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Model predicts threat of West Nile virus in moderate climate conditions
West Nile virus has in recent years been seen in European countries with a more moderate climate. Researcher at Wageningen Bioveterinary Research applied a mathematical modelling approach to assess the conditions under which WNV circulation occurs. The insights gained from the modelling contribute to the design of proactive strategies addressing the emerging health threat WNV poses in Europe, according to the researchers.
West Nile virus (WNV) is a vector-borne flavivirus that causes an increasing number of human and equine West Nile fever cases in Europe. While the virus has been present in the Mediterranean basin and the Balkans since the 1960s, recent years have witnessed its northward expansion, with the first human cases reported in Germany in 2018 and the Netherlands in 2020. WNV transmission and amplification within mosquitoes are temperature-dependent. Researchers of Wageningen Bioveterinary Research (WBVR, part of Wageningen University & Research) applied a mathematical modelling approach to assess the conditions under which WNV circulation occurs. The research was financed by the Dutch ministry of Agriculture, Nature and Food Quality.
Model
“We modelled five distinct European regions where previous WNV circulation has been observed within the Netherlands, Germany, Spain, Italy, and Greece”, explains WBVR researcher Eduardo de Freitas Costa. In the model the researchers considered the proportion of mosquito bites on birds that can carry the virus (or dilution), the number of hosts and vectors, the length of mosquito season, and temperature data in specific European regions where WNV has been observed previously.
Reproduction rate
The researchers observed that the number of days in which the basic reproduction number (R0) is above 1, increased over the last 40 years in all five regions. In the Netherlands, the number of days in which the R0 is above 1, is 70 percent lower than in Spain. The temperature in Greece, Spain and Italy allowed for circulation under low vector-host ratios, and at a high dilution. On the other hand in the Netherlands and Germany, given the observed daily temperature, the thresholds for circulation requires a lower dilution and higher vector-host ratios.
Introduction window For the Netherlands, a short window of introductions between late May and mid-June would result in detectable outbreaks. “Our findings revealed that the temperate maritime climate of the Netherlands allows WNV circulation primarily during warmer summers, and only under high vector-host ratios.”