Equine Neurological Disorders: Diagnosis and Management

Equine neurological disorders affect the brain, spinal cord, and peripheral nerves of horses, producing clinical signs that range from subtle gait changes to recumbency and death. This article covers the most common neurological conditions seen in horses, the diagnostic approach including cerebrospinal fluid analysis and serology, and practical management strategies for horse owners and veterinarians. Early recognition and accurate diagnosis are critical for appropriate treatment decisions and prognosis.

At a Glance

The table below summarizes the most common equine neurological disorders, their causes, key diagnostic methods, and typical clinical presentations.

Clinical Presentation and Recognition

Horses with neurological disease present with a wide range of clinical signs depending on the region of the nervous system affected and the severity of the condition. The Merck Veterinary Manual provides comprehensive information on neurological diseases affecting horses, including diagnostic approaches and treatment considerations (Merck Veterinary Manual, www.merckvetmanual.com). The World Organisation for Animal Health addresses animal health and welfare standards that apply to the management of infectious neurological diseases in horses (World Organisation for Animal Health, Animal Health and Welfare, www.woah.org/en/what-we-do/animal-health-and-welfare).

Gait Abnormalities

Ataxia, or incoordination, is one of the most common presenting signs of neurological disease in horses. Ataxia can be symmetric or asymmetric and may affect all four limbs or be more pronounced in the pelvic limbs. Horses with ataxia may stumble, cross their limbs, or have a wide-based stance. The severity of ataxia is typically graded on a scale from 0 (normal) to 5 (recumbent). Owners should observe horses at walk and trot on a firm, level surface, noting any limb interference, knuckling, or swaying. Turning in tight circles often accentuates pelvic limb ataxia.

Cranial Nerve Deficits

Cranial nerve dysfunction can manifest as facial paralysis, head tilt, difficulty swallowing, tongue weakness, or abnormal eye movements. These signs help localize the lesion within the brainstem. Horses with EPM frequently show asymmetric cranial nerve deficits. To assess cranial nerve function, observe the horse's ability to blink in response to a hand motion near the eye, check for symmetrical ear and lip movement, and note any drooping of the lower lip or eyelid.

Muscle Abnormalities

Muscle fasciculations, tremors, atrophy, or weakness may be observed. West Nile Virus infection characteristically causes fine muscle fasciculations, particularly around the muzzle and neck. Muscle atrophy along the topline is a common finding in chronic EPM cases. Palpation of the epaxial muscles and gluteal muscles can reveal asymmetry or wasting. Owners should document the location and progression of any muscle changes.

Behavioral Changes

Depression, aggression, circling, head pressing, or other behavioral abnormalities may indicate forebrain involvement. These signs require prompt veterinary evaluation. Horses with forebrain disease may also show compulsive walking, yawning, or changes in appetite. Document the onset, frequency, and triggers of any behavioral changes.

Common Neurological Disorders

Equine Protozoal Myeloencephalitis

Equine Protozoal Myeloencephalitis is one of the most frequently diagnosed neurological disorders in horses in the Americas. The condition is caused primarily by the protozoan parasite Sarcocystis neurona, though Neospora hughesi has also been implicated. The Veterinary Clinics of North America: Equine Practice published a comprehensive review of EPM in 2022, covering pathogenesis, diagnosis, and treatment approaches (Equine Protozoal Myeloencephalitis, The Veterinary clinics of North America. Equine practice, 2022, pubmed.ncbi.nlm.nih.gov/35810151).

The life cycle of Sarcocystis neurona involves opossums as definitive hosts and various intermediate hosts. Horses are considered aberrant hosts, meaning the parasite does not complete its life cycle within the horse. Infection occurs when horses ingest feed or water contaminated with sporocysts shed in opossum feces. Management practices that reduce opossum access to feed storage areas and water sources can decrease exposure risk.

Clinical signs of EPM are typically asymmetric and can include ataxia that may be more pronounced on one side, muscle atrophy particularly along the topline, cranial nerve deficits such as facial paralysis or difficulty swallowing, head tilt or circling, and lameness that shifts between limbs. The asymmetric nature of EPM helps distinguish it from many other neurological conditions.

A study examining the seroprevalence of Sarcocystis neurona and its association with neurologic disorders in Argentinean horses found that exposure to the parasite is widespread, though not all exposed horses develop clinical disease (Seroprevalence of Sarcocystis neurona and Its Association With Neurologic Disorders in Argentinean Horses, Journal of Equine Veterinary Science, 2014, doi.org/10.1016/j.jevs.2014.06.002). This finding underscores the importance of diagnostic testing to confirm active neurological disease instead of relying solely on serological evidence of exposure.

West Nile Virus Encephalomyelitis

West Nile Virus is a mosquito-borne flavivirus that can cause severe neurological disease in horses. The virus was first identified in the United States in 1999 and has since become endemic in many regions. Clinical signs of WNV encephalomyelitis include fever, ataxia, muscle fasciculations particularly around the muzzle and neck, hyperesthesia, and progressive weakness that can lead to recumbency. The onset of clinical signs typically occurs 5 to 15 days after infection.

