Equine Herpesvirus 1 and 4 Respiratory Disease or Abortion
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.
Limit spread while seeking adviceSeparate affected animals when practical and obtain veterinary guidance on contacts, cleaning and normal mixing.
Emergency presentation is possibleAtaxia, urine dribbling, tail weakness or inability to rise requires immediate EHM and biosecurity assessment.
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 types 1 and 4 are widespread viruses capable of establishing lifelong latent infection after initial exposure. EHV-4 most often causes respiratory disease, particularly in young horses. EHV-1 can cause respiratory illness, late-pregnancy abortion, neonatal death and neurological disease. Stress or immune change can reactivate latent virus, allowing shedding without a new external source.
Respiratory infection produces fever, dullness, reduced appetite, nasal discharge, pharyngitis and sometimes cough. Signs overlap with influenza and cannot reliably identify the virus. EHV-1 can spread through the bloodstream and damage small blood vessels in the pregnant uterus, leading to abortion that may occur without the mare appearing very ill. Foals infected before birth may be born weak and develop rapidly fatal respiratory disease.
Virus spreads through respiratory secretions, close contact, contaminated equipment and tissues or fluids from an abortion. Aborted fetus, placenta and bedding can contain a large viral load. EHV-1 neurological disease is addressed separately because its monitoring and care differ from uncomplicated respiratory infection.
Most respiratory cases recover with rest, but abortion outbreaks can cause major losses. Vaccination can reduce respiratory disease and abortion risk but does not eliminate infection, latency or all shedding. Prevention depends on grouping pregnant mares appropriately, limiting new introductions, rapid isolation of fever and meticulous handling of abortion material. Because latent infection is common, an outbreak does not necessarily reflect a recent biosecurity failure by one identifiable horse.
Both viruses establish lifelong latency and may reactivate during stress. EHV-4 is primarily respiratory, while EHV-1 also causes abortion, neonatal death and neurological disease. Abortion can occur without warning after maternal respiratory signs have resolved, and the fetus, placenta and fluids contain high viral loads that can contaminate people, equipment and the foaling area.
Screening & Early Detection
Temperatures should be recorded for horses exposed to a suspected EHV case, as fever may precede other signs. Affected horses are isolated and movements stopped according to veterinary biosecurity advice. The history should identify recent travel, arrivals, contact groups, pregnancy stage, abortions and any subtle neurological change.
Early diagnosis commonly uses PCR on a nasopharyngeal swab together with citrated whole blood or buffy-coat testing. The nasal sample assesses respiratory shedding, while blood can detect cell-associated viraemia. Intermittent shedding and sampling time affect sensitivity, so a negative result is interpreted with the clinical picture. Paired serology may support retrospective diagnosis but is less useful for immediate movement decisions.
After abortion, the fetus and intact placenta should be secured away from other horses and submitted through the veterinarian. PCR, virus isolation and characteristic tissue changes provide diagnosis; maternal serology alone has limited value. Exposed horses continue temperature and clinical monitoring for the prescribed period. Ataxia, urine dribbling, tail weakness or inability to rise requires immediate escalation for possible EHM.
Treatment & Management
Uncomplicated respiratory EHV infection receives rest, hydration, palatable feed and anti-inflammatory medication when needed for fever and comfort. Antimicrobials are reserved for secondary bacterial infection. Exercise resumes gradually after fever and respiratory signs resolve, allowing time for airway recovery. Antiviral medication may be considered in selected high-risk or complicated cases, but evidence, timing and dosing require specialist veterinary judgement.
Biosecurity is part of treatment at property level. Affected, exposed and clean groups are separated; equipment is dedicated; people work from lowest to highest risk; and movements resume only under veterinary or regulatory guidance. Abortion material and contaminated bedding are handled with protective clothing, removed safely and the area cleaned before disinfection. Pregnant mares should not share exposure with travelling young stock.
Vaccination schedules are reviewed for future risk, including products intended to reduce EHV-1 abortion, but vaccination does not treat an infected horse or guarantee prevention. A mare that aborts needs examination for retained membranes, uterine disease and systemic complications in addition to outbreak control. Prognosis is good for routine respiratory disease; neonatal infection, abortion loss and neurological progression carry much more serious consequences.
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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