The fourth (4th) case of West Nile Virus in California in 2024 was confirmed in San Joaquin County. On August 8, 2024, a vaccinated 9-year-old Quarter Horse gelding in San Joaquin County displaying neurologic signs was confirmed positive for West Nile Virus. The gelding is alive and recovering.
Diagram showing mosquito to bird as vectors then to horse and to human as incidental hosts.
West Nile Virus may cause a wide range of clinical illness ranging from mild 'flu-like' signs to encephalitis (inflammation of the brain) that may be fatal to both humans and horses.
© 2020 by Department of Health
The third (3rd) case of West Nile Virus in California in 2024 was confirmed in Sacramento County.On August 2, 2024, an unvaccinated 3-year-old Thoroughbred gelding in Sacramento County displaying neurologic signs was confirmed positive for West Nile Virus. The gelding is alive and recovering.
The second (2nd) case of West Nile Virus in California in 2024 was confirmed in Yuba County. On July 25, 2024, a 16-year-old undervaccinated Quarter Horse gelding in Yuba County displaying neurologic signs was confirmed positive for West Nile Virus. The gelding was euthanized due to severity of clinical signs.
The first (1st) case of West Nile Virus in California in 2024 was confirmed in Placer County. On July 25, 2024, a 6-year-old Andalusian cross stallion in Placer County with an unknown vaccine history displaying neurologic signs was confirmed positive for West Nile Virus. The stallion is alive and recovering.
West Nile Virus
West Nile Virus (WNV) is a mosquito-borne virus first detected in the United States (U.S.) in the New York City area in 1999. Since 1999, the virus has spread throughout the U.S. and Canada, infecting birds, humans, horses, and other animals. As of 2015, more than 27,000 horses in the U.S. have been infected since the disease was first identified. The virus is maintained in the wild bird population and is spread between birds by mosquitos. Birds are considered the natural reservoir for WNV since high levels of virus circulate in their bloodstream. Mosquitos acquire WNV in blood meals from infected birds and pass it on to other birds, animals, and people. Mosquitos that feed on an infected horse or human have not demonstrated the ability to ingest enough of the virus to transmit it to other animals or humans; therefore, horses and humans are considered “dead end hosts.”
Clinical Signs
West Nile Virus may cause a wide range of clinical illness ranging from mild” flu-like” signs to encephalitis (inflammation of the brain) that may be fatal to both humans and horses. While horses are susceptible to WNV infection, many infected horses do not develop clinical illness and recover uneventfully.
Consult a veterinarian if your horse exhibits any of the following clinical signs:
- Fever
- Incoordination, especially in rear limbs, causing stumbling and falling
- Generalized weakness, muscle twitching, seizures or coma
- Drooping lips and lip smacking, head drooping, grinding teeth
- Hypersensitivity to touch or sound
- Recumbency (inability to rise)
Diagnosis
Any horse displaying abnormal behavior or neurologic signs should be examined by a veterinarian to rule out neurologic diseases, such as WNV, Equine Herpes Virus-1, Equine Protozoal Myeloencephalopathy, Eastern/Western Encephalomyelitis and Rabies.
Blood samples should be collected by a veterinarian and sent for diagnostic testing. The WNV IgM capture ELISA test is a specific test that detects acute WNV infection in animals, is usually positive within six (6) days post infection and can remain positive for up to two (2) months post infection. A positive WNV IgM Capture ELISA test is indicative of recent infection. Unexposed animals vaccinated for WNV will test negative on the WNV IgM capture ELISA test.
Treatment and Prognosis
Currently, there is no specific treatment for WNV. Supportive care includes administration of anti-inflammatory drugs and intravenous fluids. Recumbent WNV positive horses are at a higher risk of dying or requiring euthanasia. For horses exhibiting clinical signs of WNV, the case fatality rate is approximately 33%; however, many infected horses will fully recover following infection.
Prevention
Vaccination and mosquito control minimize the risk of WNV infection in horses. Although the number of WNV infected horses has declined over the last five years, WNV remains an important disease in unvaccinated horses. The American Association of Equine Practitioners (AAEP) recommends incorporation of a WNV vaccine as an annual core vaccine in equine vaccination protocols. Currently, there are four United States Department of Agriculture (USDA)-licensed WNV vaccines available for horses in the U.S. Horse owners should consult with their veterinary practitioner to ensure current WNV vaccination status of their horses.
Mosquito Control
Minimizing horse exposure to mosquitoes during the peak mosquito feeding periods at dawn and dusk decreases the risk of horse exposure to WNV. Application of mosquito repellant can also effectively reduce the number of mosquito bites on horses. Mosquito control efforts to eliminate mosquito breeding sites are also important. Methods to eliminate mosquito breeding sites include:
- Draining unnecessary standing water found in wheelbarrows, tires, etc.
- Cleaning water containers at least weekly (i.e., bird baths, plant saucers)
- Scheduling pasture irrigation to minimize standing water
- Keeping swimming pools optimally chlorinated and draining water from pool covers
- Stocking of water tanks with fish that consume mosquito larvae (Contact local mosquito control for assistance) or use mosquito “dunk” available at hardware stores.
Press release by CDFA