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West Nile Virus
Ainsley Yuknis, Research Assistant
Katherine Kamminga, Ph.D., Assistant Professor of Entomology
General Information
West Nile Virus (WNV) is a mosquito-borne virus that primarily spreads during the summer months. While most infections show no symptoms, the virus can lead to severe neurological health issues, including encephalitis, in both humans and horses.
Transmission Cycle
Symptoms and Severity
| Impact Level | Frequency | Clinical Presentation |
|---|---|---|
| Asymptomatic | 80% | No visible symptoms; most people are unaware that they are infected |
| Mild | 19% | Presents as a mild cold or flu-like symptoms; rarely diagnosed |
| Severe | 1% | Requires hospitalization and can lead to permanent neurological damage or even death |
WNV does not spread directly from person to person, but by a specific biological cycle:
- Reservoirs: Birds act as the primary "reservoir" for the virus.
In Humans
- Vectors: Mosquitoes become infected when they feed on infected birds, then transmit the virus to other animals through bites.
- Headaches and fever.
- Inflammation of the brain and spinal cord.
- Vision problems or coma in severe cases.
- Dead-End Hosts: Humans and horses can contract the virus, but they do not develop high enough viral levels to pass it to other humans, animals, or back to mosquitoes.
Note: Most infected persons develop long-term immunity.
In Horses
- Rare Cases: Transmission can occur through blood transfusions or organ transplants, though this is rare.
- Encephalomyelitis: Inflammation of the brain with loss of coordination (ataxia), teeth grinding, muscle spasms, or blindness.
- Mortality: Approximately 30% of horses that develop encephalomyelitis do not survive.
Prevention and Protection
Rainy and warm weather increases mosquito populations and infection risks. There is no vaccine or specific treatment for humans; prevention is the only defense.
Action Steps:
- Eliminate Mosquito Habitat: Empty troughs, puddles, and old tires that collect rain water.
- Biological Control: Introduce mosquito-eating fish into small, contained ponds.
- Vaccination: Ensure horses are vaccinated annually. Human vaccines are not available.
Tracking Outbreaks
Stay informed during the summer months to understand the risk level in your county:
- ArboNET: The CDC’s interactive map displays reported cases across the country.
- Local Monitoring: Check the local news, published reports, or the health department for information on virus activity, e.g., testing of WNV in birds and mosquitoes. Be alert when excessive dead birds are seen, and cases of horse WNV disease are reported.
Sample outbreak map of Kentucky via CDC.gov
Scan QR code or visit CDC.gov during the summer months to track current case numbers by location.
Sources
Castillo-Olivares, J., & Wood, J. (2004). West Nile virus infection of horses. Veterinary research, 35(4), 467-483.
Centers for Disease Control and Prevention. (n.d.). ArboNET Surveillance System. https://www.cdc.gov/mosquitoes/php/arbonet/index.html.
Figueiredo, L. T. M. (2007). Emergent arboviruses in Brazil. Revista da Sociedade Brasileira de Medicina Tropical, 40(2), 224-229.
Gould, C. V., Staples, J. E., Guagliardo, S. A. J., Martin, S. W., Lyons, S., Hills, S. L., Petersen, L. R. (2025). West Nile virus: a review. JAMA.
Ng, T., Hathaway, D., Jennings, N., Champ, D., Chiang, Y. W., & Chu, H. J. (2003). Equine vaccine for West Nile virus. Developments in biologicals, 114, 221-227.
Ronca, S. E., Ruff, J. C., & Murray, K. O. (2021). A 20-year historical review of West Nile virus since its initial emergence in North America: Has West Nile virus become a neglected tropical disease?. PLoS neglected tropical diseases, 15(5), e0009190.
For more information, contact:
Ainsley Yuknis Research Assistant Ainsley.Yuknis@kysu.edu
Katherine Kamminga
Katherine Kamminga Assistant Professor of Entomology Katherine.Kamminga@kysu.edu
Acknowledgments
Thank you to Professor Alexander Lai, Jennifer Rydzewski, and the DuPage County Forest Preserve for serving as reviewers for this info sheet.