Over the last several years there have been numerous outbreaks of Equine Herpes Virus (EHM) myeloencephalopathy infections in horses. Many of the outbreaks originated at shows or events where numerous horses were stabled together for a few days.
The cause of the outbreaks is Equine Herpes Virus-1 (EHV-1). Usually the virus causes respiratory signs including fever, poor appetite, coughing, nasal discharge and lethargy. It can cause abortion and neonatal death. In rare circumstances the virus may produce neurologic signs known as myeloencephalopathy. Loss of bladder and anal tone, mild incoordination, stumbling, hindlimb paralysis, and recumbency are signs of equine myeloencephalopathy.
Transmission from one horse to another comes from direct contact (nose to nose) or by contact with infected discharges carried on boots, buckets, blankets, trailers, hands, aborted fetus etc. The incubation period (time from infection to exhibiting clinical signs) is 2-14 days. Most horses have been infected with the respiratory form before the age of 2 years. Previous infection does not protect the horse from reinfection later in life. Some horses have a carrier or latency state in which they test positive but have no clinical signs. Stress may cause a recrudescence of the disease or result in shedding of the virus.
The virus can be treated with anti-viral drugs, which will lessen the severity of clinical signs if caught in the very early course of the disease. However, treatment is usually supportive (bladder drainage, anti-inflammatory drugs, sling) until the horse recovers on it own. If the horse becomes recumbent, the prognosis is poor.
Although vaccination for EHV-1 does give some protection against the respiratory form, it does not protect against the neurologic form. Following some simple management practices while at home and away can help protect your horse from infection. New horses on the farm, or horses that are returning to the farm after co-mingling with other horses should be kept away from horses on the farm for 21 days.
While at a horse show and when returning take your horse’s temperature every day. Contact a veterinarian the first day your horse has a fever. If EHV-1 is suspected, a nasopharyngeal swab can be obtained and submitted for tests to determine of EHV-1 is to blame. Horses exposed to EHM will be quarantined for 21 days following clinical signs to prevent spread of the disease.