West nile virus (WNV) is a zoonotic mosquito-borne flavivirus which is endemic in the United States (U.S.) and Canada, Africa, Europe, the Middle East, and West Asia. From 1999 through 2016, there have been 27,726 confirmed cases of WNV in horses in the United States (USDA APHIS, 2017).

WNV is an arbovirus which is maintained in a primary enzootic cycle involving mosquitoes (Culex spp.) and wild migratory birds. Migratory birds may carry the virus to new regions and the mosquitoes spread it. In temperate regions, mosquitoes hatch in the late spring and feed on birds through the spring and summer so that the virus population in birds and mosquitoes is much amplified by late summer/early autumn. At that time, certain populations of mosquitoes which serve as “bridge vectors” also feed on humans and horses and other mammals, as well as on birds, creating a secondary cycle that can lead to clinical disease outbreaks of WNV in humans, unvaccinated horses, and possibly other animals.

What are the symptoms of WNV in horses?

Clinical signs of WNV are often highly variable, including their duration. Of those horses that survive the course of the disease and appearing as if they fully recovered, 30% will have a relapse in clinical signs within 7 to 10 days. Overall, 30% of horses affected by WNF will eventually progress to complete paralysis of one or more legs. Most horses end up getting euthanized for humane reasons or they may suddenly die. The most common symptoms found in horses infected with WNV include:

• Fever: Mild to moderate fever of 102-103°F (38.6-39.4°C) concurrent with loss of appetite and depression.


• Muscle fasciculations: Fine and course fasciculations of the muscles of the horse's face and neck are most common. Fasciculations often involve all four legs and body; sometimes they occur on the muzzle and eyelids. When severe, fasciculations may impede the horse from performing normal activities such as walking, eating, and interacting with other horses and people.
• Personality changes: Often involving complete change in personality of the horse. Horses that are usually quiet may have periods of hyperexcitability and apprehension, sometimes to the point of aggression. Horses that are usually aggressive may become oddly compliant and quiet. During periods of hyperexcitability, affected horses will sometimes develop a narcolepsy-like sleep disorder.
• Cranial nerve defects: Frequently are abnormal for short periods of time. Tongue weakness, head tilt and muzzle deviation are the most common. Difficulty breathing as a result of nervous system dysfunction can sometimes occur as a result of respiratory failure and gastrointestinal disturbances.
• Spinal abnormalities: Horses may develop ataxia and paresis involving different combinations of forelimbs or hindlimbs, or by a flaccid paresis that is localized to one or more legs. The condition may only last a short period, or horses may suddenly become recumbent.

How do horses get WNV?

WNV is transmitted to horses through mosquitoes. Other arthropods such as ticks and lice might also have minor roles in transmission. Over 25,000 clinical cases of WNV have been diagnosed in horses. Cases occur during peak mosquito activity months in northern climates and is year-round in tropical and subtropical climates.

What is the incubation period for WNV?

The incubation period for WNV is 3 to 15 days. The majority of infected horses don't present any clinical signs. The mortality rate is approximately 1 in 3 clinically affected unvaccinated horses.

How is WNV diagnosed?

The diagnosis of WNV is currently based history (whether or not the horse has received the vaccination), clinical signs, and laboratory tests which demonstrate one or more of the following results:

• Virus isolation or reverse transcriptase–polymerase chain reaction (RT-PCR) detection of WNV from tissue, blood, or cerebrospinal fluid.
• A four-fold increase in plaque reduction neutralization test (PRNT) antibody titers between paired serum samples taken two weeks apart (in temporal association with clinical signs of disease).
• Detection of IgM antibody to WNV by IgM-capture ELISA.
• A neutralizing titer of more than 1 : 10 by PRNT in a single serum sample.