Equine herpesvirus-1 (EHV-1) is a highly contagious virus that can cause serious illness in horses, ranging from mild respiratory disease to life-threatening neurological complications and reproductive loss. As a horse owner or caregiver, recognizing the early signs of EHV-1 and knowing when to seek immediate veterinary attention can make a critical difference in your horse’s outcome. This article provides a practical symptom checklist to help you identify potential EHV-1 cases and understand the urgency of various clinical presentations.
EHV-1 spreads rapidly through direct contact, shared equipment, and respiratory droplets, making it a concern for both individual horses and barn operations. The virus can manifest in three primary forms: respiratory infection (the most common), neurological disease (equine herpesvirus myeloencephalopathy or EHM), and abortion in pregnant mares. Understanding which symptoms warrant an immediate veterinary call versus those requiring monitoring is essential for protecting your horse’s health and preventing transmission to other horses in your facility.
Understanding EHV-1 and Its Three Forms
EHV-1 typically has an incubation period of 2 to 10 days, meaning an exposed horse may show no signs for up to two weeks after contact with the virus. Once symptoms appear, they can progress rapidly, particularly in young horses, immunocompromised individuals, or those under stress. The virus is most contagious during the acute phase when respiratory signs are present, but shedding can continue for weeks even after clinical recovery.
The three recognized forms of EHV-1 disease each present distinct symptom patterns and carry different levels of urgency:
- Respiratory form: The most common presentation, affecting the upper and lower respiratory tract
- Neurological form (EHM): Involves the central nervous system and spinal cord; can be rapidly progressive
- Abortive form: Causes pregnancy loss in mares, typically in the second and third trimester
Respiratory EHV-1: The Most Common Form
Classic Respiratory Symptoms
Respiratory EHV-1 typically begins with upper respiratory signs similar to a common cold. Watch for these key indicators:
- Nasal discharge (clear to yellowish, may become purulent)
- Fever, typically 101.5 to 105.5 degrees Fahrenheit
- Cough (usually mild to moderate, often dry initially)
- Lethargy and decreased appetite
- Enlarged submandibular lymph nodes (swelling under the jaw)
- Conjunctivitis (redness or discharge from eyes)
- Watery eyes
When to Call the Vet: Respiratory Form
Call your veterinarian immediately (same day or within a few hours) if your horse shows:
- Any combination of fever (above 101.5 F) and nasal discharge
- Harsh, labored breathing or respiratory distress
- Fever lasting more than 3 to 5 days
- Signs of secondary bacterial pneumonia (increased respiratory rate above 40 breaths per minute, crackles heard on lung auscultation, or worsening cough after initial improvement)
- Severe lethargy or inability to stand
Even uncomplicated respiratory EHV-1 requires prompt veterinary evaluation to confirm the diagnosis, assess severity, and rule out other serious conditions. Early professional assessment allows your vet to begin appropriate supportive care and implement isolation protocols if needed.
Neurological EHV-1 (EHM): The High-Urgency Form
Recognizing Neurological Symptoms
EHM can develop independently of respiratory signs or emerge as respiratory illness wanes. Some horses may show minimal or no respiratory symptoms before neurological signs appear. This form is an emergency and requires immediate veterinary intervention. Classic EHM symptoms include:
- Ataxia (loss of coordination or wobbly gait), starting in the hind limbs
- Weakness in the hindquarters or all four limbs
- Proprioceptive deficits (knuckling over of hooves, stumbling)
- Paresis (partial paralysis) or plegia (complete paralysis)
- Inability to rise or reluctance to stand
- Recumbency (lying down and unable to stand)
- Loss of tail tone
- Loss of bladder or rectal control (incontinence)
- Fever (though may be absent or brief)
- Behavioral changes or altered mental status
EHM Progression and Urgency
EHM can progress from subtle neurological signs to complete hindquarter paralysis within 24 to 72 hours. A horse that appears mildly uncoordinated in the morning may be unable to stand by evening. This rapid progression makes early recognition critical.
Call your veterinarian immediately (emergency call) if your horse shows any of the following:
- Any degree of ataxia or incoordination, especially in the hind limbs
- Weakness or dragging of limbs
- Inability or reluctance to bear weight on one or more limbs
- Inability to rise after lying down
- Loss of bladder or rectal control
- Behavioral changes combined with fever or respiratory signs
- Trembling or shivering of muscles
Do not delay calling your vet while waiting to see if the signs improve. EHM is a medical emergency, and every hour can affect the horse’s prognosis and recovery potential.
Abortion Associated with EHV-1
Pregnancy Loss Symptoms
EHV-1 can cause abortion in pregnant mares, particularly in the second and third trimester (approximately 4 months through term). Abortion may occur with or without preceding respiratory or systemic signs. Key indicators include:
- Sudden expulsion of the fetus and placenta
- Vaginal discharge or hemorrhage following an unexpected delivery
- Incomplete expulsion of the placenta (retained placenta)
- Signs of premature labor or colicky behavior in a pregnant mare
- Fever or systemic illness in a pregnant mare
- A “sick” appearance in a pregnant mare with no obvious explanation
When to Call the Vet: Abortion Form
Call your veterinarian immediately if a pregnant mare aborts or shows signs of impending abortion. Retained placenta is a serious complication requiring urgent treatment. Additionally, the aborted fetus and placenta should be submitted for diagnostic testing (PCR or viral culture) to confirm EHV-1, which has public health and barn management implications.
