Taking into account climate effects on West Nile Virus transmission risk for decision-making in Quebec
Background: In 2002, major human epidemics of West Nile Virus (WNV) were reported in five
cities in the North East region of North America. The present analysis examines the climatic
conditions that were conducive to the WNV epidemic, in order to provide information to public
health managers who eventually must decide on the implementation of a preventive larvicide
spraying program in Quebec, Canada. Two sets of variables, the first observed in the summer of
2002 and the second in the preceding winter were analysed to study their potential as explanatory
variables for the emergence of the virus at epidemic levels.
Results: Results show that the climatic conditions observed in the year 2002 have contributed to
the emergence of the virus and can be observed once every forty years on average. The analysis
has shown that the 2002 events observed in several North East North American cities are
characterized by two main variables: the number of degree-days below -5°C in the winter (DD-5)
and the number of degree-days greater than 25°C in the summer (DD25).
Conclusion: In the context of a declining rate of human and aviary infection to WNV, this element
contributed to the decision to suspend the use of preventive larvicides in the province of Quebec
in 2006 and for the foreseeable future. The second part of this study indicates that it is very
important to estimate the risk that extreme values can be observed simultaneously in the summer
and in the winter preceding the appearance of the virus. The proposed models provide important
information to public health officials, weeks before the appearance of the virus, and can therefore
be useful to help prevent human epidemics.
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