A case study on the treatment of West Nile virus caused encephalomyelitis in horses under stable conditions documented the clinical progression and management of affected animals (Treatment of west nile virus caused encephalomyelitis in horses at stable conditions: Case study, Magyar Allatorvosok Lapja, 2019, api.elsevier.com/content/abstract/scopus_id/85065513697). The study highlighted the importance of supportive care and the variable recovery outcomes depending on the severity of neurological deficits at presentation.

Vaccination is the primary preventive measure for WNV. Horses that have been vaccinated or previously infected typically develop immunity, though booster vaccinations are recommended annually or more frequently in high-risk areas. Mosquito control measures, including eliminating standing water, using insect repellents, and stabling horses during peak mosquito activity at dawn and dusk, reduce exposure risk.

Equine Herpesvirus-1 Myeloencephalopathy

Equine Herpesvirus-1 is a highly contagious virus that can cause respiratory disease, abortion, and neurological disease. The neurological form, known as EHV-1 myeloencephalopathy (EHM), results from vasculitis and thrombosis within the spinal cord and brain. EHM can occur sporadically or as outbreaks in equine populations. The virus is spread through direct contact with infected horses, aerosolized respiratory secretions, and contaminated equipment or personnel.

Clinical signs of EHM include acute onset of ataxia often symmetric, urinary incontinence and bladder atony, decreased tail tone, fever preceding neurological signs, and recumbency in severe cases. The fever often occurs 24 to 48 hours before neurological signs develop, making temperature monitoring an important screening tool during outbreaks.

The American College of Veterinary Internal Medicine provides resources and consensus statements on the diagnosis and management of EHV-1 infections, including biosecurity recommendations for outbreak control (American College of Veterinary Internal Medicine, www.acvim.org). Biosecurity measures for EHV-1 include isolating affected horses, using dedicated equipment, implementing traffic control on the premises, and monitoring temperatures of all exposed horses.

Equine Dysautonomia (Grass Sickness)

Equine dysautonomia, commonly known as grass sickness, is a degenerative condition affecting the autonomic nervous system. The disease is most commonly reported in the United Kingdom and parts of Europe, though cases have been identified in other regions. The Veterinary Clinics of North America: Equine Practice published a review of equine dysautonomia in 2018, covering current understanding of the condition (Equine Dysautonomia, The Veterinary clinics of North America. Equine practice, 2018, pubmed.ncbi.nlm.nih.gov/29398183).

The exact cause of grass sickness remains unknown, though evidence suggests a toxin-mediated mechanism. Clinical signs reflect autonomic dysfunction and include dysphagia (difficulty swallowing), ptosis (drooping eyelids), colic and gastrointestinal stasis, patchy sweating, muscle tremors, and weight loss. The disease is classified as acute, subacute, or chronic based on the speed of onset and severity of signs. Acute cases often progress rapidly to recumbency and death, while chronic cases may survive with ongoing supportive care.

Pituitary Pars Intermedia Dysfunction

Pituitary pars intermedia dysfunction, also known as equine Cushing's disease, is a common endocrine disorder in older horses that can present with neurological signs. The condition results from a pituitary adenoma that causes excessive production of proopiomelanocortin-derived peptides. A 2023 review in the Veterinary Journal addressed the diagnosis of equine pituitary pars intermedia dysfunction, highlighting the importance of endocrine testing in affected horses (Diagnosis of equine pituitary pars intermedia dysfunction, Veterinary journal (London, England : 1997), 2023, pubmed.ncbi.nlm.nih.gov/37805159).

Clinical signs of PPID include hirsutism (long, curly coat that fails to shed), lethargy and poor performance, laminitis, muscle wasting, neurologic signs such as ataxia or weakness, and increased thirst and urination. The neurological signs in PPID are typically mild and progressive, often improving with medical management of the underlying endocrine condition.

Neurologic Disorders of Neonatal Foals

Neonatal foals are susceptible to a unique set of neurological disorders that require specialized diagnostic and management approaches. The Veterinary Clinics of North America: Equine Practice published a review of neurologic disorders of neonatal foals in 2005, covering conditions such as neonatal encephalopathy, sepsis-associated encephalopathy, and congenital anomalies (Neurologic disorders of neonatal foals, Veterinary Clinics of North America Equine Practice, 2005, doi.org/10.1016/j.cveq.2005.04.006).

Common neurological conditions in neonatal foals include neonatal encephalopathy (perinatal asphyxia syndrome), bacterial meningitis, congenital defects of the central nervous system, metabolic disorders such as hypoglycemia or electrolyte imbalances, and trauma during birth. Foals with neurological signs require intensive monitoring and supportive care, including thermoregulation, nutritional support, and seizure management.

Diagnostic Approach

Clinical Examination and Neurological Assessment

The diagnostic approach to a horse with suspected neurological disease begins with a thorough history and complete physical examination. The neurological examination should be systematic and include assessment of mental status and behavior, cranial nerve function, gait and posture, proprioception and conscious proprioception, spinal reflexes, and muscle tone and atrophy.

