Equine Herpesvirus (Rhinopneumonitis) in Horses

Key Takeaways

Equine herpesvirus 1 (EHV-1) and equine herpesvirus 4 (EHV-4), also referred to as rhinopneumonitis (or “Rhino”),  are the most common and widespread forms of herpesvirus to affect horses. 

• Herpesviruses are a major cause of respiratory diseases and, in mares, can cause abortion up to 12 weeks after infection

• Symptoms of EHV usually include fever, nasal discharge, cough, and loss of appetite

• In the neurological form, paralysis and loss of bladder function can develop

• Diagnosis is confirmed by isolating the virus through specialized testing, blood samples, and nasopharyngeal swabs

• There is no specific treatment for equine herpesvirus

• Affected horses must be isolated and rest, nursing care, and medications such as antipyretics help alleviate symptoms

A Closer Look: What is EHV in Horses?

EHV is highly infectious. It is transmitted between in-contact horses and can even spread between horses as they are handled by humans. Due to the highly contagious nature of EHV, affected horses must be isolated immediately. Strict observation of good biosecurity practices helps prevent EHV as well as other infectious diseases.

In most cases, EHV-1 and EHV-4 remain dormant in infected horses. These horses carry the virus in an inactive state and can  infect other horses they come in contact with.

Horses with poor prognosis and/or the neurological form of EHV are often euthanized. It might take months for a horse to recover from the neurological form and in most cases there are long-term neurologic deficits.

Risk Factors

EHV is highly contagious and can affect any horse. In foals, outbreaks of respiratory disease occur annually due to a strain of EHV-1, and in mares, EHV-4 can lead to abortion 2 to 12 weeks after infection. Being an extremely contagious infection, it is common to have seasonal outbreaks in big stables where many horses live. 

The neurological form of EHV can lead to 

Uncoordinated movement (ataxia)

• Paralysis of the hind legs 

• Loss of bladder and tail function

In severe cases, it might develop into quadriplegia and death.

Possible Causes

The cause of EHV is a highly contagious viral infection.

Main Symptoms

The respiratory form (rhinopneumonitis) is the most common and symptoms include:

• Fever


Poor appetite

• Lethargy

• Enlarged lymph nodes

Nasal discharge

• Noisy breathing

• Difficulty eating

Testing and Diagnosis

It is impossible to diagnose equine herpesvirus based on clinical signs alone, as they are indistinguishable from other respiratory diseases. A correct diagnosis is only reached with a PCR test or by isolating the virus in samples taken with a nasal swab or a blood sample.

PCR testing also helps to differentiate between the neuropathological strain and other forms of the infection.

Steps to Recovery

There is no specific treatment for EHV-1 and 4. Symptoms can be alleviated with medications, such as antipyretics, and with rest and nursing care. Nursing care is often intensive, including IV fluid therapy, slinging recumbent horses in a standing position, and providing assistance with bowel and bladder functions. Nursing care is fundamental to avoiding pneumonia, congestion, and other complications. Antibiotics are prescribed for secondary bacterial infections.

The prognosis depends on the strain of herpesvirus affecting the horse. 

When symptoms develop, they may last up to three weeks. 21 days is the minimum isolation period for contagious horses. Mares that abort due to equine herpesvirus recover rapidly and maintain fertility. 

The neuropathological strain has a more dramatic prognosis. Even with treatment, recovery might take up to 3 months and sometimes neurological deficits persist. 

In more severe cases, horses might be euthanized due to an extremely poor prognosis.


Quarantine of symptomatic horses is critical to prevent larger outbreaks in a herd. Strategies include:

• Avoiding nose-to-nose contact with unfamiliar horses

• No sharing of buckets between horses

• Quarantining all new arrivals for a minimum of 2 weeks

It is impossible to completely prevent EHV infections. Vaccines decrease the severity of symptoms and  help minimize the risk of contracting the virus. Vaccinations must be administered according to veterinary recommendations to maximize efficacy, but even appropriately administered vaccines do not protect against the neurologic form of EHV.

Isolation of affected horses is necessary for about 21 days.

Aborted fetuses must be removed immediately as they are highly contagious. 

It is recommended to avoid stressing the herd as sometimes a latent infection might reemerge in stressful conditions.

It is imperative to wash carefully after handling infected horses as the virus can spread to horses through people’s hands and clothing.

Is EHV Common in Horses?

EHV-1 and EHV-4 are the most common forms of equine herpesvirus.

Typical Treatment

• Isolation

• Nursing care

• Medication

• Vaccination

• Stall rest

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