Rhinopneumonitis in Horses: “Rhino Flu” Transmission, Risk Factors & Treatment

Rhinopneumonitis in horses, commonly called “rhino flu” or “rhino,” is a contagious respiratory disease caused by equine herpesviruses (EHV-1 and EHV-4). These viruses are widespread in horse populations around the globe, and most horses are exposed at some point in their lives.

While many cases of rhinopneumonitis are mild and resemble a common cold, outbreaks can spread quickly through barns, shows, and training facilities if biosecurity measures are not in place. Fever is often the earliest sign, followed by nasal discharge, coughing, lethargy, and reduced appetite.

Most horses recover fully with rest and supportive care. However, certain strains, particularly EHV-1, are monitored more closely due to their potential risk of neurological disease or reproductive complications in pregnant mares.

Understanding how rhinopneumonitis spreads, what symptoms to watch for, and how vaccination and biosecurity reduce risk can help protect individual horses and limit barn-wide disruption.

Rhinopneumonitis in Horses

Rhinopneumonitis, or equine herpes viral respiratory disease, is a common respiratory illness in horses that is caused by equine herpesviruses (EHV-1 and EHV-4). Often referred to as “rhino” by owners, rhinopneumonitis usually affects the respiratory system and often causes mild to moderate illness, especially in young horses. ^[1]

Horses with rhinopneumonitis may develop a fever, nasal discharge, cough, and appear tired or off their feed. The disease spreads easily where horses are in close contact, such as boarding barns, breeding farms, shows, and training facilities.

Stress from travel or changes in routine can increase the risk of illness. ^[2]

Although most horses recover with rest and supportive care, rhinopneumonitis can disrupt training schedules and, in some cases, lead to complications or outbreaks within a barn.

Clinical Signs of Rhinopneumonitis in Horses

The clinical signs of rhinopneumonitis vary depending on a horse’s age, immune status, and level of stress, but most cases involve mild to moderate respiratory illness. After exposure, the virus begins to replicate in the body, with early infection typically developing within the first 2 to 5 days. ^[2]

Fever is usually the earliest and most consistent sign of infection and may appear before any obvious respiratory symptoms. Because of this, monitoring body temperature is one of the most effective ways to detect illness early, particularly in barns with multiple horses.

Common symptoms of rhino in horses include: ^[1][3][4][5]

• Fever
• Nasal discharge, which may start clear and become thicker over time
• Coughing
• Lethargy or decreased energy
• Reduced appetite

Some horses may show only mild signs, while others appear more noticeably unwell. Young horses are more likely to develop obvious respiratory symptoms. Adult horses may have subtle signs but can still shed the virus and infect others.

When to Call the Veterinarian

Contact your veterinarian promptly if your horse develops signs of rhinopneumonitis or another viral respiratory disease, such as:

• A persistent or high fever
• Worsening nasal discharge
• Severe or persistent cough
• Difficulty breathing
• Complete lack of appetite or thirst

Immediate veterinary attention is needed if you notice neurological signs such as:

• Incoordination
• Weakness
• Difficulty standing
• Changes in behavior

Early evaluation can help confirm the diagnosis, guide supportive care, and reduce the risk of complications or spread to other horses.

Causes

Rhinopneumonitis is caused by equine herpesviruses, most commonly equine herpesvirus type 1 (EHV-1) and equine herpesvirus type 4 (EHV-4).

These viruses mainly affect a horse’s upper respiratory tract, including the nose, throat, and airways. EHV-4 is the most frequent cause of routine respiratory disease, while EHV-1 is less common but can be associated with more serious complications. ^[1][3][6]

Horses become infected when they inhale virus particles from the air or come into contact with nasal secretions from an infected horse. The virus can also spread through shared equipment, hands, clothing, and grooming tools. Once inside the body, the virus multiplies in the lining of the respiratory tract, leading to fever and cold-like signs.

A key feature of equine herpesviruses is their ability to remain dormant (inactive) in a horse’s body after the initial infection. Horses that appear healthy can carry the virus for life, and stress such as travel, weaning, illness, or heavy training can cause the virus to reactivate and spread to other horses. ^[1]

EHV-1 vs. EHV-4: What’s the Difference?