The Merck Veterinary Manual provides detailed guidance on performing a neurological examination in horses, including specific tests for each component of the nervous system (Merck Veterinary Manual, www.merckvetmanual.com). Key components of the examination include observing the horse at rest and in motion, testing limb placement and hopping responses, assessing tail tone and anal sphincter tone, and evaluating cranial nerve function through menace response, pupillary light reflex, and facial sensation.

Cerebrospinal Fluid Analysis

Cerebrospinal fluid (CSF) analysis is a critical diagnostic tool for many equine neurological disorders. CSF can be collected from the atlanto-occipital or lumbosacral spaces. Analysis typically includes total protein concentration, nucleated cell count and differential, cytology, and specific diagnostic tests such as PCR or antibody testing. CSF collection requires sedation and aseptic technique, and samples should be handled promptly to maintain cell integrity.

A study on molecular detection of Sarcocystis neurona in cerebrospinal fluid from 210 horses with suspected neurologic disease demonstrated the utility of PCR testing for confirming EPM diagnosis (Molecular detection of Sarcocystis neurona in cerebrospinal fluid from 210 horses with suspected neurologic disease, Veterinary Parasitology, 2021, doi.org/10.1016/j.vetpar.2021.109372). PCR testing offers high specificity for detecting parasite DNA in CSF, though sensitivity may vary depending on the timing of sample collection relative to infection.

Biofluid markers of equine neurological disorders have been reviewed from human perspectives, highlighting the potential for novel diagnostic approaches using CSF and blood biomarkers (Biofluid Markers of Equine Neurological Disorders Reviewed From Human Perspectives, Journal of equine veterinary science, 2020, pubmed.ncbi.nlm.nih.gov/32067661). These markers may improve diagnostic accuracy for conditions where current testing has limitations.

Serology and Antibody Testing

Serological testing plays an important role in diagnosing infectious neurological diseases. Common serological tests include IgM capture ELISA for West Nile Virus, serum neutralization tests for EHV-1, immunofluorescent antibody tests for Sarcocystis neurona, and complement fixation tests for various viral agents.

Interpretation of serological results requires consideration of vaccination history, timing of sample collection relative to disease onset, and regional disease prevalence. A single positive serological result may indicate exposure instead of active disease, particularly for EPM where seroprevalence can be high in endemic areas. Paired acute and convalescent samples collected 2 to 4 weeks apart can demonstrate rising antibody titers consistent with recent infection.

Advanced Imaging

Advanced imaging techniques such as magnetic resonance imaging (MRI) and computed tomography (CT) are increasingly available for equine patients. These modalities can identify structural lesions, inflammatory changes, and neoplastic processes within the central nervous system. However, availability is limited to specialized referral centers, and general anesthesia is typically required. MRI is particularly useful for evaluating the cervical spine in horses with suspected cervical vertebral stenotic myelopathy and for identifying intracranial lesions.

Electrodiagnostic Testing

Electrodiagnostic testing, including electromyography (EMG) and nerve conduction studies, can help localize lesions within the peripheral nervous system and muscles. These tests are particularly useful for differentiating neurological from musculoskeletal causes of gait abnormalities. EMG can detect denervation potentials and myotonic discharges that indicate specific types of neuromuscular disease.

Skeletal Muscle Biopsy

Skeletal muscle biopsy can provide valuable diagnostic information in horses with suspected neuromuscular disease. The Veterinary Clinics of North America: Equine Practice published a review of skeletal muscle biopsy in 2025, discussing indications, techniques, and interpretation (Skeletal Muscle Biopsy, The Veterinary clinics of North America. Equine practice, 2025, pubmed.ncbi.nlm.nih.gov/39609140). Biopsy samples are typically collected from the semimembranosus, semitendinosus, or gluteal muscles and submitted for histopathology, histochemistry, and sometimes electron microscopy.

Diagnostic Decision Making

The table below outlines the diagnostic tests most appropriate for each common neurological disorder.

Management Strategies

Supportive Care

Supportive care is a cornerstone of management for horses with neurological disorders. Key components include a safe, padded environment to prevent self-trauma, adequate nutrition and hydration which may require assisted feeding or intravenous fluids, bladder management for horses with urinary incontinence, physical therapy to maintain muscle mass and joint mobility, and pain management as indicated. Recumbent horses require frequent turning to prevent pressure sores and respiratory complications. Slings or supportive harnesses may be used for horses that are weak but able to bear some weight.

Specific Treatment Approaches

Treatment varies depending on the specific neurological disorder diagnosed. For EPM, antiprotozoal medications are the mainstay of therapy. The Veterinary Clinics of North America: Equine Practice published a review of therapeutics for equine protozoal myeloencephalitis in 2017, discussing available treatment options and their efficacy (Therapeutics for Equine Protozoal Myeloencephalitis, Veterinary Clinics of North America Equine Practice, 2017, doi.org/10.1016/j.cveq.2016.12.001). Treatment duration typically extends for several weeks, and response is monitored through serial neurological examinations.

