Equine Herpesvirus Myeloencephalopathy
Learn what the condition is, how it may be detected early, how it is treated or managed, and which breeds or species are linked to it.
Condition snapshot
A concise guide to the condition’s pattern, detection, management and urgency.
Emergency presentation is possibleNew ataxia, urine retention, tail weakness or difficulty rising requires immediate veterinary and biosecurity action.
Immediate movement controlSeparate the suspect horse, stop movements and contact a veterinarian before sampling or moving exposed horses.
This snapshot is a general guide, not a diagnosis or treatment plan. New, severe or worsening signs require veterinary assessment.
About the condition
Equine herpesvirus myeloencephalopathy, commonly abbreviated EHM, is the neurological form of infection associated with equine herpesvirus type 1. The virus can damage the small blood vessels supplying the spinal cord and brain, producing inflammation, clotting and areas of nervous-tissue injury. EHM can occur during an EHV-1 respiratory or abortion outbreak, but a neurological case may be the first obvious warning on a property.
Fever often precedes neurological signs by several days. Early changes can include a weak or unsteady hindlimb gait, dragging the toes, difficulty backing, reduced tail and anal tone, urine dribbling or an overfull bladder. Disease may progress to inability to stand. The hindquarters are usually more affected than the forelimbs, although severe cases can become recumbent and develop cranial-nerve or respiratory complications. Alertness is often retained, which can make profound physical disability especially distressing.
Not every horse infected with EHV-1 develops EHM. Viral strain, magnitude and duration of viraemia, age and host response influence risk. Vaccination supports control of respiratory and abortion disease but is not proven to prevent every neurological case. EHM is not diagnosed from incoordination alone; trauma, cervical spinal disease, toxicities and other infectious encephalitides can resemble it.
The welfare and biosecurity consequences are both serious. Affected horses may shed virus, while apparently healthy exposed horses can become febrile and infectious. Prognosis varies with neurological grade and the ability to remain standing. Horses that stay ambulatory frequently recover, although improvement can take weeks or months. Recumbency, bladder paralysis and complications of prolonged lying substantially worsen the outlook.
Neurological disease follows vascular injury in the spinal cord and brain rather than direct destruction of motor neurons. Hindlimb ataxia, urinary retention, tail weakness and recumbency can appear during or after fever, although not every exposed horse becomes neurological. Because affected horses may shed EHV-1, a single suspect case requires immediate movement control even before laboratory confirmation.
Screening & Early Detection
During a suspected EHV-1 event, all horses in the affected group should have rectal temperatures recorded twice daily and be observed for gait change, urine retention, tail weakness and difficulty rising. New fever should trigger immediate separation and veterinary notification. Movements, direct contacts, shared equipment and the order in which clinical signs appeared should be documented.
Veterinary neurological examination localises the lesion and grades severity. Diagnosis commonly uses real-time PCR on a deep nasal or nasopharyngeal sample together with EDTA or citrated whole blood, because respiratory shedding and cell-associated viraemia are not identical. Sampling time affects sensitivity; a negative result does not automatically exclude EHM in a compatible outbreak. Cerebrospinal fluid may show increased protein or xanthochromia and can be tested where collection is safe.
Competing causes of ataxia require consideration, including cervical compression, trauma, equine protozoal myeloencephalitis in endemic regions and other notifiable encephalitides. Testing and horse handling must follow jurisdictional biosecurity instructions. Exposed animals remain monitored for the full veterinarian-directed risk period, even if an initial test is negative or respiratory signs are absent.
Treatment & Management
There is no treatment that reliably reverses established spinal-cord vascular injury. Care focuses on limiting inflammation, supporting the horse safely and preventing complications while nervous function recovers. Anti-inflammatory medication, intravenous fluids and antiviral treatment may be considered according to timing, severity and specialist advice. Antimicrobials do not treat EHV-1 and are reserved for a documented secondary bacterial problem.
Ambulatory horses need deep non-slip bedding, controlled assistance and protection from falls. Bladder function is checked repeatedly; catheterisation and medication may be needed when urine retention or overflow develops. A recumbent horse requires intensive nursing, regular repositioning, pressure-sore prevention, limb protection, sling assessment and monitoring for muscle and respiratory injury. Sling use is not benign and should be supervised by an experienced team.
The affected premises requires strict separation of sick, exposed and unexposed groups, dedicated equipment, hand and clothing hygiene and controlled movement under veterinary or regulatory direction. Horses are cared for from lowest to highest risk. Recovery exercises begin only when strength and coordination permit safe progression. Decisions about prolonged treatment must include welfare, ability to stand and realistic nursing capacity. Survivors may retain weakness or urinary dysfunction, but many ambulatory cases regain useful function with time.
Breeds or species affected
Akhal-Teke
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Anglo-Arabian
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Arabian
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Barb
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Caspian
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Kathiawari
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Marwari
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Shagya Arabian
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Thoroughbred
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American Quarter Horse
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American Paint Horse
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Appaloosa
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Australian Stock Horse
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Brumby
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Waler
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Morgan
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Standardbred
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American Saddlebred
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Tennessee Walking Horse
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Missouri Fox Trotter
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Rocky Mountain Horse
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Peruvian Paso
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Criollo
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Mustang
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Canadian Horse
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Andalusian
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Lusitano
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Lipizzaner
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Friesian
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Dutch Warmblood
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Hanoverian
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Holsteiner
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Oldenburg
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Westphalian
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Trakehner
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Selle Français
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Belgian Warmblood
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Danish Warmblood
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Swedish Warmblood
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Swiss Warmblood
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Australian Warmblood
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Irish Sport Horse
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Cleveland Bay
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Württemberger
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Zangersheide
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Knabstrupper
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Orlov Trotter
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Camargue
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Icelandic Horse
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Pony of the Americas
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Quarter Pony
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Miniature Horse
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Clydesdale
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Shire
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Percheron
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Belgian Draft
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Suffolk Punch
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American Cream Draft
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Ardennes
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Breton
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Boulonnais
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Comtois
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Dutch Draft
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North Swedish Horse
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Norwegian Fjord
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Haflinger
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Gypsy Cob
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Irish Draught
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Welsh Pony and Cob
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Connemara Pony
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New Forest Pony
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Dartmoor Pony
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Exmoor Pony
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Shetland Pony
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Highland Pony
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Fell Pony
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Dales Pony
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Australian Pony
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Finnhorse
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Black Forest Horse
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