West Nile Virus


As you know West Nile is here and may be here to stay. Thankfully, there are many things that you can do to help protect your horses.


West Nile encephalitis is an infection of the brain brought on by West Nile Virus.
The West Nile virus was first discovered in an adult woman in the District of Uganda in 1937. It was not until 1957 that the virus became recognized as a cause of severe human meningoencephalitis (inflammation of the spinal cord and brain). It was discovered in horses in the early 1960’s where it was found in Egypt and France.
North America saw the first appearance of the West Nile virus in humans and horses around 1999. To date, there have been 447 horses in 42 counties (including San Diego) that were confirmed cases of equine illness caused by West Nile virus in California alone of these, 199 have died or had to be euthanized. There have also been 2,917 cases of birds in 52 counties that were West Nile virus positive.

Life/Transmission Cycle

The basic transmission cycle of the virus begins when mosquitoes feed on infected birds. The mosquitoes then transmit the virus to humans and animals while biting to consume blood. The virus is stored in the mosquito’s salivary glands. During the bite the virus can be injected into the human or animal, where it may multiply and cause illness.
Severe illness or death in horses can result if the virus, once it has multiplied in the blood system, crosses the blood brain barrier and infects the brain. Once in the brain it causes inflammation and interferes with normal central nervous system functions.

So far there is no documented evidence of transmission of the virus from person-to person, animal-to-person or horse-to-horse. However, horses with the virus should be isolated.

Symptoms of West Nile virus

Equine symptoms include ataxia (uncoordinated movement), weakness of limbs, recumbency (leaning or laying down), muscle fasciculation (muscle twitches) and sometimes death. A fever has been detected in less than 25 percent of confirmed cases; therefore a fever is not necessarily a symptom of the disease. Birds that are infected may die and can then be tested for West Nile.


Many horses that do survive after being diagnosed with West Nile require nursing care around the clock. These horses will often not return to the same level of competition that they were at before contracting the disease.

Prevention of West Nile virus

Collected data indicates that the majority of horses recover from the virus with proper treatment. Taking precautions is imperative in order to reduce the risk of contracting West Nile virus. Some recommendations to reduce the risk of infection for your horses include:
Vaccinate your horses semi- annually for the West Nile virus.
Using insect repellent (those containing permethrin are safe and efficacious.)
Eliminating standing water (i.e. in old tires, wheelbarrows, plastic pools)
Keep livestock watering troughs, birdbaths, and any other necessary standing water clean. Using Mosquito Dunks is also a good way to prevent mosquitoes in standing water..

If you find a dead bird the number to call to report it for possible West Nile testing
(858) 694-2888. They are primarily interested in raptors, such as blue jays, crows and ravens.

For more information on West Nile virus visit http://www.westnile.ca.gov or http://www.cdfa.ca.gov/ahfss/ah/wnv_info.htmTo set up an appointment to vaccinate your horses for West Nile please call our office (619) 659-3532.