EHV-1 stands for equine herpesvirus-1, a common pathogenof the upper portion of the respiratory tract in horses. It is common enough that you almost certainly have immunized your horse against it each time you vaccinate against rhinopneumonitis. EHV-1, along with EHV-4, are the two herpesvirus strains that cause the most respiratory disease in horses and, like influenza virus, these viruses cause high fevers, lethargy, an initially clear nasal discharge that turns purulent, lymph node enlargement and swelling in the throatlatch area, occasional limb edema, and a predisposition for secondary bacterial bronchitis or bronchopneumonia. So the answer to this question is, in most instances, that the symptoms of EHV-1 are those of respiratory infection and are indistinguishable from EHV-4 infection or influenza. Testing of blood and nasal secretions is necessary to distinguish among viruses that affect the equine upper respiratory tract. EVH-1 differs from these other respiratory tract viruses in its propensity to affect two other major body systems—the central nervous system and the reproductive tract. EHV-1 can cause abortion in pregnant mares, in storms if a herd is unvaccinated. Making the headlines most recently, however, is the neurologic form of EHV-1 infection. This manifestation, although infrequent, can be life-threatening and is the disease entity people usually refer to when discussing EHV-1. The signs of neurologic EHV-1 infection range from mild hind limb ataxia (lack of coordination) to urine dribbling and inability to void the bladder properly, loss of sensation around the tailhead and thighs, weakness in the hind limbs severe enough to cause difficulty in rising, and weakness severe enough that the horse may dog sit or lie down without being able to stand up at all. Owners should know that transmission takes place via direct or indirect contact with the nasal discharge or aerosolized droplets coughed up and liberated into the stable air by an infected horse. The two best strategies for protecting your horse are to limit exposure and maintain appropriate immunization among the permanent residents on a given premises. To limit exposure, select a boarding facility that has a mostly permanent resident population and that practices good quarantine protocol for incoming horses. At the stable and especially at shows, prevent nose-to-nose contact with other horses and do not allow your horse to drink from a community water trough or contact feed or water buckets from other horses. Don’t pet or interact with other horses, and don’t lend out your grooming implements or feed and water tubs. Wash and disinfect your trailer after shows and, if you transport horses commercially via land or air, ask whether the vehicles or boxes are disinfected with antiviral agents between shipments. At any stable, newly arrived horses should be segregated from the resident horse population for no less than 14 days, and preferably 21 days. This allows sufficient time for diseases to complete their incubation time and manifest as disease before the horse has been turned out into a group pasture. With regard to vaccination, one of the most intimidating aspects of EHV-1 infection is that immunization does not appear to confer protection against the neurologic form of the disease. In fact, it has been proposed that vaccination induces high antibody levels which, although helpful and an important part of the immune response in most instances, in this scenario contribute to the problem by binding with virus particles to form small complexes that damage and block capillaries of the spinal cord. When this happens, neurons in the spinal cord that received blood flow from a damaged capillary are injured by ischemia (no blood flow) in the same manner as stroke occurs in the brain. This most often affects the lower spinal cord segments, and the resulting clinical signs are ataxia, with or without urinary incontinence, penile paralysis, or loss of hind limb strength Immunity following a herpesvirus infection lasts for three to four months. Because the neurologic form of EHV-1 already has affected many horses in multiple states in 2011 and has caused the deaths of at least a dozen horses, I recommend that owners make an appointment to review their horses’ immunization status with their veterinarian and talk with him or her about making a contingency plan that can be implemented if a horse develops fever or nasal discharge.

This question was answered by Kim A. Spraybery, D.V.M., a board-certified medicine specialist.