Permissive Summer Temperatures of the 2010 European West Nile Fever Upsurge

Abstract

BackgroundIn the summer of 2010, Europe experienced outbreaks of West Nile Fever (WNF) in humans, which was preceded by hot spells. The objective of this study was to identify potential drivers of these outbreaks, such as spring and summer temperatures, relative humidity (RH), and precipitation.MethodsPearson and lag correlations, binary and multinomial logistic regressions were used to assess the relationship between the climatic parameters and these outbreaks.ResultsFor human morbidity, significant (<0.05) positive correlations were observed between a number of WNF cases and temperature, with a geographic latitude gradient: northern (“colder”) countries displayed strong correlations with a lag of up to four weeks, in contrast to southern (“warmer”) countries, where the response was immediate. The correlations with RH were weaker, while the association with precipitation was not consistent. Horse morbidity started three weeks later than in humans where integrated surveillance was conducted, and no significant associations with temperature or RH were found for lags of 0 to 4 weeks.ConclusionsSignificant temperature deviations during summer months might be considered environmental precursors of WNF outbreaks in humans, particularly at more northern latitudes. These insights can guide vector abatement strategies by health practitioners in areas at risk for persistent transmission cycles.

Climate change impacts on West Nile virus transmission in a global context

“…Among other variables, the average maximum relative humidity was associated with vector population dynamics [60]. A recent analysis detected correlations between morbidity in humans and weekly relative humidity in Europe and western Asia [22]. All these studies found that air temperature is a better predictor for increasing disease cases than air humidity.…”

“…However, it was shown that the replication cycle is completed more quickly in mosquitoes at higher temperatures [38,39], while a clear association was found between extreme heat and outbreak intensity in humans [4,18,22,35,[40][41][42][43]. At the same time, it is important to note that, in some cases, extremely high temperatures begin to slow down mosquito activity.…”

“…Increased temperatures cause an upsurge in the growth rates of vector populations, decrease the interval between blood meals, shorten the incubation time from infection to infectiousness in mosquitoes, accelerate the virus evolution rate and increase viral transmission efficiency to birds [22,23,30,32,35,36].…”

“…The unprecedented upsurge in the number of human WNF cases in summer 2010 was accompanied by extremely hot spells in southeastern Europe and Eurasia [22]. According to the World Meteorological Organization [73], this warming peaked in this past decade ending in 2010, which was also one of the three hottest years ever recorded.…”

“…According to the World Meteorological Organization [73], this warming peaked in this past decade ending in 2010, which was also one of the three hottest years ever recorded. An analysis of the climatic drivers of the 2010 outbreak found the ambient temperature to be the most important [22]. The impact of climatic variables on the endemization of WNV in Europe and western Asia has been reviewed recently by Paz & Semenza [23].…”