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Weather factors in the prediction of western equine encephalitis epidemics in Manitoba

Published online by Cambridge University Press:  15 May 2009

R. F. Sellers  and
A. R. Maarouf
R. F. Sellers
Affiliation:
Agriculture Canada, Health of Animals Laboratory Division, Halldon House, 2255 Carting Avenue, Ottawa, Ontario K1A 0Y9
A. R. Maarouf
Affiliation:
Environment Canada, Canadian Climate Centre, 4905 Dufferin Street, Downsview, Ontario M3H 5T4
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Summary

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Cases of western equine encephalitis in horses in 1987 in western USA and Manitoba, Canada were examined by backward trajectory analysis of winds. Culex tarsalis mosquitoes infected with western equine encephalitis virus could have been carried on southerly winds from Texas and Oklahoma to northern LTSA and from there to Manitoba. The presence of the Polar front over North Dakota and Minnesota at the end of July would have led to the landing of Cx. tarsalis in Montana and Wisconsin and prevented further carriage into Manitoba. Temperatures in southern Texas during the winter months (average daily maximum temperatures 19.7 °C and higher) would have permitted continuous transmission of western equine encephalitis virus by Cx. tarsalis in this area.

Weather factors involved in outbreaks from 1975–88 were analysed to see if epidemics in Manitoba (23 or more cases in horses) could be predicted. The conditions for epidemics could be defined as follows: (a) the number of cases in horses in LTSA was 98 or more, (b) winds were southerly with speeds 45 kmh-1 or higher, and (c) counts of Cx. tarsalis females/light trap per day were 3·2 or higher. There were 3 or fewer cases in Manitoba, when the number of cases in USA was 27 or less, even when Cx. tarsalis counts were higher than 3·2. With Cx. tarsalis counts below 3 and/or unsuitable winds, or the Polar front further south, the number of cases in Manitoba was between 0 and 17, even when the number of cases in USA was from 38–172. Without information on the extent of infection further south, the weather variables would probably be more useful in excluding the possibility of an epidemic in Manitoba than in predicting one.

Type
Research Article
Information
Epidemiology & Infection , Volume 111 , Issue 2 , October 1993 , pp. 373 - 390
Copyright
Copyright © Cambridge University Press 1993

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