Several herpesviruses can infect horses, but rhinopneumonitis refers specifically to the respiratory form of equine herpesvirus infection. This is an important distinction, as equine herpesviruses can affect horses in different ways depending on which strain of the virus is involved.

Rhinopneumonitis is most commonly caused by two closely related viruses: equine herpesvirus type 1 (EHV-1) and equine herpesvirus type 4 (EHV-4). These two viruses were once thought to be the same, but research has shown they are distinct and can affect horses in slightly different ways. ^[1][7][8]

Both EHV-1 and EHV-4 are contagious and can cause similar clinical signs, including fever, nasal discharge, and coughing. Because these signs overlap, it is not possible to determine which virus is involved based on clinical signs alone. ^[3]

EHV-4 is the most common cause of routine respiratory rhinopneumonitis and is responsible for most typical outbreaks, particularly in young horses. EHV-1 occurs less often in routine respiratory cases but is monitored more closely. While it can cause the same respiratory illness as EHV-4, EHV-1 also has higher potential to affect other body systems, which is why outbreaks involving EHV-1 are managed with additional caution. ^[5]

Because EHV-1 and EHV-4 cannot be reliably distinguished based on clinical signs alone, laboratory testing is often used during respiratory outbreaks to guide appropriate biosecurity and management decisions.

Transmission Routes

Understanding how rhinopneumonitis spreads helps explain why outbreaks can occur even in well-managed barns. It also underscores the importance of early isolation and good hygiene when a horse develops a fever or respiratory signs.

Rhinopneumonitis spreads easily from horse to horse, especially in barns or facilities where horses are kept close together. The virus is most often spread through the air when an infected horse coughs, snorts, or has nasal discharge. ^[4]

It can also spread through direct contact with infected secretions or indirectly through contaminated hands, clothing, and shared equipment such as buckets, grooming tools, and tack. ^[4]

Risk Factors

Some horses and situations increase the likelihood of rhinopneumonitis spreading or causing illness.

Risk factors for rhino in horses include: ^{[1][4][9][10]}

• Age: Young horses, including foals, weanlings, and yearlings, are at higher risk because their immune systems are still developing. They may have limited prior exposure to equine herpesviruses and can shed higher amounts of virus when infected, increasing the risk of transmission within a group.
• Shared housing: Boarding barns with frequent horse movement can increase exposure risk. New arrivals may introduce the virus to resident horses, and close contact in shared airspace allows respiratory secretions to spread easily between stalls.
• Shared facilities: Training facilities, shows, sales, and competitions bring together horses from multiple locations. Direct nose-to-nose contact, shared equipment, and contaminated surfaces can facilitate viral spread, particularly when horses return home after exposure.
• Stress: Travel, changes in routine, and other stressors can weaken a horse’s immune system and may allow the virus to reactivate in dormant cases. These horses can then spread the virus to others, even if they show only mild signs.
• Lack of quarantine: Mixing horses from different locations without an isolation period increases the likelihood of introducing the virus into a barn. Horses may shed EHV before showing clinical signs, so failing to quarantine new or returning horses can allow silent transmission to occur.

Diagnosis

Rhinopneumonitis is often suspected based on a horse’s clinical signs, travel and competition history, and known exposure to other horses, particularly in settings where respiratory illness or fever has recently occurred.

Fever is usually one of the earliest signs, sometimes appearing before nasal discharge or coughing. In barns with multiple horses, a sudden increase in fevers or respiratory illness may raise concern for an outbreak. ^[11]

A veterinarian can confirm the diagnosis using laboratory testing. This typically involves collecting a nasal swab or blood sample and testing it for equine herpesvirus using PCR (polymerase chain reaction) testing. This testing confirms whether EHV is present and what strain is causing symptoms, which helps guide management decisions, especially in group housing situations. ^[1][4]

Horses showing neurologic signs may require additional evaluation to assess the risk of equine herpesvirus myeloencephalopathy (EHM).

