Respiratory problems are frequent health issues for horses, and are often associated with microbial infections. For horse owners preparing for spring competitions or the birth of foals, a review of respiratory diseases is timely. Owners need to recognize early signs of respiratory infections to minimize the spread to other horses and to initiate prompt veterinary treatment. The impact of these infectious respiratory diseases can be minimized by strategic vaccination and smart management practices.
Respiratory diseases in horses are similar to those in humans, which can help owners recognize, control, and prevent these diseases. Just as in humans, some respiratory diseases are much more contagious than others and can spread rapidly. The best approach is to treat any horse with fever and nasal discharge as potentially contagious until a veterinarian can prove otherwise.
Influenza has the greatest impact on horses because it spreads rapidly due to the potential of viruses to be transmitted in respiratory aerosols (like human sneezes) as well as the very short incubation period of one to three days. Natural immunity to the virus is short-lived (several months), meaning a horse can be re-infected quickly.
A horse that develops influenza usually acts unwell, often doesn't eat, and has a high fever, watery nasal discharge, and a cough. A few can develop secondary bacterial pneumonia or inflammation of the heart muscle, both of which can be fatal. The cough might persist for days. A rapid diagnostic test is available to identify the source of the respiratory signs.
Treatment includes keeping the horse comfortable and reducing high fevers. As a general rule, horses should be rested one week for every degree of fever at the peak of the illness. Influenza can spread by direct contact, by people in contact with infected horses, and by respiratory droplets over short distances.
Equine herpesvirus (EHV) type 4 infections can be clinically identical to influenza, but might be accompanied by conjunctivitis (inflammation of the membrane covering the eye) or mild limb swelling. The fever is typically slightly lower and the cough is less persistent. Equine herpesvirus type 1 also produces mild respiratory signs, but also causes abortion or neurologic disease. It can be spread by contact with nasal secretions, or in the case of abortion, by contact with the fetus, placenta, or placental fluids.
Horses or facilities that have contact with outside horses are ideal candidates for a respiratory virus vaccination program for both influenza and herpesviruses. After a horse recovers from clinical signs, herpesvirus becomes latent and can cause disease later when the horse is stressed, similar to herpes cold sores in people.
Strangles can also cause serious illness and spread rapidly from horse to horse. This bacterial infection, caused by Streptococcus equi, is similar to strep throat in humans. Affected horses develop swollen lymph nodes (like tonsils) that can abscess and rupture. Fever, poor appetite, and thick whitish/yellow nasal discharges can be seen. The nasal discharge is a source of infection to other horses. The lymph nodes might enlarge rapidly in a few horses, greatly narrowing the horse's upper airway and necessitating an emergency tracheostomy. Most horses recover with little treatment, but some develop potentially fatal complications of purpura hemorrhagica (also called bastard strangles, which is abscessation of lymph nodes in other parts of the body).
The organism is readily isolated from the nasal discharge, or pus from an abscessed lymph node. Strangles vaccines reduce the severity of illness, but might produce some side effects. These risks should be considered before vaccination.
Rhodococcus equi, which causes pneumonia, affects foals and can spread rapidly through a farm. This bacterium is inhaled from dust or ingested and causes abscesses in the lungs, accompanied by fever. No vaccines are currently available, but administration of antibody-rich plasma to very young foals on farms with chronic problems has been known to help.
Other microbes can cause respiratory illness, but with much less impact. Equine viral arteritis (EVA) initially might resemble an EHV infection. EVA also can cause abortion, and exposed stallions might become carriers, shedding the virus in their semen.
Rhinoviruses can cause mild "cold-like" signs in horses, but horses recover quickly.
Julia H. Wilson, DVM, Dipl. ACVIM, is an Associate Professor of Veterinary Population Medicine and Division Head of Large Animal Medicine at the University of Minnesota.