EHV-1 is one of the most contagious of the infectious equine diseases. Knowing how it’s spread and what to watch for are two of the key elements in helping prevent an outbreak amongst your herd.

An EHV Overview

The science of EHV virus strains is fairly complex. They consist of what are called “enveloped double-stranded DNA viruses.” There are five equine herpesvirus strains, with EHV-1 and EHV-4 being the most damaging.

Both of the aforementioned are common in the Americas and abroad. The nature of this disease allows it to be spread quite readily, making it possible for large outbreaks to occur with exposure originating from only one carrier. The incubation time for EHV-1 can last anywhere from 2 – 10 days, meaning a horse can be infected and asymptomatic for more than a week.

EHV-1 is sometimes still referred to as the “equine abortion virus,” as it can cause spontaneous abortion of in-utero foals. EHV-4, on the other hand, tends to show up in foals and yearlings as a respiratory illness often referred to as “equine rhinopneumonitis virus.” It’s important to note that EHV-1 can present with respiratory signs and EHV-4 can also cause abortion. Both of these strains have also been known to cause neurological disease. Because EHV-1 is more prominent and tends to be more devastating, it will be the focus of this article.

Signs and Symptoms of EHV-1

EHV-1 can cause respiratory, reproductive and neurological disease in adult horses, yearlings and foals. Some symptoms are easier to detect than others, making it difficult to diagnose in some cases.

Respiratory Symptoms

Horses suffering from respiratory EHV-1 will display a number of different symptoms. They can be mild to severe. Signs and symptoms for respiratory EHV-1 include:

  • Coughing
  • Lack of appetite
  • Fever between 102° and 107° F that persists for anywhere from 1 day to 1 week
  • Depression
  • Nasal Discharge

Neurological Signs

Horses with the neurological form of EHV-1 will display a handful of symptoms that can include:

  • Hindlimb weakness or paralysis
  • Lack of coordination
  • Inability to control the bladder or tail
  • Numbness throughout the skin of the tail and hindlimb area
  • Inability to rise from a recumbent position

Abortion Symptoms

Unfortunately, there are no obvious signs that a mare is going to abort a pregnancy due to the EHV-1 virus. It can happen in conjunction with respiratory or neurological EHV-1, or without any outward signs that something is amiss.

Diagnosing and Treating EHV-1

If a veterinarian suspects EVH-1, depending on the state they are practicing in, they may or may not be required to report their suspicion of infection to the state’s veterinary governing body. Regardless of reporting requirements, a veterinarian should test for the virus to aid in treatment and help prevent a widespread outbreak.

Diagnosis and Testing

Subjective and objective observation and examination of a horse displaying symptoms will allow a veterinarian to determine if EVH-1 is suspected. If it is, EVH-1 can be tested for via a nose/throat swab from the sick horse (and any horses that may share common areas or facilities with it). Other means of testing can include tissue samples from an aborted fetus or blood tests from the diseased horse. These may also include paired blood samples that will allow for detection of antibody levels.

It is important to note that asymptomatic horses can still be carriers and shed the virus. If a symptomatic horse is suspected of EHV-1, it would be prudent to test all horses that may have come into contact with that horse, or shared common areas. Horses that have recovered from EHV-1 can still be carriers; the virus can be latent for years, but re-triggered when the horse is stressed or otherwise immunocompromised.

Treatment

Because EVH-1 is a virus, supportive care is essential. Antibiotics will not be effective against the virus; most horses do not experience secondary infections as a result of EVH-1, which would negate the need for antibiotic therapy altogether.

  • Horses that are experiencing symptoms of EVH-1 may benefit from the use of non-steroidal anti-inflammatory drugs to lessen inflammation and pain, while helping reduce fever. Nutritional support may also be warranted for horses displaying inappetence.
  • In the case of horses with neurological symptoms, it is important to try and keep them from lying down for prolonged periods of time. If a lack of coordination/balance is an issue, keeping them in a well-padded space, such as a stall with thick shavings can help prevent injury.
  • Recovery for uncomplicated cases may be achieved in as little as a few weeks, whereas, more complicated or compromised horses have a prolonged recovery time and higher rate of mortality. Horses that are suffering from the neurological form and are unable to stand have a much poorer prognosis than those that are able to stand and move around.

How EHV-1 is Transmitted

The highly contagious nature of EHV-1 makes it difficult to control in crowded or regularly frequented facilities. The most common means of transmission from one horse to another include:

  • Indirect contact with the infected horse(s) through communal buckets, troughs or waterers are a major source of infection. Small bodies of water such as puddles or ponds could also harbor EHV-1 easily. Nasal discharges on handlers’ clothing or through tack or turn-out sheets and blankets can also pose a risk to uninfected horses.
  • Direct contact between infected and uninfected horses poses the greatest risk. This can occur through nose-to-nose contact or through nasal secretions being introduced to the mucosal tissues of an uninfected horse via other direct contact.
  • Aerosolized viruses pose a threat when an infected horse sneezes or snorts within close proximity to an uninfected horse.
  • The possibility of transmission through aborted fetuses, placental material or associated fluids is also a risk. If a fetus is aborted, it is advisable to move pen mates to another enclosure and properly clean the area, removing all expelled tissues and fluids and using approved solutions to kill the virus.

Prevention

There is no absolute way to completely prevent exposure or infection from EHV-1. There are, however, methods that can greatly reduce the possibility of horses being exposed to or contracting the disease from infected animals.

  • Vaccinating healthy horses can reduce disease risk. While vaccinations are never an absolute means for preventing any disease, they can reduce the possibility of a horse contracting EHV-1, or at least lessen symptoms and virus shedding if infected, when used properly.
  • Quarantine of new horses for 3 – 4 weeks is essential in helping prevent infection from not only horses carrying EHV-1, but other contagious diseases, as well.
  • Keep compromised or pregnant mares well away from new arrivals or any horse suspected of possible exposure.
  • Reduce stress when possible – latent viruses can become active if the immune system becomes taxed due to stress or other illnesses or injuries.
  • All facilities with a horse testing positive for EHV-1 should remain in quarantine for at least three weeks. This includes all horses – not just symptomatic animals.
  • Good husbandry should be practiced. Washing hands thoroughly after each horse, using an approved antimicrobial footbath and changing out of contaminated clothing before working with healthy animals is essential.
  • Remove bedding and uneaten hay; it’s recommended that this material be burned to avoid accidental exposure of healthy horses.
  • Clean all stalls, fences, alleys and other areas the infected horse(s) have come in contact with using a phenol-based disinfectant. Follow labeling directions to ensure maximum efficacy.

EHV-1 is highly infectious, but with careful handling practices, it may be avoided.

For information on current outbreaks of infectious equine diseases including EHV-1, check out http://www.equinediseasecc.org/outbreaks.aspx . This comprehensive listing shares all outbreaks and status, in addition to the origin of disease, if known.  

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