For viral encephalitides such as WNV and EHV-1, treatment is primarily supportive, as no specific antiviral therapies are approved for horses. Anti-inflammatory medications may be used to reduce central nervous system inflammation, though specific protocols should be determined by the attending veterinarian. Horses with EHV-1 may benefit from anticoagulant therapy to address the thrombotic component of the disease, though evidence for efficacy is limited.

Biosecurity Measures

Biosecurity is critical for preventing the spread of infectious neurological diseases. The World Organisation for Animal Health provides guidelines for biosecurity measures applicable to equine facilities (World Organisation for Animal Health, Animal Health and Welfare, www.woah.org/en/what-we-do/animal-health-and-welfare).

Key biosecurity measures include isolation of affected horses for a minimum of 21 to 30 days depending on the disease, dedicated equipment and personnel for affected animals, disinfection protocols for stalls and equipment using appropriate disinfectants effective against the specific pathogen, vector control for mosquito-borne diseases including elimination of breeding sites and use of insect repellents, and vaccination programs for preventable diseases. Traffic control on the premises, including limiting visitor access and implementing footbaths, reduces the risk of disease introduction and spread.

Vaccination Strategies

Vaccination is available for several neurological diseases of horses, including West Nile Virus, Eastern and Western Equine Encephalomyelitis, Equine Herpesvirus-1 (respiratory and abortion forms), and rabies. The Merck Veterinary Manual provides vaccination recommendations based on geographic region, horse age, and use (Merck Veterinary Manual, www.merckvetmanual.com). Core vaccines recommended for all horses include rabies, Eastern and Western Equine Encephalomyelitis, and West Nile Virus in endemic areas. Risk-based vaccines such as EHV-1 are recommended based on individual horse exposure risk and use.

Practical Implementation Steps for Horse Owners

Step 1: Recognize Early Signs

Horse owners should be familiar with early signs of neurological disease, including subtle changes in gait or coordination, behavioral changes such as depression or aggression, difficulty eating or drinking, unexplained lameness or stumbling, and muscle tremors or fasciculations. Any horse that suddenly becomes reluctant to move, shows a change in tail carriage, or develops urinary incontinence should be evaluated for neurological disease.

Step 2: Document Observations

Maintain detailed records of onset and progression of clinical signs, response to any treatments administered, environmental factors such as recent travel, new horses, or changes in feed, vaccination history, and temperature and other vital signs. Video recordings of the horse's gait and behavior can be valuable for the veterinarian to assess progression and response to treatment.

Step 3: Contact Veterinarian

Contact a veterinarian promptly if neurological signs are observed. Provide the veterinarian with a description of clinical signs, duration and progression, relevant history, and biosecurity concerns if multiple horses are affected. For acute onset of severe signs such as recumbency, seizures, or inability to stand, seek emergency veterinary care immediately.

Step 4: Implement Biosecurity

If an infectious cause is suspected, implement biosecurity measures immediately. Isolate the affected horse in a separate stall or paddock at least 10 meters from other horses, restrict movement of horses on and off the property, use dedicated equipment for the affected horse including buckets, halters, and grooming tools, and practice good hygiene and disinfection including hand washing and footbaths.

Step 5: Prepare for Diagnostic Testing

Be prepared for diagnostic testing that may include blood collection for serology and PCR, CSF collection which requires sedation or anesthesia, imaging studies which may require referral to a specialty center, and muscle or nerve biopsy. Discuss the costs, risks, and expected diagnostic yield of each test with your veterinarian before proceeding.

Records and Measurements

Essential Records for Neurological Cases

Maintain the following records for horses with suspected or confirmed neurological disease: daily neurological assessment scores using a standardized grading system, temperature, pulse, and respiration recorded at least twice daily, food and water intake measured in kilograms and liters, urine and fecal output noted for frequency and character, medications administered including dose, route, and time, and response to treatment documented with specific observations.

Monitoring Parameters

Monitor the following parameters to assess disease progression and response to treatment: gait score graded 0 to 5 based on severity of ataxia, cranial nerve function assessed daily for changes in menace response, facial symmetry, and swallowing ability, ability to stand and ambulate recorded as independent, assisted, or recumbent, appetite and hydration status monitored through feed consumption and skin tent testing, body weight measured weekly using a weight tape or scale, and laboratory parameters as indicated including blood work and CSF analysis.

Common Failure Patterns in Diagnosis and Management

Diagnostic Failures

Common diagnostic failures include incomplete neurological examination that misses subtle deficits, failure to collect appropriate samples at the correct time such as collecting CSF after treatment has been initiated, misinterpretation of serological results without considering vaccination history, overreliance on a single diagnostic test instead of using a combination of tests, and failure to consider differential diagnoses such as musculoskeletal disease mimicking neurological signs.

Management Failures

Common management failures include delayed initiation of treatment while awaiting definitive diagnosis, inadequate supportive care particularly for recumbent horses, failure to implement biosecurity measures leading to spread of infectious disease, poor compliance with treatment protocols such as missed doses or premature discontinuation, and inadequate monitoring for complications including secondary infections and pressure sores.

