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Introduction
Equine Herpes Virus-1 (EHV-1) is a contagious viral disease of horses that can cause repiratory disease, abortion and occasionally neurologic disease. This disease has also been called Rhinopneumonitis. While there has been a great deal of interest in the disease during 2005-2007 due to the increased incidence of the neurologic form of the disease, it is the abortions that are the most important to financial success of the equine industry. This disease is the most frequent cause of abortion in the horse.
EHV-1 is a herpes virus. There are several types of herpes viruses in horses but EHV-1 is the most important and can cause abortion, neurologic and respiratory disease. The disease is highly contagious among horses with mares often contracting from foals which do not often so symptoms. The virus multiplies in the lungs and then spreads to the circulatory system and eventually to the placenta and fetus. Abortions occur 1 - 4 months after intial infection with the abortion usually occuring at 7 months to term. Horses can shed the EHV-1 from the onset of clinical signs until 1-2 weeks after the clinical signs are gone.
Clinical signs of mild respiratory disease in young horses, viral isolation (rarely done) using nasopharyngeal swab taken early in course of disease OR find virus in tissues (aborted fetus) and apply flourescent or immunoperoxidase stain. Serology can be used, however, vaccination can cause high titers.
In uncomplicated cases, complete recovery occurs in several weeks. However, remember that herpes infection is for life, and reactivation of the virus can cause recurrent disease or continuous shedding! Symptomatic treatment is best; avoid antibiotics unless complicated by secondary infection. The horse should be restde and gradually returned to activity. Symptomatic animals should be isolated and facilities disenfected. The facility should be quarantined for 30 days after last new case is identified.
For prevention, EHV-1 vaccines are available, but their efficacy at controlling respiratory disease is questionable. Vaccination may at least reduce severity and duration of disease compared to unvaccinated horses. Initially, 2 doses are given at 2-3 week intervals, then booster every 3 months to a year (each product is different). For abortion protection, vaccinate mares at 5, 7, and 9 months gestation with a killed virus only (be sure label says safe for pregnant mares). Failure to vaccinate pregnant mares and then EHV-1 infection is the most common cause of abortion in the horse.
Latency is an issue as it can persit for the life of an animal in the nerve ganglia (trigeminal ganglion), lymphoid tissues of the respiratory tract, and peripheral blood lymphocytes. The virus can then be activated by corticosteroids or serious stress.