Fever is the single most important early warning sign of EHV-1 infection—and it may be the only symptom you see before a horse becomes seriously ill. Any temperature at or above 101.5°F in a resting horse warrants immediate veterinary contact and isolation, particularly if the horse recently traveled, attended an event, or had contact with horses from other facilities. The disease can progress from fever to life-threatening neurological signs within 7-12 days, making early detection critical. Unlike many respiratory illnesses, EHV-1 causes minimal coughing, so waiting for a “typical sick horse” appearance means missing the crucial intervention window.
Horse owners who monitor temperatures twice daily during outbreak periods and avoid masking fever with NSAIDs give their horses—and their veterinarians—the best chance of early intervention and successful outcomes.
EHV-1 Fever Detection: Why Temperature Monitoring Is Your First Line of Defense
Normal adult horse temperature ranges from 99°F to 101.5°F (37.5°C–38.6°C), though individual horses have their own baseline. Establishing your horse’s normal when healthy provides the comparison point that makes early fever detection possible.
The fever threshold requiring action is 101.5°F. EHV-1 typically produces temperatures of 102°F or higher, with severe cases reaching 107°F. Critically, EHV-1 causes a characteristic biphasic fever—an initial spike at days 1-2, followed by a second peak at days 6-7. This second fever coincides with viremia (virus in the bloodstream), the dangerous phase that precedes neurological disease or abortion.
To properly take a rectal temperature, stand beside (never directly behind) the horse’s hindquarters. Apply lubricant to a digital veterinary thermometer, lift the tail to the side, and insert approximately 2 inches into the rectum, angling slightly to contact the rectal wall. Wait for the digital signal. During outbreaks or exposure situations, temperatures should be taken twice daily, 8-12 hours apart, at consistent times, and always before exercise or administration of any medications. Continue monitoring for a minimum of 21 days after last known exposure.
Early EHV-1 Symptoms: Subtle Warning Signs Owners Often Miss
The earliest indicators of EHV-1 infection include fever, lethargy, loss of appetite, and nasal discharge—signs easily dismissed as minor illness. Nasal discharge typically begins clear and watery (serous), progressing to thicker yellow mucoid discharge, then green-yellow mucopurulent discharge if secondary bacterial infection develops. Bilateral discharge from both nostrils indicates systemic infection.
Swollen lymph nodes beneath the jaw (submandibular) provide another early clue. To check, stand beside the horse’s head and palpate between the lower jawbones, feeling for firm swelling, heat, or pain on touch. Unlike strangles, EHV-1 causes lymph node enlargement without abscess formation—if you see thick purulent drainage, suspect bacterial infection instead.
Behavioral changes warrant attention: depression, reduced interaction with handlers, and decreased willingness to move all signal systemic illness. Extreme lethargy or a “coma-like state” indicates potential neurological involvement requiring emergency veterinary care.
EHV-1 Respiratory Signs vs. Influenza and Strangles: Key Differences
The respiratory form of EHV-1 produces surprisingly mild coughing—soft and occasional rather than the harsh, paroxysmal cough characteristic of equine influenza. This distinction matters: horse owners waiting for dramatic respiratory distress may miss EHV-1 entirely. Respiratory symptoms typically last 9-12 days in adult horses with prior exposure but can persist 2-3 weeks in young, immunologically naïve animals.
Key differentiators from similar conditions:
| Condition | Cough | Fever Pattern | Lymph Nodes | Distinctive Feature |
|---|---|---|---|---|
| EHV-1 | Soft, occasional | Biphasic (peaks days 1-2 and 6-7) | Enlarged, no abscess | Potential neurological/abortion complications |
| Equine Influenza | Harsh, prominent | Single high spike | Minimal involvement | Abrupt onset, severe cough |
| Strangles | Variable | High sustained fever | Abscess and drain | Thick purulent discharge, difficulty swallowing |
| Allergies/RAO | Chronic, exercise-triggered | No fever | Normal | Environmental triggers, improves outdoors |
The biphasic fever pattern is perhaps the most reliable clinical indicator of EHV-1. Young horses (particularly weanlings and yearlings) show more pronounced respiratory signs, while older horses with prior exposure often have mild or inapparent infections—yet can still transmit the virus.
Neurological EHV-1 Symptoms (EHM): Emergency Signs You Must Act On
Equine Herpesvirus Myeloencephalopathy (EHM)—the neurological form—typically appears 8-12 days after initial fever and represents the most dangerous manifestation of EHV-1. Once neurological signs begin, they progress rapidly over 24-48 hours before stabilizing. The mortality rate ranges from 30% to 50%, with some severe outbreaks reaching 75%.
Neurological signs appear in a characteristic progression:
The earliest indicator is loss of tail tone—a completely flaccid, limp tail that offers no resistance when lifted. Next comes hindlimb incoordination: stumbling, toe-dragging, and a “drunken” gait affecting the rear legs more severely than the front. Horses may appear sedated when walking. Urine dribbling or retention signals bladder dysfunction, a classic EHM sign—check whether the stall is normally wet and whether the horse has been observed urinating.
The “dog-sitting” posture—where a horse sits on its hindquarters like a dog because the rear limbs can no longer support its weight—indicates severe neurological compromise. Finally, recumbency (inability to rise) marks a critical threshold. Horses recumbent for more than 24 hours have significantly diminished survival prospects and often require euthanasia due to complications including pressure sores, pneumonia, and bladder rupture.
Risk factors for developing EHM versus just respiratory disease include age (horses over 20 years face 50-70% risk compared to about 10% for horses under 15), being female, experiencing higher fevers, and infection with the neuropathogenic D752 viral strain.