Early diagnosis is important for limiting the spread of the virus. Identifying infected horses allows for prompt isolation, increased biosecurity measures, and monitoring of other horses in the barn. Even horses with mild signs may be tested during an outbreak to help prevent further transmission.

Treatment

There is no specific antiviral cure for rhinopneumonitis, so treatment focuses on supportive care and management while the horse’s immune system clears the infection. Most horses recover fully with rest and appropriate care. ^[4]

Key aspects of treatment and management include: ^[2][4][11]

• Rest: Horses should be removed from training or work until they have fully recovered. Returning to work too soon can delay recovery or increase the risk of relapse.
• Monitoring: Daily temperature checks help track recovery and identify horses that may be newly infected.
• Hydration and nutrition: Ensuring access to fresh water and encouraging normal feed intake supports immune system function during recovery.
• Supportive veterinary care: In some cases, anti-inflammatory medications may be used to reduce fever and discomfort. Antibiotics are not effective against viruses but may be prescribed if a secondary bacterial infection is suspected.

Isolation of affected horses is a critical part of management. Horses showing signs of rhinopneumonitis should be kept separate from healthy horses, with dedicated equipment and limited human contact to reduce the risk of spread.

Prognosis

The course of rhinopneumonitis can vary depending on the horse’s age, immune status, and level of stress, but most horses follow a similar general pattern.

Fever often appears first and may last several days. Respiratory signs such as nasal discharge or coughing may persist for one to two weeks, and mild fatigue can continue even after visible signs improve. During this time, rest is critical to allow the immune system to fully clear the infection. ^[4]

In otherwise healthy adult horses with no underlying medical conditions, full recovery from uncomplicated respiratory EHV infection is common. Clinical signs typically resolve within two to three weeks, although some horses may require additional time before returning to their previous level of performance.

Recovery time can be longer in foals, older horses, pregnant mares, or horses with pre-existing health concerns such as poor body condition, concurrent respiratory disease, or compromised immune function. Horses experiencing complications, including secondary bacterial infections or neurologic involvement, may have a more guarded prognosis and require extended monitoring and care.

Although many horses appear brighter within a short period, returning to work too soon can delay recovery or increase the risk of relapse.

Potential Complications of Rhinopneumonitis

Most horses with rhinopneumonitis experience a self-limiting respiratory illness and recover fully with appropriate care. However, as with many infectious diseases, complications can occur in some cases.

One possible complication is a secondary bacterial infection of the respiratory tract. Viral infections can weaken the horse’s natural defenses, allowing bacteria to take hold in irritated airways. Horses with prolonged nasal discharge, persistent fever, or worsening cough may require veterinary reassessment. ^[2]

In young, stressed, or immunocompromised horses, recovery may take longer, and clinical signs may be more pronounced. Extended periods of rest may be needed to ensure full recovery before returning to training. ^[2]

In some cases, particularly with EHV-1, infection can lead to neurological disease or reproductive complications in pregnant mares. These outcomes demonstrate why equine herpesvirus outbreaks are managed carefully, even when initial signs appear mild. ^[12][13][14]

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Prevention

Preventing rhinopneumonitis focuses on reducing exposure to the virus, supporting the horse’s immune system, and limiting spread when exposure does occur. Because equine herpesviruses are widespread and many horses carry the virus without showing signs, prevention is about risk management rather than complete elimination.

Vaccination

Vaccination plays an important role in reducing the severity of respiratory illness and limiting viral shedding. Vaccines for EHV-1 and EHV-4 are commonly included in routine equine vaccination programs, particularly for young horses and those who travel or are housed in group settings. ^[1]

While vaccination does not completely prevent infection, vaccinated horses are less likely to develop severe clinical signs and may recover more quickly. Vaccination schedules should be tailored to the individual horse based on age, exposure risk, and veterinary guidance.

Biosecurity & Hygiene

Robust biosecurity practices are essential, especially in barns with multiple horses or frequent movement on and off the property.

Best practices to reduce the risk of spread include: ^[1][15][16]

• Isolating horses with fever or respiratory signs
• Avoiding shared water buckets, feed tubs, and grooming equipment
• Washing hands or changing gloves between handling horses
• Having separate handlers for infected and non-infected horses
• Cleaning and disinfecting commonly used equipment
• Limiting nose-to-nose contact between unfamiliar horses

New arrivals should ideally be monitored and, when possible, kept separate from resident horses for a short period to allow time for signs of illness to appear.

