Viral Respiratory Disease in Horses

June 9, 2008 (published) | June 2, 2014 (revised)

Respiratory disease is common at horse shows and sale events when many horses congregate and are mixed together.  The most common causes of respiratory disease are viral and involve equine influenza, equine herpes virus 1, and equine herpes virus 4.  Equine herpes virus is referred to as EHV, and is the same disease previously called rhinopneumonitis or in many cases, just rhino.  This term is no longer used because we now know an actual rhino virus can also affect horses and herpes is a totally different virus. 

During the past decade, there have been a number of outbreaks of herpes virus 1 that causes not only a respiratory disease but a neurological disease as well as abortion.  However, the neurological herpes disease is the one that is associated with the most concern as this is due to a mutation in the virus, and there is a concern if this virus is being shed routinely at equine events.  A group of vets at Colorado State studied horses at four horse shows and tested them for influenza, EHV 4 and EHV 1.  Results indicated all three viruses were found in some horses at all horse shows tested although the levels were low.  Actual viral infection due to an influenza outbreak at the shows was only found at one show in Florida.  Most of the horses shedding viruses were less than 2 years old. This study proved that young horses attending equine events did fairly commonly shed respiratory viruses.  Because of this, we recommend that you vaccinate your horses for equine influenza, EHV 1, and EHV 4 before taking your horse to a show.  Vaccination does not provide 100% protection against the diseases but hopefully decreases shedding of the viruses.  Unfortunately, there is no vaccine licensed to prevent the neurological form of EHV 1.      

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