EHV-1 Symptoms: Early Warning Signs and When to Call the Vet

Equine herpesvirus-1 (EHV-1) is a highly contagious respiratory virus that affects horses of all ages and can progress rapidly from mild symptoms to serious complications. Many horse owners miss the earliest warning signs of EHV-1 infection because the initial symptoms appear subtle and may resemble other common equine illnesses. Understanding the earliest indicators of EHV-1 infection–before your horse develops obvious clinical disease–is critical to stopping viral spread, supporting recovery, and preventing severe neurological complications that can develop within days of infection onset.

The key to protecting your horse and your barn lies in recognizing the very first signs of illness. EHV-1 often begins with fever and behavior changes that many owners attribute to stress, weather changes, or minor illness. This article focuses specifically on the early warning signs that should trigger immediate veterinary attention, the biphasic fever pattern characteristic of EHV-1, and how to distinguish these subtle symptoms from normal variations in your horse’s daily health.

Understanding EHV-1 Infection Timeline

EHV-1 spreads through respiratory secretions, direct contact, and fomites (contaminated equipment, clothing, and surfaces). After exposure, the incubation period typically ranges from 2 to 10 days before the first symptoms appear. During this window, your horse may be shedding virus and exposing other horses in your barn without showing any signs of illness. Once symptoms begin, they can escalate quickly, making early detection essential.

The infection progresses through distinct phases. The respiratory phase begins with fever and mild upper respiratory signs. If your horse’s immune system controls the virus at this stage, recovery may proceed smoothly. However, if viral replication continues unchecked, the virus can spread to the central nervous system, potentially causing neurological disease that ranges from subtle incoordination to complete paralysis.

The Biphasic Fever: Your First Real Clue

One of the most distinctive and earliest signs of EHV-1 infection is a biphasic fever pattern–meaning your horse’s temperature rises, may drop briefly, and then rises again. This pattern is not typical of many other equine illnesses and should immediately raise suspicion for EHV-1.

How to Recognize Biphasic Fever

• First fever spike: Temperature rises to 101.5-104.0 F (38.6-40.0 C), typically lasting 24 to 48 hours
• Apparent recovery period: Temperature drops, sometimes appearing nearly normal for 12 to 24 hours, causing owners to believe the horse is recovering
• Second fever spike: Temperature rises again, often to the same range or higher, accompanied by worsening respiratory or neurological signs

Many horse owners take a single temperature reading when a horse seems unwell, find it elevated, and then assume recovery is underway if the next day’s temperature is lower. This false sense of security during the apparent recovery period is dangerous. Vigilant owners should measure temperature every 4 to 6 hours during any unexplained fever to detect the biphasic pattern early. If you observe two distinct fever spikes separated by a brief interval of normal or near-normal temperature, contact your veterinarian immediately and suspect EHV-1 until proven otherwise.

Subtle Behavioral Changes Before Fever Appears

Fever is not always the first sign. Some horses show behavioral and attitude changes before their temperature becomes elevated. These early signs are easy to overlook because they mimic stress, fatigue, or minor discomfort from many causes.

Pre-Fever Behavioral Indicators

• Lethargy and depression: Your horse appears unusually quiet, stands for extended periods with head low, and shows diminished interest in food or the environment
• Appetite changes: Reduced interest in hay or grain, or slower eating pace than normal
• Increased eyelid drooping (ptosis): Eyes appear partly closed without obvious irritation or injury
• Inattention: Your horse seems withdrawn, unresponsive to sounds or activities that normally catch their attention
• Reluctance to exercise: Decreased energy during turnout or riding, unusual stumbling or clumsiness
• Hollow appearance: Loss of belly definition over just 24 to 48 hours as the horse eats less and stops drinking normally

If your horse displays any combination of these signs–especially if they develop acutely (suddenly, within hours) rather than gradually over days–measure their temperature immediately. Even if temperature is normal, document the signs and call your veterinarian. EHV-1 can cause subtle neurological signs before fever becomes obvious, and early intervention may prevent progression to severe disease.

Early Respiratory Signs

While EHV-1 is classified as a respiratory virus, the earliest respiratory signs are often extremely mild and easy to dismiss. Owners frequently expect obvious nasal discharge or coughing, but EHV-1 may begin with signs so subtle that they seem insignificant.

