West Nile Virus Encephalomyelitis

TL;DR. West Nile Virus Encephalomyelitis is a serious, mosquito-borne viral infection in horses that causes severe inflammation of the brain and spinal cord, leading to life-threatening neurological signs like incoordination and muscle tremors.

Mosquitoes are most active during dusk and dawn, making these peak times for potential West Nile Virus transmission.

What is it?

West Nile Virus (WNV) Encephalomyelitis is a serious infectious disease of horses caused by the West Nile virus, a member of the Flavivirus family. The term "encephalomyelitis" refers to inflammation of both the brain (encephalo-) and the spinal cord (myelitis). When the virus enters a horse's system, it targets the central nervous system, disrupting the normal transmission of nerve impulses and leading to profound neurological dysfunction.

First identified in the Western Hemisphere in 1999, West Nile Virus has since become established across North America. The virus is transmitted primarily through the bite of infected mosquitoes. Mosquitoes acquire the virus by feeding on infected birds, which serve as the natural reservoir hosts. When an infected mosquito subsequently bites a horse, it injects the virus into the horse's bloodstream.

Horses, like humans, are considered "dead-end" hosts for West Nile Virus. This means that while horses can become severely ill from the infection, the level of virus circulating in their blood is too low for mosquitoes to pick up and transmit to other animals. Consequently, an infected horse cannot directly transmit the virus to other horses, humans, or companion animals. However, the presence of an infected horse indicates that virus-carrying mosquitoes are active in the immediate area, posing a threat to other unvaccinated horses and humans nearby.

Causes & risk factors

The sole cause of West Nile Virus Encephalomyelitis is the West Nile virus. The primary risk factor for contracting the disease is exposure to infected mosquitoes, particularly species belonging to the Culex genus. These mosquitoes are most active during the warmer months of the year, typically from mid-summer through late autumn, or year-round in warmer, subtropical climates.

Environmental factors play a significant role in the prevalence of the disease. Areas with standing water—such as poorly drained pastures, stagnant ponds, uncleaned water troughs, clogged gutters, and discarded tires—provide ideal breeding grounds for mosquitoes. Weather patterns, such as heavy rainfall followed by warm temperatures, can cause mosquito populations to surge, dramatically increasing the risk of transmission.

There are no documented breed predispositions for West Nile Virus Encephalomyelitis. Any horse, regardless of breed, age, or sex, is susceptible to the virus if they are unvaccinated and exposed to infected vectors. However, older horses or those with compromised immune systems may be at a higher risk for developing more severe clinical signs and experiencing a poorer outcome.

Signs to watch for

The clinical signs of West Nile Virus Encephalomyelitis can develop rapidly, often progressing over a matter of hours or days. The severity of these signs depends on the extent of the inflammation within the central nervous system.

Cardinal Signs

• Ataxia: This is the most common and characteristic sign of the disease. Affected horses exhibit incoordination, stumbling, swaying, or a clumsy, uncoordinated gait. They may cross their legs while walking, have difficulty turning, or stand with their legs spaced widely apart to maintain balance.
• Muscle fasciculations: These are involuntary muscle twitches or tremors, most commonly observed around the muzzle, face, neck, and shoulders. These fine ripples under the skin are highly suggestive of West Nile Virus infection.

Common Signs

• Fever: A elevated body temperature is common, though it is often transient and may have resolved by the time severe neurological signs become apparent.
• Weakness: Horses may display generalized weakness, particularly in the hind limbs, leading to toe-dragging or difficulty standing up from a resting position.

Occasional Signs

• Altered mentation: Some horses show changes in behavior or consciousness. This can range from mild depression, lethargy, and sleepiness to hyperexcitability, sensitivity to touch or sound, or apparent blindness.
• Recumbency: In severe cases, horses may become unable to stand (recumbent). A horse that goes down and cannot rise is in a critical state.

Ataxia, or severe incoordination, is a cardinal sign of West Nile Virus Encephalomyelitis in horses.

EMERGENCY WARNING: Any sudden onset of neurological signs, such as stumbling, muscle twitching, or difficulty standing, is a veterinary emergency. If your horse exhibits these signs, contact your veterinarian immediately. Do not attempt to force a severely uncoordinated or thrashing horse to move, as this poses a significant safety risk to both the horse and handlers.

How vets diagnose it

Diagnosing West Nile Virus Encephalomyelitis requires a combination of a thorough physical and neurological examination, a history of potential mosquito exposure, and specific laboratory testing. Because the clinical signs of WNV closely mimic other serious neurological diseases—such as Rabies, Equine Protozoal Myeloencephalitis (EPM), and Eastern or Western Equine Encephalomyelitis (EEE/WEE)—accurate diagnostic testing is essential.

• IgM Capture ELISA (Gold Standard): The immunoglobulin M (IgM) capture enzyme-linked immunosorbent assay is the gold standard diagnostic test for active West Nile Virus infection in live horses. This blood test detects IgM antibodies, which are produced rapidly by the horse's immune system in response to an active or very recent infection. Because IgM antibodies do not persist long-term, a positive result confirms active infection rather than historical exposure or standard vaccination.
• Cerebrospinal Fluid (CSF) Analysis: Your veterinarian may perform a spinal tap to collect cerebrospinal fluid from the space surrounding the spinal cord. Analysis of this fluid typically reveals signs of active inflammation, such as elevated protein levels and an increased number of white blood cells (mononuclear pleocytosis).
• RT-PCR (Reverse Transcription Polymerase Chain Reaction): This molecular test can detect the genetic material (RNA) of the virus. While RT-PCR can be performed on blood or CSF, the virus is often cleared from these fluids by the time neurological signs appear, which can lead to false-negative results. RT-PCR is highly reliable when performed on brain or spinal cord tissue post-mortem to confirm the diagnosis.

