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Equine Rhinopneumonitis (Herpes Virus) Overview
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Equine Rhinopneumonitis (Herpes virus) is the generalized name for Equine herpesvirus type 1 (EHV-1) and Equine herpesvirus type 4 (EHV-4) can each infect the respiratory tract, Infection of the respiratory tract with EHV-1 and EHV-4 typically first occurs in foals in the first weeks or months of life, but recurrent or recrudescent clinically apparent infections are seen in weanlings, yearlings, and young horses entering training, especially when horses from different sources are commingled. Equine herpesvirus type 1 causes epidemic abortion in mares, the birth of weak nonviable foals, or a sporadic paralytic neurologic disease (myeloencephalopathy) secondary to vasculitis of the spinal cord and brain. Because both viruses are endemic (widespread) in most equine populations, most mature horses have developed some immunity through repeated natural exposure; thus, most mature horses do not develop serious respiratory disease when they become infected but may be a source of exposure for other susceptible horses. In contrast, horses are not protected against the abortigenic or neurologic forms of the disease, even after repeated exposure, and mature horses are in fact more commonly affected by the neurologic form of the disease than are juvenile animals. This disease is complicated because like other strains of the virus, Equine Herpes has a varied period of symptom dormancy (not showing symptoms) while it is still able to infect other horses.
Equine Rhinopneumonitis Transmission: Both EHV-1 and EHV-4 spread via aerosolized secretions from infected coughing horses, by direct and indirect contact with nasal secretions, and, in the case of EHV-1, contact with aborted fetuses, fetal fluids, and placentae associated with abortions. Young horses are most often affected when they are brought together in groups. Equine Rhinopneumonitis is spread between coughing horses or on the hands of unknowing people. An upper respiratory infection is a common result of virus invasion. Uncomplicated cases will resolve on their own in a week or two. Occasionally the illness will leave the horse susceptible to bacterial pneumonia or a guttural-pouch infection.
Equine Rhinopneumonitis Symptoms: Symptoms vary in severity from sub-clinical (unnoticeable) to severe and is characterized by fever, lethargy, anorexia, nasal discharge, cough, respiratory disease, abortion, or neurological signs. One thing that all herpes viruses have in common is their ability to live within their host indefinitely. Once a horse is infected with Equine Rhinopneumonitis, it seems that infection can be reactivated later when the horse is stressed. The neurological form of Equine Rhinopneumonitis is less common. A high fever precedes the development of signs ranging from loss of coordination and weakness to paralysis. These horses may recover with nursing care, but it can take anywhere from a few days to several months. Another common result of rhino infection is abortion in pregnant mares. Some mares will carry their fetus to term, only to deliver a weak sickly foal with slim chances of survival.
Equine Rhinopneumonitis Treatment: Primary indications for use of equine herpesvirus vaccines include prevention of EHV-1-induced abortion in pregnant mares, and reduction of signs and spread respiratory tract disease (rhinopneumonitis) in foals, weanlings, yearlings, young performance and show horses that are at high risk for exposure. Many horses do produce post-vaccinal antibodies against EHV, but the presence of those antibodies does not ensure complete protection. Consistent vaccination appears to reduce the frequency and severity of disease and limit the occurrence of abortion storms but unambiguously compelling evidence is lacking. Management of pregnant mares is of primary importance for control of abortion caused by EHV-1.