Prevention Failures

Common prevention failures include incomplete or delayed vaccination schedules, poor vector control allowing mosquito breeding on the premises, inadequate biosecurity for new horses including failure to quarantine, failure to isolate sick horses promptly, and lack of awareness of regional disease risks and seasonal patterns.

Limitations and Professional Escalation Criteria

Limitations of Diagnostic Testing

Diagnostic testing for equine neurological disorders has several limitations. Sensitivity and specificity vary by test and disease, with some tests having high false-negative rates. CSF collection carries risks including hemorrhage, spinal cord trauma, and post-collection complications. Serological results may be difficult to interpret in vaccinated horses due to antibody interference. Advanced imaging requires general anesthesia and specialized equipment that may not be readily available. Some diseases, such as grass sickness, have no definitive antemortem test, and diagnosis relies on clinical signs and exclusion of other conditions.

When to Escalate to a Specialist

Escalate care to a veterinary neurologist or internal medicine specialist when diagnosis remains unclear after initial testing, clinical signs are severe or rapidly progressive, advanced imaging such as MRI or CT is required, the horse fails to respond to appropriate treatment after a reasonable trial period, or multiple horses are affected suggesting an outbreak requiring coordinated management. Specialists can offer advanced diagnostic capabilities and treatment options not available in general practice.

When to Consider Euthanasia

Euthanasia should be considered when the horse is recumbent and unable to rise for more than 24 to 48 hours despite appropriate supportive care, severe unresponsive pain is present, quality of life is significantly compromised with loss of normal behaviors and interactions, the prognosis for recovery is poor based on the specific diagnosis and severity of signs, or there is risk of prolonged suffering without realistic chance of improvement. The Merck Veterinary Manual provides guidance on humane euthanasia considerations for horses with neurological disease (Merck Veterinary Manual, www.merckvetmanual.com). Discuss the decision with your veterinarian, considering the horse's comfort, dignity, and likelihood of meaningful recovery.

Welfare and Safety Context

Animal Welfare Considerations

Neurological disorders can cause significant suffering in horses. Welfare considerations include pain management for conditions that cause discomfort such as laminitis in PPID or muscle pain in EPM, prevention of self-trauma through environmental modifications including padded stalls and protective headgear, maintenance of social contact when possible to reduce stress, provision of appropriate nutrition and hydration including assisted feeding when necessary, and regular assessment of quality of life using standardized scoring systems.

Human Safety Considerations

Horses with neurological disease can be unpredictable and may pose safety risks to handlers. Safety considerations include use of appropriate restraint when handling affected horses, awareness of the risk of falling or collapsing particularly during examination and treatment, biosecurity precautions to prevent disease transmission to other horses and potentially to humans for zoonotic diseases such as rabies, proper disposal of biological waste including contaminated bedding and carcasses, and reporting of reportable diseases to appropriate authorities.

Regulatory Considerations

Some equine neurological diseases are reportable to animal health authorities. The World Organisation for Animal Health provides guidelines for reporting and controlling infectious neurological diseases (World Organisation for Animal Health, Animal Health and Welfare, www.woah.org/en/what-we-do/animal-health-and-welfare). Reportable diseases may include rabies, equine encephalomyelitis (Eastern, Western, Venezuelan), West Nile Virus in some jurisdictions, and Equine Herpesvirus-1 in some jurisdictions. Veterinarians are responsible for understanding and complying with local reporting requirements.

Practical Decision Framework for Differentiating Common Equine Neurological Disorders

A structured decision framework helps veterinarians and horse owners systematically differentiate between common neurological disorders when clinical signs overlap. The framework below integrates history, physical examination findings, and initial diagnostic test results to guide clinical reasoning before advanced testing is completed. This approach reduces diagnostic delays and supports timely treatment decisions.

Step 1: Assess Onset and Progression

Record the time from first observed sign to veterinary examination. Acute onset (hours to 24 hours) suggests vascular events, trauma, or viral infections such as EHV-1 myeloencephalopathy or West Nile Virus encephalomyelitis. Subacute onset (2 to 7 days) is typical for EPM and bacterial meningitis. Chronic progressive signs over weeks to months point toward PPID, cervical vertebral stenotic myelopathy, or neoplasia. The Merck Veterinary Manual provides guidance on recognizing patterns of disease onset in horses with neurological signs (Merck Veterinary Manual, www.merckvetmanual.com).

Document whether signs are static, improving, or worsening. Rapid deterioration over 12 to 24 hours warrants emergency evaluation and immediate biosecurity measures. Stable or slowly progressive signs allow more time for diagnostic testing and consultation.

Step 2: Evaluate Symmetry of Neurological Deficits

Asymmetric ataxia, muscle atrophy, and cranial nerve deficits are hallmark features of EPM. The 2022 review in Veterinary Clinics of North America: Equine Practice emphasizes that asymmetric involvement helps distinguish EPM from many other neurological conditions (Equine Protozoal Myeloencephalitis, The Veterinary clinics of North America. Equine practice, 2022, pubmed.ncbi.nlm.nih.gov/35810151). Perform a thorough cranial nerve examination, noting any asymmetry in menace response, facial symmetry, tongue tone, and swallowing ability.