The IgM capture ELISA is the gold standard diagnostic test used to confirm active West Nile Virus infection.

Treatment options

There is no specific antiviral medication available to cure West Nile Virus Encephalomyelitis. Treatment is entirely supportive and aimed at reducing central nervous system inflammation, controlling pain, and preventing secondary complications while the horse's immune system fights off the virus.

First-Line Anti-Inflammatory Therapies

• Non-Steroidal Anti-Inflammatory Drugs (NSAIDs): Medications such as Flunixin Meglumine are commonly administered to control fever, alleviate pain, and reduce mild to moderate inflammation within the nervous system.
• Corticosteroids (Glucocorticoids): In acute or severe cases, potent anti-inflammatory agents like Dexamethasone may be utilized. Corticosteroids are highly effective at rapidly reducing severe swelling and inflammation in the brain and spinal cord, though they must be used cautiously under strict veterinary supervision.
• Dimethyl Sulfoxide (DMSO): Often administered intravenously as a diluted solution, DMSO acts as a powerful anti-inflammatory agent and free radical scavenger. It helps reduce cerebral edema (brain swelling) and pressure within the central nervous system.

Supportive Nursing Care

For horses with moderate to severe neurological deficits, intensive nursing care is vital to survival. This includes:

• Fluid Therapy: Intravenous fluids may be required if the horse is unable to drink normally, ensuring proper hydration and electrolyte balance.
• Slinging: In some hospital settings, specialized slings may be used to support weak or partially paralyzed horses, helping them remain standing and preventing the muscle damage associated with prolonged lying down.
• Environmental Safety: Placing the horse in a heavily bedded stall with padded walls can prevent self-inflicted trauma if the horse is uncoordinated or thrashing.
• Frequent Turning: If a horse is completely recumbent and cannot stand, they must be rolled or turned from side to side every few hours to prevent pressure sores, lung congestion, and muscle damage.

Prognosis

The prognosis for horses showing clinical signs of West Nile Virus Encephalomyelitis is guarded. According to standard veterinary consensus, approximately 30% to 40% of horses that exhibit clinical signs of the disease die or are humanely euthanized due to the severity of the neurological damage or complications associated with prolonged recumbency.

For horses that survive the acute phase of the illness, the road to recovery can be long, often taking several weeks to months. While many horses make a full recovery, a significant portion of survivors (up to 20-30%) may suffer from residual, permanent neurological deficits. These deficits can include persistent mild incoordination, muscle weakness, or localized muscle wasting. In some cases, these lingering deficits may render the horse unsafe to ride or work, affecting their long-term utility and quality of life.

Prevention

Fortunately, West Nile Virus Encephalomyelitis is highly preventable through a combination of vaccination and environmental management.

Vaccination

Vaccination is the single most effective tool for protecting horses against West Nile Virus. Several highly effective vaccines are commercially available. The American Association of Equine Practitioners (AAEP) classifies the West Nile Virus vaccine as a core vaccine, meaning it is recommended for all horses in North America.

• Primary Series: Unvaccinated horses require an initial primary series of two doses administered several weeks apart.
• Boosters: Annual booster vaccinations should be administered in the spring, prior to the onset of peak mosquito season. In regions with year-round mosquito activity or high-risk environments, veterinarians may recommend bi-annual (twice-yearly) boosters.

Mosquito Control (Vector Mitigation)

Reducing exposure to mosquitoes is critical to minimizing infection risk. Owners should implement the following management practices:

• Eliminate Standing Water: Regularly empty, clean, and refill water troughs, buckets, birdbaths, and wading pools at least once a week. Drain stagnant puddles, clear clogged gutters, and remove items that can collect rainwater, such as old tires or tarps.
• Use Larvicides: For standing water sources that cannot be drained, use EPA-approved mosquito dunks containing Bacillus thuringiensis israelensis (Bti) to kill mosquito larvae.
• Stabling: Keep horses indoors in screened stalls during peak mosquito feeding times, which are typically dawn and dusk.
• Air Movement: Install high-velocity fans in horse stalls to disrupt mosquito flight patterns, as mosquitoes are weak fliers.
• Repellents: Apply equine-approved insect repellents containing pyrethrins or permethrins daily, and consider the use of fly sheets and masks to provide a physical barrier.

When to call your vet

Because West Nile Virus Encephalomyelitis can progress rapidly and carries a guarded prognosis once clinical signs appear, early veterinary intervention is critical.

Contact your veterinarian immediately if you observe any of the following signs in your horse:

• Sudden stumbling, tripping, or uncoordinated movement (ataxia)
• Involuntary twitching or trembling of the muscles around the muzzle, face, or neck
• Unexplained weakness, particularly in the hindquarters, or dragging of the toes
• A sudden change in behavior, such as extreme lethargy, depression, or hypersensitivity
• Inability to stand up or difficulty rising from the ground

Do not wait to see if the signs improve. Prompt diagnosis and aggressive anti-inflammatory therapy offer the best chance of a successful outcome.

Sources

• Reed, S. M., Bayly, W. M., & Sellon, D. C. (Eds.). Equine Internal Medicine. Saunders.
• Smith, B. P. (Ed.). Large Animal Internal Medicine. Mosby.
• American Association of Equine Practitioners (AAEP). Equine Vaccination Guidelines: West Nile Virus.