EHV-1 in Pregnant Mares: Abortion Risks, Warning Signs, and Prevention
EHV-1 abortion typically occurs with no premonitory signs—the expelled fetus is often the first indication of infection. This makes prevention through vaccination and biosecurity far more critical than reactive treatment. Abortions occur 14 to 120 days after infection, with 90% happening within 60 days. The highest-risk period is 7-11 months of gestation (the last trimester), peaking at 8-10 months.
Mares infected late in pregnancy may deliver live but infected foals. These neonates may appear normal initially or present with weakness, respiratory distress, and jaundice. Most infected foals die within 1-3 days despite intensive care, succumbing to progressive respiratory failure.
Pregnant mares should receive EHV-1 vaccine (specifically products licensed for abortion prevention such as Pneumabort-K or Prodigy) at the 5th, 7th, and 9th months of gestation. However, vaccination does not guarantee protection—biosecurity measures including separating pregnant mares from the general population and preventing contact with transient boarders or show horses remain essential.
When to Call the Vet for EHV-1: Exact Temperature and Symptom Thresholds
Call immediately for any of the following:
- Temperature ≥101.5°F in a horse at rest (or ≥101.0°F if on NSAIDs)
- Any fever in a horse that recently traveled or attended an event
- Nasal discharge progressing from clear to colored
- Any neurological sign: stumbling, weak tail, difficulty walking, urine dribbling
- Dog-sitting posture or inability to rise (emergency)
- Combination symptoms: fever plus lethargy, fever plus nasal discharge, fever plus loss of appetite
Before calling, isolate the febrile horse immediately—minimum 30 feet from other horses, ideally 60 feet or more. Cease all movement of horses on and off the property. Prepare travel history (past 14-21 days), vaccination records, temperature logs, and timeline of symptom onset.
Deadly EHV-1 Mistakes: What Owners Do Wrong During Outbreaks
The most dangerous error is administering NSAIDs (such as phenylbutazone or flunixin) that mask fever. During outbreak monitoring, temperatures must reflect the horse’s true status—a horse on NSAIDs showing 101.0°F may actually have significant fever. Since fever often precedes viral shedding and is frequently the only early indicator, masking it prevents identification of infected horses before they spread the virus. Horses under investigation should remain off NSAIDs for at least 24 hours before temperature readings are considered reliable.
Other costly mistakes include: sharing equipment between horses (buckets, thermometers, grooming tools), failing to take temperatures twice daily during exposure periods, moving horses before the 21-day isolation period completes, not immediately isolating febrile horses, continuing corticosteroids (which suppress immune function and may reactivate latent virus), and failing to submit aborted fetuses and placentas for diagnostic testing.
Proper biosecurity requires dedicated equipment for isolated horses, PPE for handlers (gloves, shoe covers, clothing changes), and the discipline to care for isolated horses last or with complete decontamination between animals.
Symptom timeline: what to expect
| Timeframe | What Happens |
|---|---|
| Day 1-2 | Initial fever (102-107°F), may be transient or undetected |
| Day 3-5 | Fever may subside; horse appears normal but may be shedding virus |
| Day 6-7 | Second fever peak coinciding with viremia |
| Day 7-10 | Respiratory signs develop if present (nasal discharge, mild cough, lethargy) |
| Day 8-12 | Neurological signs may appear (typically on last day of fever or shortly after) |
| Week 2-12 | Abortion risk window for pregnant mares |
| Day 9-14 | Respiratory signs resolve in uncomplicated cases |
Viral shedding typically lasts 7-10 days but may persist up to 28 days in horses with EHM. Quarantine should continue for 21-28 days after the last fever or clinical sign. Critically, 60-80% of horses harbor latent EHV-1 that can reactivate during stress, meaning recovered horses remain potential carriers for life.
How to Differentiate EHM From EPM, West Nile, and Other Neurological Diseases
When a horse presents with neurological signs, veterinarians must distinguish EHM from other causes including Equine Protozoal Myeloencephalitis (EPM), West Nile Virus, Eastern/Western Equine Encephalitis, rabies, and Wobbler Syndrome. The key differentiating feature of EHM is symmetric ataxia—both hind legs affected equally—whereas EPM classically produces asymmetric signs with one side more affected than the other. EHM also typically involves multiple horses during outbreaks, whereas EPM affects individual animals sporadically.
Definitive diagnosis requires PCR testing of both nasal swabs (detecting shedding) and whole blood (detecting viremia). Both samples should be submitted together. Results can be available within days, and DNA analysis can identify whether the neuropathogenic D752 strain is involved. For abortion cases, submission of fetal tissues and placenta enables definitive diagnosis through identification of characteristic intranuclear inclusion bodies.
Final Takeaways: Recognizing EHV-1 Symptoms Before It’s Too Late
EHV-1 demands respect but yields to vigilance. The disease offers a critical window for intervention between fever onset and progression to neurological or reproductive complications—typically 7-12 days. Horse owners who commit to twice-daily temperature monitoring, immediate isolation of febrile horses, strict biosecurity, and avoiding NSAIDs that mask fever transform their odds dramatically. The biphasic fever pattern, minimal coughing, and potential for devastating neurological sequelae distinguish EHV-1 from other respiratory diseases and require a unique management approach.
When in doubt, call your veterinarian. A single temperature reading of 101.5°F in a horse with recent travel or event exposure justifies that call. Early intervention—before neurological signs appear—gives horses the best chance of survival and complete recovery. The cost of one unnecessary phone call is trivial compared to the cost of waiting one day too long.