EHV are viruses that are found in most horses all over the world. Almost all horses have been infected with the virus and have no serious side effects. It is unknown what causes some of the horses to develop serious neurological forms that may be fatal.
The two most common strains of EHV are EHV-1, which causes abortion, respiratory disease, and neurologic disease; and EHV-4, which causes respiratory disease only but can occasionally cause abortion and rarely neurological disease.
EHV-1 myeloencephalopathy (EHM) results from widespread vascular or blood vessel injury after damage to the lining of the blood vessels of the blood-brain barrier. EHM cases occur singly or can affect multiple exposed horses. They may or may not be associated with a previous or ongoing EHV-1 respiratory disease outbreak.
EHV-1 is contagious and spreads by direct horse-to-horse contact via the respiratory tract through nasal secretions. It can also spread indirectly through contact with physical objects contaminated with the virus (e.g. human hands, tack, trailers, grooming equipment, feed or water tubs, etc.).
Typically EHV-1 causes a fever peaking at day 1 or 2 and 6 or 7 of incubation. With respiratory infections, there is often serous or mucoid nasal and ocular discharge, but not a lot of coughing. With the neurological form, fever is the first sign followed by acute onset neurological signs such as incoordination, hind limb weakness, loss of tail tone, urine dribbling, inability to rise, etc.).
We routinely vaccinate your horse for Rhinopneumonitis (the "Rhino" in your Flu/Rhino vaccine) that protects against EHV-1 and EHV-4. Unfortunately, this vaccine protects for the respiratory and abortion forms of EHV, not the mutated neurological form. Therefore, it is important to have strict biosecurity at your farm and quarantine all new animals for at least 2 weeks.