Symmetric ataxia affecting all four limbs equally is more consistent with cervical vertebral stenotic myelopathy, EHV-1 myeloencephalopathy, or toxicities. Pelvic limb ataxia that is worse than thoracic limb ataxia suggests a cervical spinal cord lesion. Hindlimb weakness with normal forelimb function may indicate a lumbosacral or peripheral nerve problem.

Step 3: Check for Fever and Systemic Signs

Take the horse's rectal temperature at presentation and every 12 hours for the first 48 hours. Fever (temperature above 38.6 degrees Celsius or 101.5 degrees Fahrenheit) preceding or accompanying neurological signs strongly suggests an infectious cause. EHV-1 infection typically causes fever 24 to 48 hours before neurological signs develop, making twice-daily temperature monitoring a critical screening tool during outbreaks. West Nile Virus infection often presents with fever, lethargy, and muscle fasciculations before ataxia becomes apparent.

Absence of fever does not rule out infectious disease, particularly in chronic EPM cases or in horses that have received anti-inflammatory medications. However, afebrile horses with progressive neurological signs should be evaluated for non-infectious causes such as PPID, cervical vertebral stenotic myelopathy, or neoplasia.

Step 4: Assess Bladder Function and Tail Tone

Urinary incontinence with bladder atony is a distinguishing feature of EHV-1 myeloencephalopathy. Palpate the bladder per rectum to assess tone and distension. Observe the horse for dribbling urine, scalding of the hindlimbs, or straining to urinate. Decreased tail tone and anal sphincter tone further support a diagnosis of EHM.

EPM rarely causes urinary incontinence unless the lesion involves the sacral spinal cord segments. Horses with grass sickness may show urinary retention or incontinence as part of autonomic dysfunction, but this is typically accompanied by other signs such as dysphagia, ptosis, and patchy sweating.

Step 5: Evaluate Cranial Nerve Function

Perform a systematic cranial nerve examination. Key observations include:

• Menace response: absent or asymmetric suggests forebrain or cerebellar disease
• Pupillary light reflex: abnormal in brainstem lesions
• Facial symmetry: drooping ear, eyelid, or lip indicates facial nerve dysfunction
• Tongue tone and movement: weakness or deviation suggests hypoglossal nerve involvement
• Swallowing: difficulty or choking indicates glossopharyngeal or vagus nerve dysfunction
• Head tilt: suggests vestibular disease

EPM commonly causes asymmetric cranial nerve deficits, particularly facial nerve paralysis and tongue weakness. West Nile Virus may cause fine muscle fasciculations around the muzzle but rarely causes cranial nerve deficits. EHV-1 myeloencephalopathy typically spares cranial nerves unless the brainstem is directly affected.

Step 6: Consider Geographic and Seasonal Factors

Regional disease prevalence influences the likelihood of specific diagnoses. EPM is more common in the Americas where opossums are present. A study examining seroprevalence of Sarcocystis neurona in Argentinean horses found widespread exposure, though not all exposed horses develop clinical disease (Seroprevalence of Sarcocystis neurona and Its Association With Neurologic Disorders in Argentinean Horses, Journal of Equine Veterinary Science, 2014, doi.org/10.1016/j.jevs.2014.06.002). West Nile Virus is seasonal, with peak transmission during late summer and early fall when mosquito activity is highest. EHV-1 outbreaks can occur year-round but are more common in winter when horses are housed indoors.

Grass sickness is most frequently diagnosed in the United Kingdom and parts of Europe, with cases peaking in spring and early summer. The 2018 review in Veterinary Clinics of North America: Equine Practice discusses the geographic distribution and seasonal patterns of equine dysautonomia (Equine Dysautonomia, The Veterinary clinics of North America. Equine practice, 2018, pubmed.ncbi.nlm.nih.gov/29398183).

Step 7: Review Vaccination and Travel History

Document the horse's vaccination status for West Nile Virus, Eastern and Western Equine Encephalomyelitis, rabies, and EHV-1. Horses that are unvaccinated or overdue for boosters are at higher risk for these preventable diseases. Recent travel, attendance at shows or events, or introduction of new horses to the premises increases the risk of exposure to infectious agents.

A history of recent transport or commingling with horses from other facilities should raise suspicion for EHV-1. The American College of Veterinary Internal Medicine provides resources on biosecurity recommendations for horses returning from events (American College of Veterinary Internal Medicine, www.acvim.org).

Step 8: Perform Initial Diagnostic Testing

Based on the clinical assessment, select initial diagnostic tests. For horses with acute onset, fever, and symmetric ataxia, collect nasal swabs and blood for EHV-1 PCR and serology. For horses with asymmetric ataxia and cranial nerve deficits, collect CSF for EPM PCR and antibody testing. For horses with fever, muscle fasciculations, and ataxia during mosquito season, submit serum for West Nile Virus IgM capture ELISA.

