There are actually two different types of equine herpes virus that is found in horses. The first variety, EHV-1 is the most problematic resulting in abortions in infected mares and respiratory tract infections and disease as well as paralysis in foals. EHV-4 is also linked to causing abortions in mares but is more likely to result in respiratory disease in foals.
For mares EHV-1 and occasionally EHV-4 cause abortion within a few months of becoming infected and pregnant. This infection can occur through close contact such as between breeding mares and stallions as well as between mares and foals. Once a mare has had an abortion she should be tested for the presence of equine herpes virus and then taken out of the breeding program, even if she recovers completely. As with humans, there is no control for the equine herpes virus and it can remain dormant in the nerve ganglia and the tissues of the respiratory tract. A horse that has an outbreak of herpes virus is infected for life, even if they show no outward signs. While not contagious unless showing signs, they are considered to be carriers. Often hormonal changes when the mare comes into season, long trailering or even dramatic increases in exercise can trigger a reoccurrence of the symptoms. Mares that are stressed through other illnesses, even severe parasite infestations can start to show signs of EHV even though they have not shown signs for long periods of time.
In foals with EHV-4 the most common signs of the viral infection is a respiratory problem that typically includes coughing and a nasal discharge. A mild to severe fever may be noted in some cases but it is not always present. Serology tests can be used to confirm EHV although distinguishing between the two strains of EHV does require additional, specialized testing.
In most cases the condition is not life threatening and putting the mare or foal on restricted work and exercise and adding high quality feeds and supplements will allow the horse to heal itself. In cases where there are other complications antibiotics may be used to treat secondary infections, but they will not treat the herpes virus itself. The horse or mare and foal, while under treatment, should be isolated from all other horses, common sources of food and water as well as stable areas. Each infected horse should have his or her own isolated feed and water areas that cannot be used by other horses. Remember that close contact will spread the disease, so isolation of the horses showing signs of herpes is critical. All bedding should be burned and all brushes, tack, trailers and stable areas should disinfected and steamed to kill and external virus contaminants in the environment. Any stable that has a horse with EHV should be quarantined for 30 days from all horses after the symptoms are cleared.
There are vaccinations for both EHV-1 and EHV-4 that can be given to all horses and also to pregnant mares. Talk to your vet regarding a vaccination schedule, as each brand of vaccination requires different rates of administration and booster shot schedules.