West Nile Virus: Why Its Season Is Longer and Reach Is Wider Than Ever

West Nile Virus (WNV), once considered a localized threat, has evolved into a global public health concern. It is currently the most widespread mosquito-borne disease in the continental United States and is increasingly affecting regions across Europe, Africa, and Asia. But what’s driving this expansion?

How Climate Change is Extending West Nile Virus Season

Recent studies show that rising global temperatures and changing precipitation patterns are extending the mosquito breeding season and increasing virus transmission windows. In New York State, for example, the WNV season has lengthened by an average of 24.8 days over the past 25 years - starting earlier and ending later than before.

Warmer temperatures:

• Accelerate moquito development
• Increase biting frequency
• Shorten virus incubation periods within mosquitoes

These changes make it easier for WNV to spread and persist in new regions.

How are Ecosystems Shifting in Relation?

WNV is maintained in a bird-mosquito cycle, and climate change is altering bird migration patterns, which affects how and where the virus spreads. Birds are arriving earlier or later in breeding grounds, shifting the timing and geography of virus transmission.

Why Mosquito Habitats Are Growing in Urban Areas

Urban environments with stagnant water - like storm drains, birdbaths, and construction sites - create ideal mosquito habitats. Additionally, deforestation, irrigation, and land development are expanding mosquito-friendly zones into previously unaffected areas.

Where is West Nile Virus Spreading in 2025?

A 2024 scoping review found that WNV is now present in 59.2% of North America and 28.3% of Europe, with increasing cases in Africa and Asia. The virus’s spread is highly influenced by local climate conditions, but global warming is making more regions suitable for transmission.

What This means for Public health

For blood centers and transfusion teams, these shifts aren’t abstract, they’re reshaping your screening timelines and donor pools.

Longer seasons and wider reach mean:

• More frequent outbreaks
• Higher risk of neuroinvasive disease
• Greater strain on blood centers from surging NAT testing volumes to nationwide donor deferrals blurring regional lines

Experts recommend:

• Enhanced surveillance, like integrating blood donor data into AI-driven predictive models for early outbreak flagging
• Cross-sector collaboration (One Health approach)
• Targeted public education for donors (e.g. ‘Delay if you’ve been in a hot zone’) alongside ramped-up mosquito control to protect supply chains

Conclusion

West Nile Virus is no longer a seasonal or regional issue - it’s a climate-driven global threat. As transmission seasons lengthen and geographic boundaries blur, your team’s proactive strategies are essential to frontline defense. What’s one step your center can take today?

Learn how Grifols supports proactive WNV screening and multi-virus detection with the Procleix Arboplex Assay