Eurosurveillance has published Early start of the West Nile fever transmission season 2018 in Europe. Excerpt from the conclusion:
Several studies have shown that certain environmental factors, such as temperature and precipitation anomalies, can be predictors for WNF transmission. Elevated temperatures can increase virus replication and shorten the incubation period within the mosquito, which can facilitate virus circulation and therefore WNV outbreaks.
The underlying determinants of this early upsurge, such as specific environmental conditions in 2018, are not fully understood to date. An in-depth study of the environmental determinants would be required to assess the association of WNF and the observed climate pattern over a longer time period in Europe.
The early start of the WNF transmission season observed in 2018 should prompt early awareness raising among clinicians and public health authorities, as the observed pattern might constitute an early sign of a season with larger outbreaks if environmental conditions remain favourable for WNV transmission. The occurrence and extent of outbreaks, however, depend on numerous factors and the situation at present does not necessarily mean that a large outbreak will happen.
As the first disease onset this year occurred at the end of May, the previous assumption that the WNF season and the related period of enhanced real-time monitoring of WNF in Europe is typically from mid-June to November, may have to be reconsidered.