Frequently Asked Questions Regarding Equine Herpesvirus
What is Equine Herpesvirus?
EHV stands for Equine Herpesvirus. There are 9 different EHVs that have been identified. EHV-1 is the type that causes neurologic disease. EHV-1 can also cause respiratory disease, abortion, and neonatal death. EHV-4 most commonly causes respiratory disease, and is very uncommonly associated with abortion or neurologic disease. Almost all horses have been infected with EHV-1 by two years of age and have no serious clinical effects. Once a horse is infected, the virus can become latent (inactive) in the body resulting in a carrier state with no external signs of disease. Latent virus can be reactivated during times of stress, such as with long-distance travel or strenuous exercise. Current estimates are that at least 60% of horses have a latent EHV-1 infection.
What is EHM?
EHM is Equine Herpesvirus myeloencephalopathy, or the neurologic form of herpes. EHM cases may occur as one isolated case or as an outbreak affecting multiple exposed horses. Outbreaks of EHM may or may not be associated with an EHV-associated respiratory outbreak.
How does EHV cause EHM?
EHV can cause inflammation of the blood vessels supplying the spinal cord and brain. The neurologic signs (EHM) result from that inflammation and small blood clots that affect the nervous tissue. During an outbreak of EHM, usually only 10% of infected horses develop neurologic signs, but this proportion is occasionally much higher. We still don’t fully understand why some horses get EHM and others don’t, but it is likely a combination of factors specific to that individual horse and to the strain of EHV infecting it.
What does “wild type” strain mean?
There are two strains of EHV-1, known as D752 and N752. D752 is more frequently associated with EHM, and is therefore sometimes called the “neurotropic strain” while N752 is called the “wild type” strain, implying that it is the “normal” variant. This is misleading, however, as either strain can cause neurologic disease. Even though D752 is more likely to cause neurologic disease than is N752, the same control methods are indicated for either strain.
What are the clinical signs of EHM?
Clinical signs typically begin with fever (rectal temperature >101.5 F), usually 1-3 days after infection although it can be up to 10 days. Neurologic signs usually appear around 7 days post-infection, often just after the fever breaks, and progress rapidly over 24 to 48 hours. During outbreaks, up to 50% of horses with fever go on to develop neurologic signs. Signs may include incoordination and weakness, especially of the hindlimbs, loss of tail tone, urine dribbling, and inability to rise. Signs of brain disease, such as head tilt are seen in a few cases. The mortality rate for horses with EHM varies widely, from about 20 to 80% of the horses affected.
How is EHV spread?
Herpesvirus is primarily spread by direct horse-to-horse contact via transfer of respiratory secretions (e.g., nasal discharge). It is not known how far aerosolized virus can spread, although 30 yards is typically considered the maximal distance EHV-infected droplets can travel after a cough or sneeze. Virus can also be spread by contaminated hands, clothing, buckets, tack, grooming equipment, feed and trailers. The virus can live for up to 7 days in the typical environment, but it is easily killed by most disinfectants.
How is EHM diagnosed?
Horses with a fever and neurologic signs should be tested by your veterinarian. Currently recommended testing consists of a nasal swab and blood sample to detect virus by PCR (polymerase chain reaction) which detects the DNA of the virus. Both samples (nasal swab and blood) should be submitted to maximize the likelihood of making a correct diagnosis. Any horse that dies or is euthanized due to neurologic disease should also be submitted for a necropsy examination to determine the cause of death.
Who decides how an outbreak is managed?
EHM is a reportable disease in Florida. That means that when a veterinarian confirms that a horse is positive for EHV and showing neurologic signs (i.e., it has EHM), the state veterinarian’s office is notified. The state veterinarians are specially trained to deal with animal disease prevention, surveillance, and control programs. They determine the necessity for quarantine or restricted movement of animals.
Is there a treatment for EHM?
Treatment is primarily supportive. This includes anti-inflammatory and anticoagulant drugs, and intravenous fluids if the horse is having trouble drinking. Nursing care is also extremely important if the horse is unable to rise. Antibiotics are not effective against EHV, but can be used if there is a secondary bacterial infection. Antiviral drugs such as valacyclovir may prevent infection in exposed horses if given before the onset of fever.
My horse is vaccinated, so he is safe, right?
Unfortunately, no. While there are several vaccines available that provide protection against respiratory disease and abortion, none is labeled for protection against the neurologic form and there is no evidence for such protection.
If you have more questions or suspect your horse has Equine Herpesvirus, contact the UF Large Animal Hospital or your veterinarian.