the end of the month. Nine of nine Bald Eagles examined were infected with WNV. To the best of our knowledge, this is the largest single raptor mortality event since WNV became endemic in the USA. Because the majority of the eagles affected were found after onset of below-freezing temperatures, we suggest at least some of the Bald Eagles were infected with WNV via consumption of infected Eared Grebes or horizontal transmission at roost sites.

Keywords: Utah; West Nile virus; bald eagle; disease outbreak; eared grebe.

Utah counties with reported mortality of birds at Great Salt Lake in December 2013.

Location of sick or dead birds during the Great Salt Lake WNV mortality event are color coded in red for bald eagles and yellow for eared grebes. The counties in Utah are labeled in white.

Cumulative mortality of bald eagles over time during the Great Salt Lake WNV mortality event.

The number of bald eagle deaths in the Great Salt Lake area during December 2013 are plotted against dates.

Table 1. Bird. BAEA, Bald Eagle, EAGR, Eared Grebes. County, county of collection. Date Collected, date when bird was initially collected. WNV RT-PCR results from Brain, Kid/Spl (kidney/spleen pool), Liver, Feather pulp, TS (tracheal swab), CS (cloacal swab) are listed as P, positive (Ct < 40), N (Ct = 0), or – (not done). WNV Isolated, tissues from which a WNV has been isolated is listed, N is no virus isolated.

Daily maximum and minimum air temperature record at the NOAA Great Salt Lake meteorological station.

The maximum (filled circles) and minimum (open circles) daily temperatures at the Salt Lake City Meteorological Station from October 1-December 31, 2013 are plotted against dates. The average temperature (10°C) for resumption of mosquito activity is marked in red.