Two distinct viruses, equine herpesvirus type 1 (EHV-1) and equine herpesvirus type 4 (EHV-4), cause two different diseases, both of which are known as rhinopneumonitis. Both cause respiratory tract problems, and EHV-1 may also cause abortion, foal death, and paralysis. Infected horses may be feverish and lethargic, may lose their appetite and experience nasal discharge and a cough.
Young horses suffer most from respiratory tract infections and may develop pneumonia secondary to EHV-1. Rhinopneumonitis is spread by aerosol and by direct contact with secretions, utensils, or drinking water. Virus may be present but inapparent in carrier animals.