Equine Herpesvirus, also known as rhinopneumonitis, is a highly contagious virus that causes respiratory disease, abortion, and intermittent outbreaks of neurologic disease in horses. It has two forms, EHV-1 and EHV-4, which are spread by 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 aborted placentas. Like herpesviruses in other species, these viruses establish latent infection in the majority of horses, which do not show clinical signs but may experience reactivation of infection and shedding of the virus when stressed. Those factors seriously compromise efforts to control the disease and explain why outbreaks of EHV-1 or EHV-4 can occur in closed populations of horses.
Special attention to biosecurity is key to prevention of EHV in a horse population in conjunction with vaccination. Minimizing or eliminating contact between non-exposed horses and the secretions of infected horses can help control the spread. Primary indications for use of equine herpesvirus vaccines include prevention of EHV-1-induced abortion in pregnant mares, and reduction of signs and spread of 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-vaccination 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.
Clinical Signs of Illness
Symptoms that should alert horse owners to the possibility of neurologic EHV-1 infection include fever, weakness and incoordination, and urine dribbling or inability to urinate. Horses with these symptoms should be examined immediately by a veterinarian. Suspect horses should be isolated from healthy horses and tested for EHV-1.
Upon detection of clinical signs suggestive of EHV, the veterinarian may choose to take a nose and throat swab (nasopharyngeal) of the horse, blood sample, or tissue from the aborted fetus for detection of virus in the tissues. Paired blood samples for detection of antibody levels may also be taken. Treatment involves supportive care and treatment of the symptoms. Non-steroidal anti-inflammatory drugs are commonly used to reduce fever, pain and inflammation. In uncomplicated cases, complete recovery will occur in a few weeks. Horses with neurological disease have variable recovery rates depending on severity of the clinical signs. The prognosis is poor if the horse is unable to stand for an extended period of time. The horse should be rested until fully recovered and gradually returned to work.
Although humans can’t be infected by EHV-1, they can aid in spreading it to their horses or other horses in their care. Therefore, owners of affected horses should wash and disinfect their hands and change their clothes before coming into contact with healthy horses to prevent the potential spread of these infectious organisms.