Many horse owners throughout the state have concerns regarding the potential spread of the equine herpes virus 1 (EHV-1) and equine herpes myeloencephalopathy (EHM).
The worry is even greater for those that travel and compete in various horse event centers, stables, clubs and horse communities within Utah and surrounding states.
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?
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.
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.
Dr. Barry Pittman, Utah State Veterinarian at the Utah Department of Agriculture and Food (UDAF), has issued some words of caution and advice for horse owners in their efforts to keep their animals safe.
“First and foremost, there have been no confirmed cases of EHV-1 or EHM in the State of Utah,” Dr. Pittman emphasizes, “However, considering the cases in surrounding states, the nature of the virus, and the co-mingling and stress levels of horses in event circuits, we are certainly susceptible to future cases.”
The Equine Disease Communication Center (EDCC, @ http://www.equinediseasecc.org/ ) has reported the following March 2019 cases of EHV-1 and EHM cases:
There have been sporadic reports of these diseases in April, as well, in Nevada, Idaho, California, Wyoming, Texas, and Canada.
Pittman said that increased biosecurity measures by organizers of Utah events, the postponement and cancellation of some events, and some horse owners electing to not participate in certain events has likely been a factor in no positive cases in Utah this year.
“Continued situational awareness concerning cases and quarantines in surrounding states, events epidemiologically linked to horses that may have been exposed, and diligence in monitoring individuals’ horses’ health will all help control the spread,” Dr. Pittman suggested.
To reiterate the uniqueness of this virus, nearly all horses become infected with EHV-1 within their first two years. The initial exposure generally occurs in foals from contact with their dams (female parent). The virus can then become inactive in the horse’s body, setting up a life-long carrier state.
Horses of any age that are carriers of EHV-1 do not show any external signs of disease when the virus is in an inactive form. However, the virus can be reactivated during times of stress, such as strenuous exercise, long-distance transport, or at weaning.
“This means there will never be an ‘all clear, no further risk’ as long as horses are brought together from multiple geographic locations,” Dr. Pittman stated. He is encouraging horse owners to seek information from reliable sources, contact event managers from events they plan to enter, increase health checks of their horses, and stay at home for now if they don’t feel comfortable traveling with their horses.
Veterinarians across the state are encouraged to call the State Veterinarian Office if they suspect they may have a case of EHV-1 or EHM and/or have collected and submitted laboratory samples to test for these viruses.
Portions of this story were sourced from the following website: https://largeanimal.vethospitals.ufl.edu