An outbreak of equine herpes virus has created quite a stir in the horse world recently. Several horses that attended the National Cutting Horse Association’s Western National Championships in Ogden, Utah this May later developed neurologic signs and then tested positive for Equine Herpesvirus-1. Several shows have subsequently been cancelled, some farms have been quarantined and a few of the affected horses unfortunately euthanized as a result of the outbreak.
EHV-1 has been classified into 5 different strains. EHV-1 and EHV-4 are the ones most commonly associated with disease in horses and are quite common in the equine population across the United States. Most horses are thought to be infected at a young age and in most cases the virus survives in the horse in a latent stage, in effect ‘hiding’ from the immune system and avoiding complete elimination. Then during times of stress or immune suppression the virus can emerge and cause clinical disease in the individual. Individuals with clinical disease shed high levels of virus into the environment serving as a source of exposure to other horses. The virus is transmitted by nasal secretions and aerosolization of the virus by coughing. When other horses are exposed to high levels of the virus shed by infected individuals they too often become infected and the process continues.
Three clinical manifestations of disease are described. Respiratory disease, commonly referred to as rhinopneumonitis or ‘rhino’, is the most widespread. Horses typically spike a fever and have nasal discharge and a cough. Because the disease is a result of viral infection, treatment is mostly supportive. Antibiotics may be required if a secondary bacterial infection is suspected. Most horses will clear the infection in 7-14 days. Secondly, abortion in the last trimester of pregnancy is frequently associated with EHV-1 infection. Vaccination of mares is recommended at 5, 7 and 9 months of gestation as a means of prevention. Lastly, myeloencephalopathy (disease of the brain and spinal cord) is described as a less common manifestation of EHV-1 infection. While this neurologic form is less common it can be the most severe resulting in neurologic compromise and potentially death of the affected horse. The neurologic form of EHV-1 is most commonly associated with a mutated strain of the virus. Currently available vaccines seem ineffective in preventing this neurologic form of EHV-1. Therefore, disease can spread quickly amongst horses at shows and other events due to low immunity. Treatment of affected individuals is also challenging. Given that the infection is viral in nature, antibiotics are of little benefit. Intravenous fluid support and anti-inflammatory treatment are commonly used but are of limited value. Some research has been conducted in the use of immune stimulants and anti-viral treatments but at this point, no clearly effective treatment has become available.
The ability for rapid spread of the disease and lack of an effective vaccine coupled with the frequent severity of symptoms and difficulty in treatment make the neurologic form of EHV-1 potentially devastating should it become widespread. For this reason, the current outbreak is being taken very seriously and confirmed cases handled cautiously in an effort to minimize the spread of this disease across the country. Research is ongoing into developing better treatment and prevention strategies.
Dr. Darren Loula, DVM, is owner of Christian County Veterinary Service, LLC, a mobile large animal vet clinic.