Herpes Virus in Horses

September 26, 2005 (published) | September 6, 2011 (revised)

Recently the equine world has been dominated by information about equine herpes virus 1. Part of this information has been correct but unfortunately a lot of misinformation has also been available, especially on the Internet. Today I am briefly going to cover the facts about the disease, although there are still lots of things that we do not know about this virus. Equine herpes virus causes three different syndromes: respiratory disease, abortion, and neurological disease.

Most horses are exposed to this herpes virus when they are very young and the virus is able to remain dormant in the horse's body and not produce any clinical signs until some trigger causes the virus to become active. There are two basic strains of the virus, the regular strain that causes respiratory disease and a more pathogenic strain that causes neurologic disease that can lead to death. However, even the regular strain can cause nervous system disease, so the problem is not that simple. There are vaccines to prevent herpes virus disease in horses but these vaccines are not 100% and there is no vaccine approved to prevent the nervous system disease. However, there are some vaccines that have a higher amount of antigen than others and it is recommended to use these vaccines to booster horses before exposure to other horses when an outbreak is occurring. The vaccines are not expected to prevent disease but to decrease viral shedding. If a horse has already been exposed to the virus, vaccination is not recommended. The most important factor in prevention of the disease is isolation and quarantine of the infected horses because the disease is very contagious.

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