Managing Stress

Stress can weaken the immune system and trigger reactivation of dormant herpesvirus in previously infected horses. Minimizing stress where possible can help reduce the risk of illness and viral shedding. ^[2]

Common stressors include:

• Transport and travel
• Weaning
• Heavy training schedules
• Changes in environment or routine
• Nutritional deficiencies

Gradual transitions, appropriate conditioning, and adequate rest can help support immune function, particularly during high-risk periods such as show seasons or training changes.

Monitoring

Early detection plays a key role in preventing outbreaks. Regularly monitoring horses for fever—especially after travel or exposure to new horses—can allow for early isolation before respiratory signs become widespread.

To measure a horse’s rectal temperature, you will need a thermometer and a lubricant. If lubricant is unavailable, water also works.

Stand beside the horse’s hindquarters, positioned next to the back leg and facing toward the tail rather than the head. This stance reduces your risk of injury if the horse kicks. With one hand, gently move the tail to the side. Some horses may resist or clamp their tail down, so maintain steady, calm pressure until you can safely proceed.

With your other hand, insert the lubricated thermometer into the rectum. Angle the tip slightly toward the side of the rectal wall, rather than straight into the center. This helps ensure you are measuring body temperature and not the temperature of manure.

Rectal Temperature Check Photo: Dr. Madison Ricard, DVM, PhD, DACVP, PAS

If using a traditional mercury thermometer, hold it in place for approximately one minute. ^[17] Because mercury thermometers can break and pose a safety risk, digital models are generally preferred. If a mercury thermometer must be used, attach it to a string or clip so it can be retrieved easily if dropped or drawn further into the rectum.

Digital thermometers will signal when the reading is complete, typically with a beep.

Table 1. Summary of horse body temperature guidelines

What to Do If Rhinopneumonitis Is Suspected in a Barn

Early action is one of the most effective ways to limit the spread of rhinopneumonitis within a barn. A single horse with a fever or mild respiratory signs can be the first indication of a developing outbreak.

Horses showing signs of illness should be isolated immediately, and a veterinarian should be contacted to assess the situation and determine whether diagnostic testing is needed.

Because the virus spreads through airborne droplets and direct contact, strict hygiene and disinfection practices are critical. This includes hand hygiene, cleaning shared equipment, and limiting unnecessary contact between horses. ^[1][4]

During an outbreak, horses that are sick or have been in close contact with affected animals may need to remain isolated for an extended period, often several weeks after the last horse has recovered. Temporary movement restrictions, such as postponing travel, shows, or new arrivals, may also be necessary. ^[4][19][20]

New horses, or those returning from other facilities, should be isolated before mixing with resident horses to reduce the risk of introducing infection. Minimizing stress during this time is important, as stress can trigger reactivation of latent virus.

Pregnant mares should be housed separately from young horses and those in active training whenever possible. ^[6]

Clear communication among barn staff, owners, and veterinarians helps ensure that precautions are followed consistently. While these measures may feel disruptive, early and thorough management can significantly reduce the length and impact of an outbreak.

Frequently Asked Questions

Here are some frequently asked questions about rhinopneumonitis in horses:

Summary

Rhinopneumonitis is a common equine respiratory disease caused by equine herpesviruses (EHV-1 and EHV-4). Most horses are exposed at some point in their lives, and illness is usually mild to moderate.

• The most common signs include fever, nasal discharge, coughing, lethargy, and reduced appetite
• Fever is often the earliest sign, making temperature monitoring an important early detection tool
• The disease spreads easily between horses, especially in barns, shows, and training facilities, through airborne droplets, direct contact, and shared equipment
• Horses with mild or no signs of rhinopneumonitis can still spread the virus
• Most horses recover fully with rest and supportive care, including isolation, monitoring, hydration, and veterinary guidance
• Prompt isolation of sick horses and good hygiene practices help limit outbreaks and reduce disruption in barns