Equine Herpes Virus
Equine herpes virus (EHV) is a common virus which affects horses. The disease is also known as Equine Rhinopneumonitis. This virus causes respiratory disease as well as abortions and neurologic disease. Respiratory disease is most common in young animals. Older animals can be infected and shed the virus without showing any signs of disease.
Horses with respiratory disease caused by EHV may have fever, coughing and/or nasal discharge. Some horses develop neurological signs such as incoordination, urinary incontinence and bladder distension. Severely affected horses may be unable to rise and “dog-sitting” may be observed. Neurologic disease may or may not be accompanied by respiratory disease. In pregnant mares, abortion may occur in late pregnancy or the foal may be born alive but with weakness, jaundice, respiratory distress and neurological signs and die within a few days.
The time from infection to the onset of disease is usually between four to six days. Abortions can occur from two weeks to several months after exposure to the virus. The virus is transmitted both directly (contact between horses) and indirectly (airborne virus or contaminated clothes, equipment, etc). The virus is mainly spread by aerosol droplets caused by coughing and snorting. Aborted fetuses, fetal membranes and fetal fluids are also infectious and the mares that have aborted shed the virus in their respiratory secretions.
Treatment for EHV-related disease is mainly supportive. Neurologic cases will need a safe, well-bedded stall especially if they are uncoordinated or recumbent. Horses that have difficulty urinating may need to be catheterized.
The outcome is good in the majority of cases. For horses that are recumbent for a long period of time, the prognosis is usually poor. It may take weeks or months before neurologic signs disappear completely, although in a few cases neurological problems have persisted for life.
Horses usually shed the virus for about a week after the onset of fever or neurologic disease. Once infected, animals carry the virus for life. They can shed virus periodically with or without showing signs of the disease, typically during periods of stress. This re-activation of the virus is responsible for the spread and survival of the virus in horse populations. Because the virus is so common, most horses become exposed to one or more strains of EHV at some point in their lives.
Several vaccines against EHV are available; however, vaccination does not prevent infection with EHV but does reduce the frequency and severity of clinical disease. Vaccination is generally recommended for broodmares. None of the currently available vaccines state any claim for protection against the neurologic form of EHV infection.
Any suspected outbreak of EHV should be taken very seriously and steps taken to prevent spread of the disease. If you suspect your horse is infected with EHV, consult with your veterinarian for confirmation. When EHV is detected, isolate any infected horses immediately and avoid all direct and indirect contact with other horses. Exposed horses should also be isolated as a precautionary measure. Additional biosecurity measures should be taken, such as stopping all horse traffic on and off premises where infected horses have been identified and isolating infected and exposed horses from the general population for 28 days following the onset of the last identified case. Thoroughly clean and disinfect all facilities and equipment to prevent further cases.
For more information on this and other equine health issues, please contact your local veterinarian.
Dr. Wendy Wilkins, DVM, PhD
Disease Surveillance Veterinarian
Saskatchewan Ministry of Agriculture