The equine herpes viruses EHV-1 and EHV-4 occur worldwide in the equine population. Most horses are latently infected, i.e. without clinical symptoms. Seasonally, especially in winter and spring, clinically manifest forms of EHV-1 infections may occur.

Worldwide

Horses

Transmission occurs via direct contact (droplet infection) from infected animals or indirectly via grooming utensils, equipment, hands, clothing, shoes. The pregnant mother mare can transmit the virus to the unborn foal. Healed horses remain latently infected and can sometimes excrete the virus again after reactivation.

The incubation period depends on the form of progression. Neurological symptoms usually appear 4-6 days after infection, sometimes after 24 hours, occasionally much later. Abortions often occur weeks after infection of the dam.

Clinical cases may present with three main forms:

• Rhinopneumonitis: mild respiratory symptoms such as cough, watery nasal discharge, mild fever at onset.
• (Late) abortion / neonatal death
• Myeloencephalopathy: ataxias of the hindquarters, stiff gait, hindhand weakness, convulsions, recumbency

A specific therapy is not possible. Diseased horses can only be treated symptomatically. Affected horses of a herd must be separated from healthy ones. In unaffected horses, fever should be measured twice daily. In healthy horses with existing vaccination protection, a booster vaccination is recommended. Herds with clinically affected horses should observe a quarantine period of 28 days after the last suspected infection, during which no horse may re-enter or leave the herd.

EHV-1 is not notifiable or reportable. Horses can be vaccinated against EHV-1 and EHV-4. Vaccination protection is only sufficient if all horses in a herd are vaccinated. Vaccination mainly protects against respiratory diseases caused by EHV-1 and EHV-4 and reduces the risk of EHV-1-related abortions. A protection against the occurrence of the neurological form by the vaccination could not be proven so far.

EHV-1 and EHV-4 also occur in horse herds in Austria. In spring 2021, outbreaks with a neurological course occurred in some herds.

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 with vesicles, pustules, or erosions on the vulva or penis. An infection with EHV-3 is notifiable in Austria. Affected horses are virus carriers for life and must be excluded from breeding. EHV-2 from the subfamily Gammaherpesvirinae causes inflammation of the cornea and conjunctiva. However, EHV-2 and EHV-3 are of little clinical significance. EHV-5 can cause various clinical courses such as abortions, dermatitis or systemic granulomatosis.

EHV have low tenacity in the environment (persistence less than 7 days) and are 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.

Symptomatology

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.

In the case of respiratory symptoms caused by an EHV infection, it is not possible to distinguish it from other respiratory diseases. Therefore, a detection (real-time PCR, currently the most common and fastest method) from nasal or nasopharyngeal swabs must be performed. During the acute phase, the virus genome can also be found in EDTA blood by PCR detection. In the case of EHM (Equine Herpesvirus-associated Myeloencepathy, damage to the brain), the disease can be detected in the brain, spinal cord and cerebrospinal fluid.

In EHV-related abortions, the fetus and placenta are examined for pathological changes and the virus can be detected in the fetal organs.

Retrospectively, the determination of antibodies from paired serum samples is possible for the detection of EHV infection. The first serum sample should be taken at the time of illness at the onset of the first symptoms, the second serum sample 2-4 weeks after the onset of illness and should be tested for EHV-1/-4. A titer increase of at least four confirms a disease caused by EHV-1 or EHV-4.