Equine Pathology Workbook

Equine Herpes Myeloencephalopathy

There are 9 known types of herpes virus that infect the horse.

Equine herpes virus (EHV) type

and type

are the most common strains to cause

clinical disease.

EHV type I normally causes rhinopneumonitis, but may be responsible for abortion, still birth, premature birth, and birth of non-viable foals due to severe liver damage. Mutated strains of EHV type I may be responsible for the condition called equine herpes myeloencephalopathy.

EHV type II causes pharyngitis and cytomegalovirus.

EHV type III is considered a

disease as it causes equine coital exanthomas,

a condition similar to genital herpes lesions in humans.

EHV type IV causes disease, which is called rhinopneumonitis. Rhinopneumonitis is clinically seen as a fever of 38.8 – 41.1° C, serous nasal discharge that progresses to mucopurulent discharge, cough, lethargy, anorexia and depression. Occasionally EHV type IV causes ocular lesions that may permanently impair the horse’s vision.

EHV type V causes multinodular pulmonary fibrosis and cytomegalovirus.

Equine herpes myeloencephalopathy begins as a normal EHV type I infection. Etiology of the strain specific to EHM is from the alpha-herpesvirinae family. This is a DNA virus that causes lifelong symptoms. It initially presents as rhinopneumonitis, but the infection spreads through the local into the blood vessels that supply the head and brain. EHV type I is able to cross the blood-brain barrier to infect the blood vessels of the brain and the cells of the meninges. The virus causes damage to these structures resulting in inflammation, hemorrhage and DIC. Inflammation causes the intracranial pressure to increase which, combined with the DIC, can cause ischemic necrosis of brain tissue. Degeneration of axons within the CNS can also be seen. EHV type I is transmitted through aerosol respiratory fluids, through direct or indirect contact with nasal discharge, through contact with the aborted fetus, placenta or fetal fluids from an infected mare, or through contact with an infectious neonate. Infected neonates are thought to be highly contagious and are rampant shedders of virus through most of their bodily fluids. Equine herpes myeloencephalopathy and abortion are generally seen in mature individuals, while rhinopneumonitis is primarily seen in young animals less than years of age. The virus is commonly seen in North America and Northern Europe. The clinical signs and symptoms of EHV type I infection include respiratory disease, abortion and neurological signs.

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