If a pregnant mare shows fever, lethargy, or mild respiratory signs, contact your veterinarian promptly. While not all febrile illness in pregnant mares is EHV-1, the risk to the pregnancy warrants professional evaluation and monitoring.
Secondary Bacterial Infection and Complications
Some horses with EHV-1 develop secondary bacterial pneumonia as the viral infection damages respiratory tract defenses. Watch for these warning signs indicating progression to bacterial infection:
- Fever returning or persisting after initial improvement
- Increased respiratory rate (above 40 breaths per minute at rest)
- Shallow, labored breathing
- Cough that worsens or changes character
- Yellow, green, or blood-tinged nasal discharge
- Decreased appetite or complete anorexia
- Severe lethargy or depression
These signs require immediate veterinary attention and may necessitate antimicrobial therapy and intensive supportive care.
Quick Reference Symptom Checklist
| Form of EHV-1 | Key Symptoms | Timeline | Urgency Level |
|---|---|---|---|
| Respiratory (most common) | Fever, nasal discharge, cough, lethargy, swollen lymph nodes | Symptoms appear 2-10 days after exposure; fever typically lasts 5-10 days | Call same day or within hours of fever and nasal discharge |
| Neurological (EHM) | Ataxia, hindquarter weakness, inability to stand, loss of bladder/rectal control | Can develop rapidly; progression from mild ataxia to paralysis in 24-72 hours | Emergency call immediately; do not delay |
| Abortion | Sudden pregnancy loss, retained placenta, premature labor | Most common 4 months through term; may occur without prior signs | Emergency call immediately; confirm diagnosis with fetus/placenta submission |
Isolation and Barn Management After Suspected EHV-1
If you suspect EHV-1 in your horse, immediate isolation is critical to prevent transmission to other horses. While isolation is a management matter rather than a clinical symptom, it directly affects the health of your entire barn population. Isolate the affected horse in a separate facility with dedicated equipment, separate caregiver access, and strict hygiene protocols. Consult your veterinarian on isolation duration and precautions specific to your situation.
Frequently Asked Questions
Can a horse have EHV-1 without showing respiratory signs?
Yes. Some horses, particularly those with strong immune systems, may have minimal respiratory symptoms or skip the respiratory phase entirely. Mares may abort without any prior illness signs. Additionally, the neurological form (EHM) may develop independently or after respiratory recovery. This is why fever combined with any atypical sign in a horse warrants veterinary evaluation.
How long does EHV-1 fever usually last?
In uncomplicated respiratory EHV-1, fever typically lasts 5 to 10 days, though individual variation is significant. Fever lasting longer than 10 to 14 days or fever that returns after an initial decline may indicate secondary bacterial infection or complications. Daily temperature monitoring during suspected EHV-1 helps you and your vet track disease progression.
Can I tell if my horse has EHV-1 or another respiratory illness just by looking at it?
No. EHV-1 respiratory symptoms are very similar to those of equine influenza, equine rhinovirus, and other upper respiratory pathogens. Laboratory testing (nasal swabs for PCR or viral culture) is required for a confirmed diagnosis. This is why veterinary evaluation is essential—your vet can perform appropriate diagnostic tests to identify the pathogen and guide treatment decisions.
Is EHV-1 fatal?
Respiratory EHV-1 is rarely fatal with appropriate supportive care, though secondary bacterial pneumonia can be life-threatening if untreated. EHM (neurological form) carries a more guarded prognosis; horses with severe neurological signs or complete paralysis may require euthanasia if they cannot stand or care for themselves. Early recognition and aggressive treatment improve outcomes for all forms.
What should I do if I notice symptoms in my horse?
Contact your veterinarian as soon as possible. For respiratory signs (fever and nasal discharge), call the same day. For any neurological signs (ataxia, weakness, inability to stand), call for emergency evaluation immediately. Provide your vet with details on recent horse contacts, travel, or facility introductions. Your vet will perform a physical examination, take nasal swabs if indicated, and may recommend blood work or other diagnostics to confirm diagnosis and rule out other conditions.
Key Takeaways
- EHV-1 has three primary forms: respiratory (most common), neurological (EHM; high-urgency), and abortive (causes pregnancy loss)
- Fever combined with nasal discharge warrants a same-day veterinary call; any neurological signs (ataxia, weakness) require emergency evaluation
- EHM can progress from subtle incoordination to complete paralysis within 24 to 72 hours, making early recognition critical
- Respiratory EHV-1 is confirmed by laboratory testing (PCR or viral culture) because clinical signs resemble other viral respiratory illnesses
- Horses suspected of EHV-1 must be isolated immediately to prevent transmission to other animals on your property
- Secondary bacterial pneumonia can develop during recovery; watch for returning fever, worsening cough, or increased respiratory rate
- This article is not a substitute for veterinary diagnosis or treatment. Always consult an equine veterinarian for suspected EHV-1 cases