Equine herpesvirus (EHV), or rhinopneumonitis, is a contagious infection that is endemic to horses worldwide.

EHV may cause mild to severe symptoms that usually involve the upper respiratory tract. In rare cases, it may cause neurological symptoms, abortion or death. [1]

Horses can experience latent infections of EHV where the virus lives dormant without causing symptoms. [1]

There are nine known types of herpes viruses in horses, of which EHV-1 and EHV-4 are the most common. [2] EHV-1 usually results in more severe symptoms but EHV-4 is more common.

It is estimated that almost every horse is infected with EHV-4 at some point in their life. and many may harbour the latent virus for the rest of their lives. [2][3]

Outbreaks of EHV threaten horse health and welfare and can have significant economic influence. EHV outbreaks can be costly because they can result in the cancellation of events and the isolation and treatment of infected horses. [1]

What is Equine Herpesvirus (EHV)?

EHV is a family of highly contagious DNA viruses that can cause various respiratory and neurological symptoms in horses.

EHV can spread quickly because horses may be contagious even if they are asymptomatic.

After infection, EHV may remain dormant in the horse for the rest of its life. However, the latent virus can be reactivated and result in clinical signs and viral shedding. [3]

Reactivation of EHV may be triggered by stressors such as trailering, castration, weaning, or giving birth. EHV can also be reactivated by the administration of corticosteroids. [2][3][4]

Each subsequent reactivation of latent EHV-1 or EHV-4 is usually less severe than the initial infection. [2][5]

Humans are not at risk of contracting EHV. However, humans may be responsible for spreading it between horses through contaminated hands or clothing.

Symptoms of EHV

The clinical signs of EHV usually appear 2 days after infection and typically last for 8 days. Common symptoms include: [2][4][9]

  • Watery nasal discharge which may contain mucus and pus
  • Fever
  • Swollen lymph nodes
  • Appetite loss
  • Eye discharge
  • Depression
  • Coughing

Additional symptoms may occur depending on which form of EHV the horse is infected with. Symptoms are often more severe in younger horses.

Equine Herpesvirus-1 (EHV-1)

The prevalence of EHV-1 is estimated between 13 – 88% in different populations of horses. [1][2][3][4]

Horses can become infected with EHV-1 by nose-to-nose contact with a contagious horse or by coming into contact with respiratory secretions and contaminated equipment, including tack, feed and water buckets, clothing or grooming supplies.

Most adult horses have some immunity to EHV-1 due to previous infection. However, the antibodies are short-lived and may not protect against forms of EHV-1 that can cause abortion or neurological concerns. [8]

EHV-1 is difficult to treat as EHV-1 infected cells can avoid detection from the immune system. [4]

Pathogenesis of EHV-1

EHV-1 can infect different parts of the body and cause different symptoms, which depend on the strain of the virus contracted by the horse. [5]

The virus passes into the upper airways of the horse’s respiratory system which includes the nose, pharynx (throat), and trachea. [4][6]

These airways are lined with a mucosal layer containing epithelial cells. The virus replicates in the epithelial cells causing inflammation, cell damage and respiratory symptoms. [4][6]

EHV-1 can also infect blood vessels. This can result in inflamed blood vessels (vasculitis) and the formation of blood clots that reduce oxygen delivery throughout the body (ischaemic thrombosis). [4][6]

The virus may also reach the peripheral or central nervous system, the lungs, and/or the reproductive organs. [4][7]

In horses with latent EHV-1 infection, the virus infects and remains dormant in cells of the peripheral nervous system and lymphocytes located near the respiratory system. [4]

The latent virus can be reactivated at any time causing the horse to develop clinical signs of EHV-1 and to become contagious.

Respiratory Disease

EHV-1 normally begins in the upper respiratory tract, but it can spread to the lower airways in the lungs and cause bronchopneumonia.

Severe respiratory disease caused by EHV-1 is more common in younger horses, especially weanlings. [8]

EHV-1 respiratory disease is characterized by bleeding, edema, inflamed blood vessels, and low white blood cells in the lungs. [5][9]

Symptoms typically last 9-12 days and can include: [4][8]

  • Fever
  • Swollen lymph nodes
  • Watery nasal discharge
  • Coughing

Secondary bacterial infections can occur which cause nasal discharge to contain mucus or pus. [4]

Neurological Disease

If the EHV-1 virus infects the central nervous system, it can result in a neurological disease known as Equine Herpes Myeloencephalopathy (EHM).

