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Volume 12, Number 8—August 2006

Dispatch

West Nile Virus Epizootiology, Central Red River Valley, North Dakota and Minnesota, 2002–2005

Jeffrey A. Bell*, Christina M. Brewer*, Nathan J. Mickelson*, Gabriel W. Garman*, and Jefferson A. Vaughan*Comments to Author 
Author affiliations: *University of North Dakota, Grand Forks, North Dakota, USA

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Table 2

Epizootiology of West Nile virus (WNV) within the central Red River Valley of North Dakota and Minnesota during the first 4 years of its introduction into the region*

Year Primary transmission season† Thermal accumulations (degree-days)‡ Vector abundance§ Human cases in ND¶ Seasonal MIR# Passerine seroprevalence
2002, introductory 92 days
(11 Jun–10 Sep) 1,067 230 17 0.0
(n = 5,871) No birds tested
2003, epidemic 92 days
(11 Jun–10 Sep) 1,022 21 617 5.7
(n = 5,432) 17%
(n = 82)
2004, cold 51 days
(7 Jul–1 Sep) 371 9 20 0.0
(n = 1,245) 58%
(n = 52)
2005, equilibrium? 84 days
(20 Jun–11 Sep) 867 29 86 1.3
(n = 3,123) 57%
(n = 143)

*ND, North Dakota; MIR, minimum infection rate.
†Time between first and last appearances of host-seeking Culex tarsalis mosquitoes in Mosquito Magnet traps.
‡Based on developmental threshold temperature of 14.3°C for WNV growth in Cx. tarsalis (14).
§Average number of Cx. tarsalis mosquitoes captured per trap-night in Grand Forks, ND.
¶Data from North Dakota Department of Public Health (15).
#No. of WNV-infected Cx. tarsalis mosquitoes per 1,000.

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