Equine Herpes Virus

Story originally posted by: Michael Lowder, DVM, MS

There are five herpes viruses that affect horses with an additional three that affect donkeys. Of these, two types cause the most significant disease. These are equine herpes virus type 1 (EHV-1) and type 4 (EHV-4). Both of these cause the disease known as rhinopneumonitis.

Three disease forms of the equine herpes virus infection may be identified: respiratory, abortion, and neurological. Of these, you can only vaccinate for the respiratory and abortion form of the disease. The neurological form of the disease cannot be vaccinated against.

Both EHV-1 & EHV-4 can affect the respiratory system. Infection is frequently complicated by secondary bacterial invasion. EHV-4 is mainly a respiratory pathogen, whereas EHV-1 can cause multiple diseases.

In addition to respiratory form, EHV-1 infection may also result in:

  • late term abortion of virus-infected fetuses by infected gravid mares
  • birth of weak nonviable foals
  • neurologic disease (usually paralytic)
  • myeloencephalopathy secondary to vasculitis that develops in the spinal cord and brain

The disease outbreak at the University of Findlay English Riding farm is a frightful event that could happen at any stable. A preventive medicine plan was in place at the University, and all incoming horses were required to be vaccinated a minimum of two weeks prior to their arrival. All horses at the University English facility (there are over a hundred equines stabled there) had been vaccinated for the herpes virus infection (rhinopneumonitis) prior to this terrible event.

Most of the outside horses were introduced about a week before the horses at the school became sick. It is estimated that approximately 85% of the horses developed a fever, became depressed and went off feed. Fevers as high as 106(F (41(C) were observed. The clinical signs, in addition to the above, varied among horses with some showing nasal discharge, edema of the lower limbs, lethargy, coughing and hives.

The fist horse with neurological signs showed up about four to six days after the general illness set in. Several other horses quickly followed over the next few days with about 40 horses eventually showing neurological signs. The disease was confirmed to be the neurological form of rhinopneumonitis via nasopharyngeal swabs and histopathology from a fatality.

The severity of neurological symptoms may vary. Clinical signs may include altered gait, head tilt, and weakness of the hind limbs so great that the horse goes down and cannot rise. Some horses may have difficulty passing urine and manure.

So how is the disease spread? Inhaling the virus in aerosolized secretions from infected coughing horses is one way. Exposure to nasal secretions via direct and indirect contact by contaminated equipment, feed and water can also infect others. In the case of abortions due to EHV-1, the aborted fetus, fetal fluids, and placenta may transmit the virus.

Respiratory infections are common in young horses due to EHV-1 & EHV-4 when they are stressed, i.e., weaning, yearling sales, entering training, etc. This is why we see outbreaks in closed horse populations (those which no outside horses have been allowed in). Because the virus is endemic in the horse population, most of our mature horses have developed some form of immunity to the respiratory form of the disease.

Horses do not develop immunity to the neurological and abortion forms of the disease; thus, the need for vaccinating our horses. The horseman should use a vaccine that contains both the EHV-1 & EHV-4 virus strains. However, it should be noted that the vaccines are not 100% protective.

The unique thing about equine herpes virus infection is that the virus can remain dormant in chronic carrier horses that do not show clinical signs of the disease. Some older horses may acquire the respiratory form of the disease and remain subclinical. These horses may shed the virus only when they (or their immune system) are stressed. The incubation period of the virus varies between two to ten days. It has also been shown that cattle and donkeys can serve as reservoirs of the EHV-1 virus.

Treatment is limited at best as there is no specific treatment to cure viral infections. In cases with the respiratory form of the disease, supportive care is the main therapy. Most horses are put on antibiotics to prevent secondary infections, and occasionally non-steroidal anti-inflammatory medications are administered.

Treatment of horses showing neurological signs includes steroids, non-steroidal anti-inflammatory, DMSO (dimethyl sulfoxide) and the anti-viral drug acyclovir. Acyclovir is an anti-herpes drug that is used in humans but is experimental in horses. It has shown some promise in previous equine herpes outbreaks.

How does the horseman reduce their risk of farm out breaks?

  • Quarantine all newly arrived horses for a minimum of three weeks
  • Vaccinate all farm horses
  • Foals (starting about 4 months of age) should receive two doses of a modified-live vaccine 4 to 6 weeks apart. Immunity is short lived and young horses should be re-vaccinated every three months.
  • Mature horses that are not leaving the farm may get by without the vaccine, but if the farm is open to outside horses, then they should be vaccinated every three months.
  • Performance & show horses should be vaccinated every three months.
  • Require all new arrivals to be vaccinated a minimum of three weeks prior to arrival. Have owners show proof of vaccination from their veterinarian.
  • Vaccinate pregnant mares at 5, 7, and 9 months of gestation with a killed vaccine.

Remember, although the horseman can reduce risk from the respiratory and abortion forms via a correct vaccination schedule, there is no protection against the neurological form of EHV-1.