Equine upper respiratory tract disease (URTD), caused by equine herpesvirus is a disease commonly seen in young horses. It is believed that 80% of horses get infected by 2 years of age. Equine herpesvirus 1 (EHV1) and equine herpesvirus 4 (EHV4) are important equine viruses, causing much damage to the horse industry. EHV1 strains are associated with respiratory disease, abortion, and paralysis, whereas EHV4 strains are primarily associated with respiratory disease.
Young horse are at greatest risk to be infected by herpesvirus. The herpesvirus rarely occurs in foals younger than three months of age because they still have their vaccinated mothers' immunity. The great majority of infected foals is seen in sucklings and weanlings between 4 and 12 months of age which develop what veterinarians call "foal snots". The risk of developing the URTD increases with overcrowding, heavy parasite infestation, poor nutritional state, climatic extremes, existing disease, and the intermingling of animals from different social groups.
The infection is transmitted by contact with another infected horse and its respiratory discharge. Diagnosis of herpesvirus respiratory disease in horses is performed through laboratory tests. Vaccination of young horses does not prevent the infection, but diminishes the intensity of the disease.
The outbreaks of infection are common in large broodmare farm operations, annual yearling sales events and race or show barns in which horses from different places are kept together in enclosed, confined spaces. Two- and three-year-olds usually develop an acute form of herpesvirus URTD with neurological complications and increased risk of abortion. The virus remains in an individual horse over the lifetime and periodically reactivates and transmits from horse to horse and mother to foal. The factors contributing to herpesvirus reactivation include surgery, boarding, prolonged transport, weaning, lactation, extreme weather, and stress.
Symptoms of the herpesvirus URTD vary from horse to horse and may range from mild illness to a life-threatening lung inflammation. Most common signs include watery nasal discharge that occurs during the first day of the disease. By the second or third day the nasal discharge often dries to form crust in and around the nostrils and becomes thicker and yellowish. In some horses signs may include loss of appetite (anorexia), fever, lethargy, cough, labored breathing and discharge from the eyes (conjunctivitis). In uncomplicated form of infection, the prognosis for full recovery is usually good and signs usually disappear by the end of second week. With the secondary bacterial infection, primarily by primarily by Streptococcus equi zooepidemicus, the disease is much more severe.
Even though the infection by herpesvarus affects mainly the upper respiratory tract, it may lead to other serious complications including abortion, neonatal foal death, brain, lung and eye infections.
Treatment includes medications for fever reduction and anti-inflammatory agents for reduction of respiratory tract inflammation and antimicrobials. Horses unwilling to eat or drink may require fluid-electrolyte replacement therapy.
Prevention of herpesvirus URTD is the most effective way to control the disease outbreaks. Foals become maximally susceptible to infection by 5 - 6 months of age. Vaccination against EHV-1 and EHV-4 respiratory disease is recommended as part of the preventive, herd-health program for all horses at risk for acquiring infection. Vaccination of young horses does not prevent respiratory infection, but diminishes the intensity of clinical signs and both the magnitude and duration and amount of shedding of infectious virus. Because immunity to EHV-1 and EHV-4 generated by vaccination is of short duration, frequent booster doses are necessary for maximal effectiveness.