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WIREs Clim Change
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West Nile virus, climate change, and circumpolar vulnerability

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Climate has strong impacts on the spatial ranges of vector‐borne infectious diseases as well as the timing and intensity of disease outbreaks; these and shifting challenges to human health driven by future climate change are critical concerns. Many diseases of tropical origin, including West Nile virus (WNV), are sensitive to climate and likely to change their distributions in the coming decades. The 1999 outbreak of WNV in North America is an example of rapid viral adaptation to a new geographic area while recent outbreaks in Europe demonstrate the capacity of multiple viral strains to expand rapidly. WNV is one of the most widely distributed arboviruses and has displayed high rates of mutability, adaptability, and virulence. Northward expansion of WNV is happening in Europe and North America and may make WNV an increasingly worrying health risk at higher latitudes. Circumpolar northward expansion of WNV’s enzootic range appears unlikely over the coming century—at least for sustained enzootic transmission—but isolated and ephemeral transmission events might occur if the virus were to be introduced by migrating birds during warm months. Human populations in this area are at greater risk for health impacts from WNV transmission due to limited healthcare in rural areas, higher underlying morbidity in indigenous populations, and prolonged human‐environment interactions (in populations engaging in traditional lifestyles). This review presents a multidisciplinary synthesis on WNV and climate change, potential for WNV expansion, and the vulnerability of the circumpolar north. WIREs Clim Change 2016, 7:283–300. doi: 10.1002/wcc.382

Schematic showing the West Nile virus transmission cycle between Culex mosquitoes and several representative species of passerine birds, with tangential transmission to humans and horses, which are dead‐end hosts.
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Present and future climate classifications for North America and Europe based on the Koppen–Geiger system, which categorizes global climates based on annual patterns of temperature and precipitation. The top panel depicts classes for the observed period, 1976–2000, and the bottom panel represents multimodel mean climate under scenario B1 (similar to RCP4.5 in Figure ) for 2076–2100 from the IPCC's 4th Assessment Report. Published present‐day northern limits for the key WNV vectors Culex pipiens and Culex tarsalis are shown for reference in red and blue across their respective geographic ranges. The range of C. pipiens spans most of the northern hemisphere, and C. tarsalis is limited to western North America. Note the northward expansion of Dfa and Dfb zones in North America (dark and medium purple), which are characterized by winter snows, year‐round humidity, and warm or hot summers that currently support WNV transmission each year. Areas of Europe with hot summers (indicated by abbreviations ending in “a”: Cfa, Dfa, and Csa) also have had WNV disease cases annually and these areas are expected to expand with climate change.
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Suitability of July mean temperatures for WNV transmission in North America and Europe for 1950–2000 (upper panel) and as a multimodel mean for the moderate‐warming representative concentration pathway 4.5 in 2070 from the IPCC's 5th Assessment Report (lower panel). Transmission risk levels are assigned from 1 to 5 (blue to red) following threshold definitions in the California Mosquito‐Borne Virus Surveillance and Response Plan based on experimental estimates of extrinsic incubation of WNV. Thresholds for risk correspond to the following temperatures: 1 (<13.4°C), 2 (13.4–18.3°C), 3 (18.4–22.2°C), 4 (22.3–26.1°C), and 5(>26.1°C). Both observed and future temperatures were obtained from WorldClim (http://worldclim.org), and future temperatures for RCP4.5 were averaged across the ensemble of available models.
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