A study on molecular detection of Sarcocystis neurona in CSF from 210 horses with suspected neurologic disease demonstrated that PCR testing offers high specificity for confirming EPM diagnosis (Molecular detection of Sarcocystis neurona in cerebrospinal fluid from 210 horses with suspected neurologic disease, Veterinary Parasitology, 2021, doi.org/10.1016/j.vetpar.2021.109372). However, sensitivity may vary depending on timing of sample collection relative to infection.

Decision Framework Summary Table

Record System for Decision Framework

Maintain a standardized record for each neurological case using the following template:

• Date and time of first observed sign
• Onset category: acute (less than 24 hours), subacute (2 to 7 days), chronic (more than 7 days)
• Progression: static, improving, worsening
• Temperature at presentation and every 12 hours
• Ataxia symmetry: symmetric, asymmetric, pelvic limb only
• Cranial nerve deficits: list affected nerves and side
• Bladder function: normal, distended, dribbling
• Tail tone: normal, decreased, absent
• Vaccination status: WNV, EEE/WEE, rabies, EHV-1
• Recent travel or exposure: yes or no with details
• Geographic risk factors: opossum exposure, mosquito season, regional disease prevalence
• Initial test results: PCR, serology, CSF analysis

Common Failure Patterns in Using the Decision Framework

Failure to document the exact time of onset leads to misclassification of disease progression. Owners often underestimate the duration of subtle signs. Ask specifically about changes in behavior, appetite, or performance over the past week. Video recordings of the horse at walk and trot can help establish a baseline and document progression.

Overreliance on a single clinical feature without considering the full pattern of signs leads to diagnostic errors. For example, asymmetric ataxia strongly suggests EPM, but horses with cervical vertebral stenotic myelopathy can also show asymmetric signs if the spinal cord compression is unilateral. Always consider multiple differential diagnoses and use diagnostic testing to confirm the suspected condition.

Failure to implement biosecurity measures while awaiting test results can lead to disease spread if an infectious cause is present. Isolate any horse with acute onset of fever and neurological signs until EHV-1 and other contagious diseases are ruled out. The World Organisation for Animal Health provides guidelines for biosecurity measures applicable to equine facilities (World Organisation for Animal Health, Animal Health and Welfare, www.woah.org/en/what-we-do/animal-health-and-welfare).

When to Escalate to a Specialist

Consult a veterinary neurologist or internal medicine specialist when the decision framework does not clearly point to a single diagnosis, when initial diagnostic tests are negative but clinical signs persist or worsen, when advanced imaging such as MRI or CT is needed to evaluate structural lesions, when the horse fails to respond to appropriate treatment after 7 to 14 days, or when multiple horses are affected suggesting an outbreak requiring coordinated management. Specialists can perform additional diagnostic procedures such as electromyography, nerve conduction studies, and muscle biopsy to differentiate neuromuscular from central nervous system disorders. The 2025 review of skeletal muscle biopsy in Veterinary Clinics of North America: Equine Practice discusses indications for referral when neuromuscular disease is suspected (Skeletal Muscle Biopsy, The Veterinary clinics of North America. Equine practice, 2025, pubmed.ncbi.nlm.nih.gov/39609140).

Frequently Asked Questions

What are the most common neurological disorders in horses?

The most common neurological disorders in horses include Equine Protozoal Myeloencephalitis (EPM), West Nile Virus encephalomyelitis, Equine Herpesvirus-1 myeloencephalopathy, and Equine Dysautonomia (grass sickness). The prevalence of these conditions varies by geographic region. EPM is more common in the Americas where opossums are present, while grass sickness is more frequently diagnosed in the United Kingdom and parts of Europe. The Merck Veterinary Manual provides region-specific information on disease prevalence (Merck Veterinary Manual, www.merckvetmanual.com). Cervical vertebral stenotic myelopathy (wobblers) is also common in young, rapidly growing horses, particularly Thoroughbreds and Quarter Horses.

What are the early signs of neurological disease in horses?

Early signs of neurological disease in horses can be subtle and may include mild incoordination or stumbling, behavioral changes such as depression or aggression, difficulty eating or drinking, muscle tremors or fasciculations, and unexplained lameness that shifts between limbs. Owners should also watch for changes in tail tone, urinary incontinence, and abnormal sweating patterns. Any sudden change in a horse's gait or behavior warrants veterinary evaluation. Video recording the horse at walk and trot can help document subtle abnormalities for the veterinarian.

How is EPM diagnosed in horses?

EPM is diagnosed through a combination of clinical examination findings and laboratory testing. Cerebrospinal fluid (CSF) analysis is the primary diagnostic tool, with PCR testing for Sarcocystis neurona DNA and antibody testing (Western blot or immunofluorescent antibody test) being commonly used. A study on molecular detection of Sarcocystis neurona in CSF from 210 horses with suspected neurologic disease demonstrated the utility of PCR testing (Molecular detection of Sarcocystis neurona in cerebrospinal fluid from 210 horses with suspected neurologic disease, Veterinary Parasitology, 2021, doi.org/10.1016/j.vetpar.2021.109372). Serological testing on blood can indicate exposure but does not confirm active neurological disease. A positive CSF antibody test in a horse with compatible clinical signs supports the diagnosis, while a negative test makes EPM less likely.

