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Rhinopneumonitis: Equine Herpes Virus (EHV1/4)
Equine Herpes Virus Types 1 / 4
Rhinopneumonitis Infection of Horses
Introduction
The equine herpesviruses (EHV) is a large family of the alphaherpesviruses that infects all species of equines, including horses, asses, and zebras. Though nine distinct EHV's have been found only two are considered of economic significance in domesticated equines: Equine Herpes Virus type 1 and type 4 (EHV1/4). EHV-1 and EHV-4 are common worldwide causes of fever and respiratory infections in horses. EHV-1 respiratory infection is associated with the serious complications of abortion and myeloencephalitis.
Accurate diagnosis and differentiation from other respiratory diseases is important from a management, treatment, and prognosis standpoint. This article gives an overview of these two virus's, how horses become infected, clinical signs, diagnosis and prognosis of horses infected. Also discussed are proper quarantine and prevention of EHV 1/4 in horses. This article provides important back ground information to understanding equine herpes abortion (EHA) and equine herpes myeloencephalopathy (EHM) then provides links to specific articles on these diseases.
How Does EHV Cause Disease?
Activated Latent Infection?
Understanding equine herpesvirus (EHV) infections in the horse starts with the knowledge that most horses become infected as foals, sometimes as early as the first few weeks of life, and develop latent infections that they carry with them for the rest of their lives. A long standing theory has been that EHV infections often arise in populations when the latent infection becomes active causing clinically evident disease. Activation was caused by decreased immunity do to stress or aging, or the introduction of immune naive horses. Recent epidemiological investigations have brought this theory into question as to whether this reactivation is a common cause of respiratory disease outbreaks or not. What is clear is reactivation occurs frequently enough to maintain latent EHV infection in the majority of horses of the world. One of the important questions to be answered is, "What is the nature of this latent infection with respect to immune protection from new infection and some of the serious complications?"
EHV-1/4 Most Often Enters Through the Respiratory System
The initial EHV 1/4 infection follows inhalation of infectious nasal secretions that have been aerosolized by coughing and sneezing infected horses. Sometimes horses can become infected by coming into contact with infected secretions or tissues from EHV-1 abortions. Following exposure the virus invades the respiratory epithelium. In many horses this results in erosion of the mucosal surfaces of the nostrils and sinuses of the horse leading to signs of upper respiratory disease. The virus replicates here and within the first few days the virus spreads to the respiratory tract lymph nodes. This is where most EHV-4 infections stop with the horse recovering in a few weeks following infection with virus shedding lasting 10 to 14 days after the disease first started. In the case of more serious complications like EHA or EHM shedding can continue for longer periods.
EHV-1 Usually Enters the Blood
In the case of EHV-1 infections, and very rarely in the case of EHV-4 infections, a white blood cell associated viral infection spreads from the lymph nodes to the blood creating a viremia. The virus in the blood persists for several weeks and for most EHV-1 infections this is where the virus is stopped by the immune system and the horse recovers after several weeks of infection.
EHV-1 Occasionally Invaded the Nervous System or Placenta
Occasionally the viremia may spread to and infect the blood vessels of the central nervous system or the placenta in mares during the last half of pregnancy. This infection results in damage to the microvasculature and a initiation of an inflammatory cascade which includes: vasculitis, microthrombosis, and local hemorrhage. In some cases the virus can spread from the placenta to the fetal vasculature. The damage done to the vessels causes focal necrosis of the tissues which results in the neurological and abortion forms of the disease. Typically around 10% of infected horses develop neurological signs during EHV-1 outbreaks. Abortion outbreaks can have much higher rates of effected individuals and exceed 50% of the exposed and susceptible individuals. It is important to note that often abortion occurs without prior respiratory signs and though this can happen with EHM it is less common.
Symptoms of Rhinopneumonitis
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