The fifth (5th) case of West Nile Virus in California in 2025 was confirmed in Sacramento County. On September 29, 2025, an unvaccinated 1-year-old Friesian colt in Yolo County displaying neurologic signs was confirmed positive for West Nile Virus. The colt died.
The fourth (4th) case of West Nile Virus in California in 2025 was confirmed in Yolo County. On September 23, 2025, a partailly vaccinated 9-year-old Dutch Warmblood stallion in Yolo County displaying neurologic signs was confirmed positive for West Nile Virus. The stallion is alive and recovering.
The third (3rd) case of West Nile Virus in California in 2025 was confirmed in Stanislaus County. On September 18, 2025, a vaccinated 2-year-old Quarter Horse mare in Stanislaus County displaying neurologic signs was confirmed positive for West Nile Virus. The mare is alive and recovering.
The second (2nd) case of West Nile Virus in California in 2025 was confirmed in Contra Costa County. On September 16, 2025, a vaccinated 8-year-old Arabian gelding in Contra Costa County displaying neurologic signs was confirmed positive for West Nile Virus. The gelding is alive and recovering.
The first (1st) case of West Nile Virus in California in 2025 was confirmed in Santa Barbara County. On August 29, 2025, an undervaccinated 5-year-old Quarter Horse mare in Santa Barbara County displaying neurologic signs was confirmed positive for West Nile Virus. The mare is alive and recovering.
To date for 2025, a total of five (5) horses have been confirmed positive for WNV in California. The positive horses were located in Contra Costa (1), Sacramento (1), Santa Barbara (1), Stanislaus (1) and Yolo (1) Counties. One (1) horse was unvaccinated, two (2) horses were undervaccinated, two (2) horses were vaccinated, and zero (0) horses had unknown vaccine history. Four (4) horses are alive and recovering and one (1) horse was euthanized or died.
In 2024, a total of seven (7) horse were confirmed positive for WNV in California. The positive horses were located in Los Angeles (1), Placer (1), Sacramento (1), San Diego (1), San Joaquin (1), Tehama (1) and Yuba (1) Counties. One (1) horse was unvaccinated, three (3) horses were undervaccinated, one (1) horse was vaccinated, and two (2) horses had unknown vaccine history. Six (6) horses are alive and recovering and one (1) horse was euthanized or died.
In 2023, a total of thirty-one (31) horses were confirmed positive for WNV in California. The positive horses were located in Butte (1), Fresno (2), Kern (2), Kings (1), Madera (1), Merced (2), Napa (1), Orange (1), Placer (1), Riverside (4), Sacramento (3), San Bernardino (1), San Joaquin (2), San Luis Obispo (2), San Mateo (1), Santa Clara (1), Siskiyou (1), Stanislaus (2), Tulare (1) and Yuba (1) Counties. Fifteen (15) horses were unvaccinated, four (4) horses were undervaccinated, nine (9) horses were vaccinated, and three (3) horses had unknown vaccine history. Twenty-two (22) horses are alive and recovering, eight (8) horses were euthanized or died, and one (1) horse has unknown status.
For 2022, a total of sixteen (16) horses were confirmed positive for WNV in California. The first case was confirmed on July 13, 2022 and the last case on October 26, 2022. Horses ranged in age from under 1 year to 15 years (mean 6, median 5), with 10 males and 6 females affected, with multiple breeds reprsesented, but an overrepresentation of Quarter Horse breeds/crosses (over half of confirmed cases).
The positive horses were located in Kern County (3), Kings County (1), Los Angeles (1), Nevada County (1), Sacramento County (2), San Luis Obispo County (1), San Joaquin County (1), Stanislaus County (1), Tehama County (2), and Tulare County (3). Ten (10) horses were unvaccinated, four (4) horses had unknown vaccine history, and two (2) were vaccinated. Eleven (11) horses recovered, one (1) horse died, and four (4) horses were euthanized.
For 2021, a total of thirteen (13) horses were confirmed positive for West Nile Virus in California. The first case was confirmed on June 28, 2021 and the last case was confirmed on October 6, 2021. Horses ranged in age from 1 to 14 years old (mean 4 years, median 3 years), with 9 females and 4 males affected, and multiple breeds represented.
The positive horses were located in Amador County (1), Fresno County (2), Kings County (1), Merced County (1), Sacramento County (2), San Joaquin County (3), Stanislaus County (1), and Yuba County (2). Nine (9) horses were unvaccinated, two (2) horses had unknown vaccine history and two (2) were vaccinated. Eleven (11) horses are alive, one (1) horse died, and one (1) horse was euthanized. CDFA continually monitors and investigates equine neurologic cases for the presence of WNV in California. CDFA urges horse owners to consult their veterinarian concerning a WNV vaccination program to ensure maximum protection of their horses.