Equine herpesviruses (EHVs) are double stranded DNA viruses that consist of at least nine different types of viruses that have been identified, worldwide. The EHVs are a major economic and welfare impact on all sectors of the equine industry worldwide. Similar to herpesviruses in other species, equine herpesviruses can establish latent infection in the majority of horses, which don't present with any clinical signs but may shed the virus when stressed or develop clinical signs at a later period. These latently infected horses are the principal reservoir of infection for EHV. Equine herpesvirus myeloencephalopathy (EHM) is another name for the neurologic disease associated with equine herpesvirus (EHV) infections. Cases of EHM occur singly in multiple exposed horses. They may or may not be associated with a previous or ongoing EHV-1 respiratory disease outbreak.

Alpha herpesviruses
EHV-1, EHV-3, and EHV-4 are alpha herpesviruses (members of the Alphaherpesvirinae subfamily), and are closely related, but have distinctly different disease profiles. These viruses cause respiratory infections, abortion, and neurological disease.

Equine herpesvirus type-1 (EHV-1)
EHV-1, often referred to as equine abortion virus, is associated with all three syndromes, although the virulence of individual isolates shows considerable variation. The neurologic form of EHV-1 is called Equine herpes virus myeloencephalopathy (EHM).

Equine herpesvirus type-3 (EHV-3)
EHV-3 is referred to as equine coital exanthema, which affects the external genitalia of horses.

Equine herpesvirus type-4 (EHV-4)
EHV-4 (Equine rhinopneumonitis virus) principally causes respiratory disease, and is most common among foals and yearlings. It can, however, also cause abortion and neurological disease in horses.

Upon recovery of either of the viruses associated with the respiratory tract, horses may not perform as well as they once had, resulting from a poor performance syndrome associated with nonspecific bronchial hypersensitivity and a RAO-type syndrome in older horses and IAD-type syndrome in younger horses.

Gammaherpesviruses
Equine gammaherpesviruses include equid herpesvirus 2 (EHV-2) and equid herpesvirus 5 (EHV-5). Which have both recently been associated with a number of clinical problems, including cases of abortion and neonatal mortality as well as with bacterial infections.

EHV-2 has been associated with respiratory disease, keratoconjunctivitis, pneumonia, pharyngitis, fever, enlarged lymph nodes and inappetence.

EHV-5 in horses is not always associated with clinically apparent disease; however when it does present symptoms, it can cause respiratory disease, especially equine multinodular pulmonary fibrosis, as well as lympho-plasmahistiocytic interface dermatitis, with pustules associated with intranuclear inclusion bodies and a lymphohistiocytic perivascular dermatitis, which differs markedly from the EVH-2-associated granulomatous inflammation with giant cell formation within the deep dermis.

Transmission
EHV is transmitted to horses most commonly through aerosolized droplets of respiratory tract secretions from infected coughing horses. It is transmitted by ingestion or inhalation of the droplets from surfaces, as well as direct and indirect contact with horses, fomites (equipment, vehicles, clothing) and personnel. The air around the horse that is shedding the virus can also be contaminated with infectious virus. The virus is estimated to be viable for up to 7 days in the environment under normal circumstances but remain alive for a maximum of one month under perfect environmental conditions.

Incubation period
The incubation period for EHV can be as short as 24 hours but is typically 4-6 days.