The equine herpesviruses are double-stranded DNA viruses of the Herpesviridae family. Horses are major hosts for EHV-1 through EHV-5, while donkeys are the natural hosts for ASH-1 (alt EHV-6), ASH-2 (alt EHV-7), ASH-3 (alt EHV-8), ASH-4, ASH-5, and ASH-6.
Clinical disease is mainly caused by EHV-1 and EHV-4 from the subfamily Alphaherpesvirinae.
EHV-3 is a venereal form of herpes transmitted mainly by mating and causes coital exanthema. EHV-2, from the subfamily Gammaherpesvirinae, causes corneal/conjunctival inflammation. However, EHV-2 and EHV-3 have little clinical significance. EHV-3, a venereal form of herpes, is transmitted primarily during mating. EHV-5 can cause various clinical courses such as abortions, dermatitis or systemic granulomatosis.
EHV has low tenacity in the environment (persistence less than 7 days) and is sensitive to detergents and lipid solvents. Latent infections occur due to the ability of the virus to evade the immune system. EHV-1 and EHV-4 are considered antigenically very stable and show little change in epitope structure.High viral loads are found in aborted fetal material. The viruses are also found in the cells of the respiratory tract as well as in the regional lymph nodes and, in the acute stage, in the blood.
Rhinopneumonitis presents with mild respiratory symptoms such as cough, watery nasal discharge and mild fever at onset.
Abortions usually occur in the last third of pregnancy. When infected foals are born, neonatal disease with respiratory symptoms and liver dysfunction with a poor prognosis may occur. The virus variant N752 is the main cause of miscarriage.
Myeloencephalopathy can occur both sporadically and epidemically. In this case, the viral variant D752 as well as the quantity of the pathogen play a role. Neurological symptoms are caused by a vasculitis with vascular damage and subsequent death of neuronal cells. Horses affected by myeloencephalopathy (paretic-paralytic form) show neurological symptoms mainly in the form of ataxias of the hindquarters, stiff gait and hindhand weakness after a short fever phase. In more severely affected horses, convulsions and recumbency occur. Urination and defecation may be difficult. Head nerve function deficits are also observed (tilted head, drooping of the ear, eyelid and lips due to facial paralysis). The symptoms often subside after a few days or weeks. However, fatal courses also occur, especially if the horses are stuck for more than three days. Euthanasia is then usually unavoidable.