Update on Infectious Diseases Affecting the Equine Nervous System

Update on Infectious Diseases Affecting the Equine Nervous System: EHV-1 infection has received a great deal of attention in the last decade due to several high-profile outbreaks, state-mandated quarantines, and recognition of a “neuropathogenic” strain. This alpha-herpesvirus is ubiquitous throughout the world, and most horses over the age of 2 years have been exposed. Latent infections involving the trigeminal ganglia and respiratory tract lymph nodes are common, and stressful events may trigger reactivation. Although primarily a respiratory pathogen, responsible for fever, inappetence, and nasal discharge in young horses, EHV-1 is also associated with abortions, neonatal death, and neurologic disease, known as EHV-1 myeloencephalopathy (EHM). Disease is spread via aerosolization of respiratory secretions or direct contact with infected horses or fomites. Virus moves from respiratory epithelial cells to regional lymph nodes to peripheral blood leukocytes, causing a cell-associated viremia. When virus crosses from leukocytes into endothelial cells of the central nervous system (CNS), it causes vasculitis with hemorrhage and thrombosis that result in hypoxia and ischemia in surrounding CNS tissue. Typical clinical signs include fever, paresis and ataxia (usually worse in pelvic limbs than thoracic limbs), urinary bladder paralysis with urine dribbling, and decreased tail and anal tone with fecal retention. Severely affected horses may become recumbent or show evidence of brainstem involvement.