Mild Respiratory Indicators to Watch

• Slight nasal discharge: Clear or slightly cloudy discharge from one or both nostrils; may appear only when you lead your horse around, not at rest
• Quiet cough: Occasional, sporadic cough that sounds dry or slightly productive; fewer than 5 to 10 coughs per day in early infection
• Mild congestion sounds: Subtle increased respiratory noise during exercise; may disappear at rest
• Reluctance to eat hay: Difficulty chewing hay due to throat soreness; your horse may drop hay or chew slowly without obvious coughing

The absence of heavy nasal discharge or frequent coughing does not rule out EHV-1. Many horses are infected and highly contagious before developing obvious respiratory signs, or they may never develop coughing at all. If your horse shows even mild respiratory signs along with fever or behavior changes, isolate the horse and contact your veterinarian.

Early Neurological Signs: A Critical Red Flag

Neurological signs can appear very early in EHV-1 infection, sometimes before–or instead of–obvious respiratory illness. These subtle signs are often misinterpreted as minor injuries, poor footing, or rider error. Recognizing them early is essential because neurological damage progresses rapidly once it begins.

Subtle Neurological Warning Signs

• Incoordination (ataxia) in the hind limbs: Swaying or wavering from side to side when standing still, particularly noticeable when viewed from behind; loss of precise hind-limb placement when walking
• Shortened stride: Your horse’s gait becomes choppy or restricted without obvious lameness or pain
• Stumbling or tripping: More frequent trips or stumbles than normal, especially on uneven ground or when turning sharply
• Dragging toes: Particularly the hind toes; you may see scuff marks on shoes or hooves
• Difficulty backing: Hesitation or unsteadiness when asked to back up; loss of precise hind-limb control
• Facial nerve involvement: Drooping of one side of the face, difficulty eating or drinking, drooling
• Urinary dysfunction: Inability to retract penis in stallions, dribbling urine, or fecal incontinence

Neurological signs demand immediate emergency veterinary evaluation. If your horse shows any incoordination, difficulty moving, or facial nerve changes, call your veterinarian immediately. Do not wait 24 hours to see if the sign resolves on its own. Neurological EHV-1 progresses rapidly, and early intervention with specific therapies (such as high-dose acyclovir or valacyclovir) may prevent permanent damage.

Environmental and Herd Factors That Increase Risk

Understanding when your horse is at heightened risk for EHV-1 exposure helps you implement heightened vigilance. Certain situations create ideal conditions for viral transmission and rapid spread within a barn population.

Risk Factor	Why It Matters	Your Action
Recent horse show or event participation	Exposure to horses from multiple facilities and regions; incubation period is 2-10 days post-exposure	Isolate returning horses for 14 days; monitor temperature and signs daily
New horse arrival at the barn	Quarantine period may be insufficient if the horse is in early incubation phase	Separate new arrivals for at least 4 weeks; use dedicated staff and equipment
Shared water troughs or equipment	Virus survives on surfaces and spreads via contaminated equipment and fomites	Disinfect shared items daily during any suspected illness; separate water sources
Overcrowded turnout or housing	Close proximity increases respiratory transmission risk	Increase space between horses if any show signs of illness
Mixed age groups without separation	Young horses and immunocompromised individuals are more vulnerable	Monitor younger and older horses more closely; isolate high-risk animals first

If any horse at your facility has been exposed to horses from other locations, or if you are attending multiple barns, implement strict hygiene protocols immediately: wash hands and change clothes between facilities, disinfect equipment, and monitor your horse closely for 14 days following potential exposure.

Taking Your Horse’s Temperature Accurately

Accurate temperature measurement is your best tool for early EHV-1 detection. Normal equine body temperature ranges from 98.0 to 101.0 F (36.7 to 38.3 C), with average being 99.5 F (37.5 C). Any sustained reading above 101.5 F (38.6 C) warrants investigation.

• Use a digital thermometer (rectal measurement is most reliable)
• Insert thermometer 2 to 3 inches into the rectum at a slight upward angle
• Hold in place for 30 to 60 seconds until the thermometer beeps
• Measure temperature at the same time each day for consistency
• During any illness, measure temperature every 4 to 6 hours to detect fever patterns
• Document readings in writing or on your phone so you can describe the pattern to your veterinarian

If you observe an unexplained fever above 102.0 F (38.9 C) lasting more than 12 hours, do not wait. Contact your veterinarian and describe any other signs you have noticed. Provide your temperature measurements and the timeline of symptom onset. This information is critical to rapid diagnosis and early treatment initiation.

