Diagnosis and testing for West Nile fever (West Nile encephalitis) in horses

What is West Nile fever?

West Nile fever (also known as West Nile encephalitis) is an inflammation of the central nervous system that is caused by infection by West Nile virus (WNV). WNV is distributed worldwide. It occurs naturally in parts of Africa, Asia, the Middle East, Europe, North America, Central America, Caribbean Islands, and South America. WNV was first identified in the U.S. in 1999 in New York and is currently found across the continental U.S.

How is a horse infected with West Nile virus?

People and animals can become infected from the bite of certain kinds of mosquitoes that are infected with the virus. WNV is maintained in nature in a transmission cycle between birds and mosquitoes. Horses are dead-end hosts, meaning they do not transmit the infection to other horses. Mosquitos, primarily the Culex species, acquire the virus from infected birds and transmit it to susceptible horses, humans, and other animals. WNV has been detected in over 150 species of birds in the U.S., many of which do not develop outward disease. Corvids, such as the American crow and blue jays, along with other birds like the house finch are likely efficient hosts. WNV transmission corresponds to the mosquito feeding and life cycle. Its peak is from mid-July to October in the Pacific Northwest region of the U.S. Infections subside after the first insect-killing frost.

What are the clinical signs of West Nile virus infection in individual horses?

Infection with West Nile virus does not always lead to signs of illness in people or animals. Horses are a species that is susceptible to infection with the virus. In horses that do become ill, the virus infects the central nervous system and may cause symptoms of encephalitis. Most clinically affected horses exhibit general loss of appetite, depression, and fever, along with neurological signs such as ataxia (stumbling, staggering, wobbly gait, or incoordination) and any combination of the following:

  • circling
  • hind limb weakness
  • recumbence or inability to stand
  • multiple limb paralysis
  • muscle fasciculation (twitching)
  • altered mental state
  • impaired vision
  • lip droop
  • inability to swallow
  • hyper excitability

It is important to note that not all horses with clinical signs of encephalitis have West Nile encephalitis. Other infectious and non-infectious diseases can cause a horse to have symptoms similar to those resulting from infection with WNV. If you are concerned your horse may be exhibiting signs of encephalitis, please contact your veterinarian.

How do I know if my horse has West Nile fever?

A tentative diagnosis of West Nile fever as a cause of neurological disease in horses is based upon clinical signs as described above. Confirmation of West Nile fever requires laboratory testing.

What are the laboratory tests for West Nile Fever?

  • Serological tests: Serological tests identify the horse’s immune defense (antibody) response to WNV infection. Antibody responses, identified in the blood, can occur very early after infection (IgM antibody) or late after infection (IgG antibody). WADDL uses the WNV IgM ELISA test to detect early antibody responses to WNV infection. An IgM WNV antibody response together with compatible clinical signs of encephalitis is diagnostic for West Nile encephalitis. WNV IgM ELISA is the preferred live animal test for West Nile encephalitis in horses.
  • Virus identification: WNV can be identified directly in animal tissues and fluids. The most common test used is polymerase chain reaction (PCR), which detects viral genetic material (nucleic acids) from multiple types of tissue and fluids. WADDL uses the real time PCR test for WNV. PCR is the preferred test for West Nile encephalitis in horses after death.
  • Pathology: Carcasses can be submitted to WADDL for necropsy (post-mortem examination) to diagnose West Nile fever and WNV infection using the virus identification methods listed above. WADDL pathologists will collect appropriate samples and submit them to WADDL testing laboratories for appropriate testing.

What samples are needed for laboratory identification of West Nile fever?

  • Serological tests: The optimal sample for WNV IgM ELISA is clotted blood (red top tube or serum separator tube).
  • Virus identification: Optimal samples for WNV real time PCR include the following:
    • Choanal (tracheal) swabs from corvid birds (may test tissues as well)
    • Tissue pools or individual spleen, heart, brain, liver, kidney, lung, or lymph nodes from non-corvid birds or other species (heart, spleen or lung are best for birds)
    • Brainstem from horses (levels of viremia are low in horses so blood samples are not optimal for WNV PCR)
    • Fixed paraffin-embedded tissues containing the tissues listed above
    • Whatman FTA cards inoculated with choanal swabs from corvids

How do I submit samples to WADDL for West Nile Fever testing?

We are switching from paper accession forms to an online ordering system that expedites the sample testing process.

If you don’t have an online account, please go to account request to enroll (it takes 1-2 minutes). Once approved, you’ll receive an email with a link to submit your order, or you can go directly to WADDL Client Portal.

This four-step submission process will take you through a series of questions to ensure all required information is provided, so when your package arrives, samples are moved quickly to their testing section(s). Additionally, if you have multiple animals to test, there is an upload function that will save time.

Thereafter, you may log in at any time to view the status of your order, ensure it was received, and view/print results.

Is treatment available for West Nile encephalitis in horses?

There is no specific treatment for West Nile encephalitis in horses. Supportive veterinary care is recommended. It is important to diagnose WNV because infection is an indication that mosquitoes carrying the virus are in the area and need to be controlled to reduce the infection potential in humans, horses, and other animals.

Is a vaccine available to protect against infection with West Nile virus?

A WNV vaccine for horses is readily available from your veterinarian. Because it is impossible to distinguish between vaccinated and naturally infected horses with current testing methods, it is important that vaccination records be kept updated for each horse that receives the vaccine. Horses vaccinated against other viral diseases such as Eastern, Western, and Venezuelan equine encephalitis and equine herpesvirus are not protected against infection with West Nile virus.

How can I protect my horse against infection with West Nile virus?

The strategy for protecting horses from WNV infection is essentially two-pronged:

  • Increase your horse’s immunity to WNV infection through vaccination
  • Decrease your horse’s exposure to mosquito vectors for WNV

Specific recommendations are:

  • Annual vaccination of horses is highly recommended. Detailed vaccination guidelines are available from the American Association of Equine Practitioners (AAEP)
  • Reduce exposure to potentially WNV-infected mosquitoes by use of insect repellants, fans, and screens in shelters
  • Insect vector control by eliminating mosquito habitat (stagnant water sources), regular removal of manure, and weed control

What are the official reporting requirements for West Nile fever in Washington state?

Veterinarians who diagnose potential West Nile virus cases should contact the State Veterinarian’s Office at 360-902-1878 or ahealth@agr.wa.gov.

For more information, contact WADDL directly at 509-335-9696.

Additional information about West Nile Fever can be found at:

Page revised June 2019

Categories: Horses