Investigations of the West Nile virus transmission cycle at Medicine Lake National Wildlife Refuge, Montana, 2005-2006

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Montana State University - Bozeman, College of Agriculture


Outbreaks of West Nile virus (WNV) in American white pelicans (Pelecanus erythrorhynchos Gmelin) at Medicine Lake National Wildlife Refuge, Montana prompted mosquito and WNV surveillance during 2005 - 2006 to describe the epidemiological cycle. A total of 102,118 mosquitoes representing 19 species was collected throughout the refuge from 261 trap nights. Ochlerotatus dorsalis (Meigen), a floodwater mosquito, was the most abundant (45.5%), followed by Aedes vexans (Meigen) (25.6%), Culex tarsalis Coquillett (18.8%) and Ochlerotatus flavescens (Muller) (7.3%). Seasonal light trap indices (LTI) for 2005 and 2006 were 110 and 53, respectively. A total of 4,402 Cx. tarsalis mosquitoes in 2005 was assayed for WNV in 90 pools, six of which were positive. The minimum infection rate (MIR) for the season was 1.36 per 1,000 mosquitoes. In 2006, 64 pools containing 2,810 females were tested, four of which were positive, resulting in an MIR of 1.41 per 1,000 mosquitoes. Avian infection with WNV was monitored with sentinel chickens, which were shown to be ineffective as an early warning system for WNV transmission. Mosquito blood-meal identification was used to assess which species were utilizing pelicans as a host.
Three methods were evaluated for collecting blood-fed mosquitoes. CO2-baited CDC light traps, a gasoline-powered backpack aspirator, and fiber resting pots collected 15, 74, and 0 blood-fed Ae. vexans, 20, 34, and 52 blood-fed Culiseta inornata (Williston), and 57, 27 and 35 blood-fed Cx. tarsalis mosquitoes, respectively. Heteroduplex PCR analyses identified 24 hosts from blood-fed Cx. tarsalis (n = 95), Ae. vexans (n = 2) and Cs. inornata (n = 39). Approximately half of the Cx. tarsalis blood-meals were avian, but none was from pelican; 75% of Ae. vexans were from white-tailed deer (Odocoileus virginianus Zimmerman); and 65% of Cs. inornata were from cattle (Bos taurus Linnaeus). Of 206 mosquitoes tested, only one Cs. inornata was positive for pelican. Pre-fledged pelican mortality in 2005 was greater than 400 and more than 385 in 2006. Low numbers of Cx. tarsalis in light traps in 2006 strongly suggest another mode of transmission is likely occurring in the pelican colony.




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