Equine Herpesvirus-1 Myeloencephalopathy 

Josh Zacharias, DVM, MS, DACVS, DACVSMR

One concern that anyone who travels and competes with horses should have is the potential for exposure to infectious disease. Recently, there has been an outbreak of herpesvirus myeloencephalopathy (EHM) in the western states. The USDA believes there is a correlation to a cutting event in Ogden, Utah from April 30th to May 8th. EHM is a neurologic disease caused by equine herpes virus-1 (EHV-1), the same virus that causes the respiratory disease known as rhinopneumonitis. Although EHV-1 is contained in “flu/rhino” vaccines, there is no evidence that it protects against the neurologic disease EHM. Almost all horses have been infected with EHV-1 by the time they are 2 years old and many asymptomatic horses may be latently infected. When latently infected horses become stressed, they may become symptomatic and begin to shed virus.

EHM is characterized by a sudden onset of signs which include ataxia (incoordination), paresis (weakness), and urinary incontinence. These symptoms may be preceded by a fever, viral respiratory disease, or abortion in the individual or herd. The course of the disease usually begins with a fever, lethargy, and possibly limb swelling prior to exhibition of neurologic signs. These include incoordination and potentially urine dribbling, loss of tail tone, hindlimb weakness, leaning against a wall to maintain balance, and the inability to rise or recumbency. The time from infection to evidence of clinical signs is 6-10 days.

The virus is primarily spread from horse-to-horse contact. It is shed via the respiratory tract or through an infected aborted fetus or fetal membranes. Transmission of the virus may occur from an asymptomatic horse that is shedding virus through nasal secretions. The virus can become airborne, however the exact distance needed to transmit infection is not known. As many as 90% of a population of horses that have been exposed may show clinical signs and, depending on the strain of virus involved in an outbreak, mortality rates may range from 1-40%.

For horses that have been exposed, it is important to take their temperature twice daily (taking temperatures once daily leaves a greater possibility of missing a temperature spike). A rectal temperature over 101.5°F is considered a fever. These horses should be examined by your veterinarian if there has been a possible exposure. Diagnostic tests for the disease include a nasal swab and blood sample for virus detection. Once an infected animal has been identified or suspected, quarantine to prevent further transmission is imperative. This includes physical separation of the animal and use of separate feeding and cleaning equipment. Washing hands and changing clothing between handling horses is important.

Treatment for clinically affected individuals is primarily supportive care. Intravenous fluids and anti-inflammatory drugs are commonly used. Antibiotics may be used to treat secondary infections if needed. Antiviral medications such as Valtrex (valacyclovir) are often helpful; however, the efficacy and optimal dose have not been established. These medications are also fairly expensive. Vaccine boosters may help limit the amount of circulating virus and amount of viral shedding. Some horses require urinary catheterization.

Once an outbreak has been established, horses and premises exposed may be quarantined by order of the state veterinarian, which may cause a significant economic impact. The optimal treatment is prevention of exposure, which can be difficult for horses that travel and comingle with different individuals every day, any of whom may be shedding virus even if asymptomatic. Vigilance and communication is vital. Watch for horses that seem lethargic or may have nasal discharge at an event. Keep your horse’s vaccination status current. It is important to monitor your own horses and get veterinary aid and advice earlier than later.

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Josh Zacharias, DVM, MS, DACVS