When to Call Your Veterinarian Immediately (Emergency Situations)

Some situations require emergency veterinary attention, not routine appointment scheduling:

• Fever above 103.5 F (39.7 C) lasting more than 12 hours
• Any neurological signs: incoordination, difficulty walking, facial drooping, or dragging toes
• Biphasic fever pattern (fever dropping then rising again within 24 to 48 hours)
• Inability or severe reluctance to stand or move
• Sudden onset of severe lethargy or depression with high fever
• Difficulty swallowing, excessive drooling, or inability to drink
• Paralysis of any limb or part of the body
• Any sign of urinary incontinence or inability to urinate

Your veterinarian can perform nasopharyngeal or tracheal swabs for PCR testing (polymerase chain reaction), blood tests, and neurological examination to confirm EHV-1 infection and assess severity. Early diagnosis allows for prompt initiation of antiviral therapy, supportive care, and strict isolation protocols to prevent spread to other horses in your facility.

Isolation Protocols If EHV-1 Is Suspected

If your horse shows signs suspicious for EHV-1, isolation is not optional–it is essential to protect other horses in your barn. The virus is highly contagious and spreads rapidly through respiratory secretions and contaminated equipment.

• Physically separate the suspected horse from all other horses immediately
• Assign dedicated staff and equipment (halter, lead, grooming supplies, water bucket) to the isolated horse
• Wear protective clothing (coveralls, gloves, boot covers) when handling the horse and change before approaching other horses
• Disinfect all equipment and surfaces with a dilute bleach solution (1 part bleach to 10 parts water) or quaternary ammonium disinfectant daily
• Do not share hay feeders, water troughs, or grooming supplies between the isolated horse and others
• Maintain isolation for at least 14 to 21 days after the last fever, and longer if neurological signs are present
• Consult your veterinarian on the appropriate duration of isolation based on your horse’s clinical course and test results

Frequently Asked Questions

Can EHV-1 kill a horse?

Yes, EHV-1 can be fatal, though death rates vary. Most horses with respiratory EHV-1 infection recover with appropriate supportive care. However, horses that develop severe neurological disease (particularly paralysis) have a much poorer prognosis. Some affected horses must be euthanized due to severe, irreversible neurological damage. Pregnant mares infected with EHV-1 may abort (lose pregnancy). Early recognition and treatment significantly improve outcomes, which is why early detection is so critical.

How is EHV-1 diagnosed?

Your veterinarian will diagnose EHV-1 using PCR testing on samples collected via nasopharyngeal or tracheal swabs, or blood tests. PCR is the most sensitive and specific test, particularly when collected during the first week of infection. Antibody testing (serology) may show evidence of prior infection but does not diagnose active, current infection. Your veterinarian will combine test results with clinical signs to confirm diagnosis.

Can vaccinated horses get EHV-1?

Yes, vaccinated horses can become infected with EHV-1. Vaccines reduce the risk and severity of infection, but do not provide complete protection. Vaccinated horses that become infected typically have milder disease and shorter recovery times. However, they can still shed virus and infect other horses. If you have vaccinated horses that show signs of illness, follow the same isolation and diagnostic protocols as unvaccinated horses.

How long does it take to recover from EHV-1?

Recovery from respiratory EHV-1 typically takes 2 to 6 weeks with appropriate care. However, neurological disease recovery is much slower and often incomplete. Horses with neurological EHV-1 may require weeks to months of supportive care and rehabilitation, and some never fully recover normal coordination. The severity of initial infection and speed of treatment initiation significantly impact recovery timeline and outcome.

Should I vaccinate my horse against EHV-1?

The decision to vaccinate should be made in consultation with your veterinarian based on your horse’s age, use (competition horses are higher risk), and exposure risk in your region or facility. Vaccines are available as respiratory vaccines (given intramuscularly or intranasally) and pregnant mare vaccines (to prevent abortion). Your veterinarian can recommend the best vaccination protocol for your horse. Even vaccinated horses require the same vigilant monitoring for early signs, as vaccination does not guarantee protection.

Key Takeaways

• EHV-1 infection often begins with subtle signs: lethargy, slight fever, mild nasal discharge, or behavior changes that mimic minor illness or stress
• The characteristic biphasic fever pattern–fever that drops then rises again within 24 to 48 hours–is a strong indicator of EHV-1 and demands immediate veterinary evaluation
• Neurological signs (incoordination, stumbling, facial drooping, difficulty backing) can appear early and progress rapidly; any neurological sign requires emergency veterinary attention
• Monitor your horse’s temperature daily during any illness or after potential exposure to infectious horses; document patterns and report them to your veterinarian
• Isolate any horse with suspected EHV-1 immediately to prevent spread to other horses in your barn
• Contact your veterinarian promptly if you observe unexplained fever lasting more than 12 hours, behavior changes, neurological signs, or respiratory illness
• Early recognition and veterinary intervention significantly improve prognosis and reduce the risk of severe neurological complications and death
• This article is not a substitute for veterinary diagnosis or treatment; always consult your equine veterinarian regarding your horse’s health and medical care