Some folks have expressed interest in knowing what to look for as they monitor their horses during the current EHV-1 outbreak. I think the following provides a good overview of the infection, its signs, and suggested treatments.
Equine Herpes Virus (Rhino) – Type 1
Rachel Gardner, DVM, DACVIM
Internal Medicine Specialist, B.W. Furlong & Associates
Equine Herpesvirus Type 1 (EHV-1), also commonly known as rhinopneumonitis or “rhino”, typically causes upper respiratory infections in young horses and abortions in pregnant mares. Horses affected with the respiratory form of the virus typically exhibit fever, depression, inappetance, nasal discharge, and a cough. Many animals in a group may be affected, and they will usually recover uneventfully. However, the virus is also capable of producing neurologic signs. Some horses that develop neurologic signs will exhibit evidence of respiratory infection first, although frequently these signs are not present. The neurologic form of EHV-1 is of significant concern as is has become more prevalent in recent years and is frequently associated with disease outbreaks.
Horses affected with the neurological form of EHV-1 will frequently suffer from a fever, followed by a variety of possible neurologic signs. These signs may include weakness, incoordination of the limbs, inability to urinate or pass manure appropriately, and decreased tail tone. The signs are typically most apparent in the hindlimbs and in severe cases, signs may progress to the inability to stand.
Diagnosis of EHV-1 can be accomplished using several different methods. It is frequently suspected prior to laboratory diagnosis based on the clinical signs being exhibited by the horses and possible history of exposure to the virus. Definitive diagnosis is most commonly achieved by detecting virus particles in the horses blood or respiratory secretions using a polymerase chain reaction (PCR) test.
Unfortunately, no specific treatment exists for EHV-1. Treatment, therefore, consists primarily of supportive and nursing care, as well as aggressive anti-inflammatory treatment. Horses may be treated with drugs that inhibit viral replication, such as acyclovir, however the efficacy of theses medications in horses is still speculative. The disease may progress rapidly, therefore early attempts at diagnosis and treatment are critical. Horses may recover partially or complete from the virus, however if the horse becomes recumbent (unable to stand), chances of recovery are slim. If there is any suspicion of neurologic signs or fever in a horse that may have had exposure to EHV-1, it is important to contact a veterinarian immediately. Early diagnosis important to help prevent spread of the disease, and so that other diseases, that may require a different treatment plan, are ruled out.
EHV-1 is primarily spread by respiratory secretions which may become aerosolized and spread up to 35 feet when a horse coughs or sneezes. The virus may also be spread by people or objects (such as a bit) contacting the respiratory secretions of an affected horse and then touching another horse. Luckily, the virus does not live for long periods of time in the environment and is easily killed by common disinfectants, such as a dilute bleach solution.
One of the complicating factors with EHV-1 is that horses can become latent carriers of the virus once they have been infected. Theses horses are outwardly healthy, and the virus lies quietly in a particular area of their nerves. During periods of stress, such as shipping or weaning, they may shed the disease into the environment and infect other horses. Although prevention of the disease is complicated by the potential for latent carriers, good management practices as well as a good vaccination program make transmission less likely. New arrivals onto the farm should be isolated for at least 7 days. Any horses showing signs of a fever or nasal discharge should be evaluated by a veterinarian and isolated for at least 21 days or until infectious disease has been ruled out.
Unfortunately, a perfect vaccine to prevent EHV-1 infection has not yet been developed, although researchers are currently hard at work to develop one. Therefore, current vaccine recommendations depend upon the age and lifestyle of each horse. Your veterinarian can advise you as to the best vaccine schedule for your horse. Because vaccination is not highly effective for the neurologic form of EHV-1, good management and isolation practices are the mainstay of preventing and controlling outbreaks.