The virus can cause bleeding, inflammation, and blood clots that reduce oxygen and nutrient delivery to the brain and spinal cord. [1][4]

Oxygen deprivation and inflammation can cause neurons to degenerate, which results in neurological symptoms. [4]

Symptom onset usually occurs up to 10 days after the horse is infected with EHV-1. [4][7][8][10]

Clinical signs of EHM can include [11]:

  • Uncoordinated movements
  • Paralysis in one or more limbs
  • Fecal and/or urinary incontinence
  • Laying down
  • Head tilting
  • Paralyzed tail
  • Blindness
  • Build up of fluid in lower limbs
  • Weakness

Recovery may take days to weeks depending on how severe the infection is. [5]

The prevalence of neurological EHV-1 varies between outbreaks and the strain of the virus. Some studies report that 10% of EHV-1 infections cause neurological symptoms, while others report neurological symptoms in up to 50% of cases. [1][8][12][20][21]

The following factors affect a horse’s risk of contracting the neurological symptoms associated with EHV-1: [4][5][8][11]

  • Age: the risk increases with age
  • Breed: Standardbreds, Hispanic breeds, and Draft breeds may be at higher risk, while ponies and smaller breeds may have lower risk
  • Sex: Mares (especially pregnant or nursing) are at increased risk
  • Season: Most cases occur during winter
  • Virus strain
  • Presence of fever

Abortion and Neonatal Foal Disease

EHV-1 can reach the uterus in pregnant mares, which may result in abortion of the fetus. [5]

Because EHV-1 is considered a major cause of fetal loss, the virus was previously referred to as Equine Abortion Virus. [4] One study found that 10% of all abortions in a population of thoroughbreds were attributed to EHV-1. [1][5][“22”]

A large majority of abortions caused by EHV-1 occur in the third trimester. [1][5]

EHV-1 arrives at the uterus via the bloodstream. The virus then enters the the inner lining of the uterus and causes inflamed blood vessels and blood clots. [4] The virus causes fetal membranes to detach from the endometrium, resulting in abortion.

EHV-1 can also pass through the umbilical cord to reach and fatally infect the fetus. [4]

A mare infected with EHV-1 close to term may deliver a live foal, but in most cases, the foal does not survive for more than a few days. [4] Foals born alive are often weak, jaundiced, struggle to breathe, and may have interstitial pneumonia. [5]

EHV-1 does not cause lasting infertility or affect future pregnancies, unless a latent infection is subsequently reactivated.

EHV-1 can be easy to miss in mares. However, some mares will show edema in the lower legs, lack of appetite, fever, and respiratory symptoms.

Abortions often coincide with respiratory or neurological EHV-1 symptoms. [4][5]

Effects On Male Fertility

Knowledge of how EHV-1 affects male fertility is limited and requires further research.

One study investigated EHV-1 in pony colts and stallions. Researchers found that the virus was delivered by the bloodstream to the genitalia, specifically the testes and epididymis. [7]

Infected genitalia developed inflamed blood vessels, edema, and blood clots. EHV-1 also caused some of the stallions’ sperm to be malformed, resulting in reducing fertility during the infection. [7]

Eye Infections

EHV-1 can reach the eye causing multifocal lesions on the chorion and retina, also known as chorioretinopathy. Eye infections are usually subclinical but can sometimes result in eye damage, vision impairment and/or blindness. [4]

In experiments, 50-90% of horses infected with EHV-1 contracted lesions in the eye. However, the prevalence of chorioretinopathy may differ outside of experimental settings. [12]

EHV-1 may also cause uveitis or chorioretinitis. These conditions result in painful and inflamed eyes, and may affect the horse’s vision. [8]

Equine Herpesvirus-4 (EHV-4)

EHV-4 infections are often mild and may not cause the horse to experience any symptoms at all.

If symptoms do occur, they mainly consist of respiratory symptoms. EHV-4 is rarely associated with abortions or neurological symptoms.

However, there are several strains of EHV-4 and the severity of symptoms depends on which form the horse is infected with. [2][13]

Like EHV-1, EHV-4 infection begins in the horse’s upper respiratory system. The virus multiplies in the respiratory epithelium and spreads to the lymph nodes and blood. [5]

EHV-4 infects and estab