Can horses recover from West Nile Virus?

Many horses can recover from West Nile Virus infection with appropriate supportive care. The prognosis depends on the severity of clinical signs at presentation. Horses that remain standing and are able to eat and drink have a better prognosis than those that become recumbent. Recovery may take weeks to months, and some horses may have residual neurological deficits. Vaccination is highly effective for preventing WNV infection and is recommended annually in endemic areas. A case study on treatment of West Nile virus caused encephalomyelitis in horses under stable conditions documented recovery in some affected animals (Treatment of west nile virus caused encephalomyelitis in horses at stable conditions: Case study, Magyar Allatorvosok Lapja, 2019, api.elsevier.com/content/abstract/scopus_id/85065513697).

What causes neurological disorders in horses?

Neurological disorders in horses can be caused by infectious agents (viruses, bacteria, protozoa, parasites), toxins, metabolic disorders, trauma, neoplasia, and degenerative conditions. Infectious causes include Sarcocystis neurona (EPM), West Nile Virus, Equine Herpesvirus-1, rabies, and Eastern/Western Equine Encephalomyelitis viruses. Non-infectious causes include pituitary pars intermedia dysfunction (PPID), equine dysautonomia, cervical vertebral stenotic myelopathy (wobblers), and various toxicities such as ionophore toxicity, lead poisoning, and botulism. The specific cause determines the treatment approach and prognosis.

How is EHV-1 myeloencephalopathy managed?

EHV-1 myeloencephalopathy is managed primarily through supportive care and biosecurity measures. There is no specific antiviral treatment approved for horses with EHM. Management includes anti-inflammatory medications to reduce central nervous system inflammation, supportive care for recumbent horses including padding and frequent turning, bladder management for horses with urinary incontinence including catheterization if necessary, and strict isolation to prevent spread to other horses. The American College of Veterinary Internal Medicine provides resources on EHV-1 management (American College of Veterinary Internal Medicine, www.acvim.org). Outbreak management requires coordinated efforts including movement restrictions, temperature monitoring of all exposed horses, and enhanced biosecurity protocols.

When should I call a veterinarian for a horse with neurological signs?

Call a veterinarian immediately if you observe any neurological signs in your horse, including incoordination, weakness, muscle tremors, difficulty swallowing, or behavioral changes. Neurological signs can progress rapidly, and early intervention improves the chances of a favorable outcome. If multiple horses are affected or if the horse is recumbent, this constitutes an emergency requiring immediate veterinary attention. For subtle signs such as mild stumbling or behavioral changes, contact your veterinarian within 24 hours for guidance on monitoring and when to schedule an examination.

What is the prognosis for horses with neurological disorders?

The prognosis for horses with neurological disorders varies widely depending on the specific condition, severity of clinical signs, and timeliness of treatment. Some conditions, such as mild EPM or WNV infection, have a good prognosis with appropriate treatment. Other conditions, such as severe EHV-1 myeloencephalopathy or equine dysautonomia, carry a poor prognosis. The Merck Veterinary Manual provides prognosis information for specific neurological conditions (Merck Veterinary Manual, www.merckvetmanual.com). Factors that worsen prognosis include recumbency, rapid progression of signs, involvement of multiple body systems, and delayed treatment. Early diagnosis and aggressive supportive care improve outcomes for many neurological conditions.

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References and Further Reading

• www.merckvetmanual.com
• www.acvim.org
• Merck Veterinary Manual. Merck Veterinary Manual.
• Animal Health and Welfare. World Organisation for Animal Health.
• Biofluid Markers of Equine Neurological Disorders Reviewed From Human Perspectives.. Journal of equine veterinary science, 2020.
• Equine Dysautonomia.. The Veterinary clinics of North America. Equine practice, 2018.
• Skeletal Muscle Biopsy.. The Veterinary clinics of North America. Equine practice, 2025.
• Equine Protozoal Myeloencephalitis.. The Veterinary clinics of North America. Equine practice, 2022.
• Neurologic diseases.. The Veterinary clinics of North America. Equine practice, 1985.
• Diagnosis of equine pituitary pars intermedia dysfunction.. Veterinary journal (London, England : 1997), 2023.
• Seroprevalence of Sarcocystis neurona and Its Association With Neurologic Disorders in Argentinean Horses. Journal of Equine Veterinary Science, 2014.
• Molecular detection of Sarcocystis neurona in cerebrospinal fluid from 210 horses with suspected neurologic disease. Veterinary Parasitology, 2021.
• Neurologic disorders of neonatal foals. Veterinary Clinics of North America Equine Practice, 2005.
• Treatment of west nile virus caused encephalomyelitis in horses at stable conditions: Case study. Magyar Allatorvosok Lapja, 2019.
• Therapeutics for Equine Protozoal Myeloencephalitis. Veterinary Clinics of North America Equine Practice, 2017.

This article is educational and is not a substitute for veterinary diagnosis or treatment. Contact a veterinarian for advice about an individual animal.