The equine herpes virus (EHV) is always present in the British horse population. It is thought most horses have been exposed to the disease when young, usually by the age of two and then become latent carriers of the virus. A horse can therefore be infected for life, and can intermittently discharge the virus from the nose and infect susceptible horses. The virus lays dormant in these horses before becoming active again at times of stress. It is important to understand that horses do not always show clinical signs while they are shedding the virus.
EHV has nine strains, with five (EHV-1 to EHV-5) affecting horses.
The two most common strains are EHV-1, which can cause respiratory problems, abortion in pregnant mares and, in rare cases (including the January 2020 outbreak in southern Britain), neurological problems leading to paralysis and in some cases death; and EHV-4, which most commonly causes mild respiratory problems and very occasionally a single case of abortion in a pregnant mare.
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