Respiratory Disease in Horses
By Dr. Janet Han, Great Lakes Equine Wellness Center, Inc.

Horses can develop respiratory disease from a variety of causes. Regardless of the cause, some of the first signs an owner may notice in their horse are coughing, nasal discharge, increased respiratory rate or effort, and exercise intolerance. One of the more common causes of respiratory disease is pneumonia, which is infection of the lungs. The infection is usually caused by viruses and/or bacteria, but can also be due to fungus or parasites. Horses with pneumonia will typically show signs of overall illness, which could include things like depression, decreased appetite or thirst, or a fever.

Some respiratory diseases in horses aren’t due to infections, but are instead caused by inflammation. The one most owners are familiar with is called heaves, also known as recurrent airway obstruction (RAO). This tends to show up in older horses, and the inflammation of the airways is caused by exposure to organic dust, for example dust associated with hay. The airways become obstructed because of increased mucus, spasm of the airways, and thickening of airway walls. Another type of inflammatory respiratory disease is inflammatory airway disease (IAD), which can be seen in horses of any age. Clinical signs of respiratory disease tend to be less obvious in IAD than heaves. In fact, clinical signs may not be apparent when the horse is at rest. More commonly, a horse with IAD will show exercise intolerance and poor performance. This is in contrast to a horse with heaves, who will be more likely to show increased respiratory effort while at rest. The causes of IAD, whether infectious or non-infectious, are not as well defined or understood as they are in heaves.

In order to diagnose respiratory disease in horses, as well as to come up with an effective treatment plan, several diagnostic tests should be performed. The first would be a physical exam, which would include listening to the heart and lungs and taking the temperature. The next step would be a rebreathing bag exam. This involves putting a plastic bag over the horse's nose to make them take deeper breaths. This allows the veterinarian to hear sounds from the lungs more clearly, and also to pick up any abnormal lung sounds, such as crackles or wheezes. Also, if a horse has respiratory disease, they will frequently cough during the rebreathing bag exam, and once the bag is removed, may take a longer time to recover and return to their previous resting respiratory rate and effort. During the workup, blood should also be drawn to run what’s known as a complete blood count (CBC). A CBC can help the veterinarian determine if an infection is present.

Another diagnostic test is the transtracheal wash. This procedure is useful to look for infection, and lets the veterinarian collect a sterile sample. This involves putting what’s basically a large needle into the trachea (windpipe) by going through the skin and muscles in the middle underside of the neck. Fluid is put into the trachea and then collected again with a syringe. The sample can then be analyzed to see what types of white blood cells are present and also if bacteria are present. The sample is also submitted for culture, which involves a laboratory trying to identify the bacteria and figuring out which antibiotics these bacteria will be sensitive to. These results allow the veterinarian to treat pneumonia much more effectively by ensuring the correct antibiotics are chosen. This procedure can also be performed in a less invasive way by using an endoscope. This alternative option would be useful in cases where the veterinarian feels it would be important to visually look at the trachea; however, the sample that is collected may be less sterile since the endoscope is being passed through the nose and then into the trachea, rather than just going directly into the trachea.

The other important diagnostic test in a respiratory workup is the bronchoalveolar lavage. This procedure is useful for assessing how much inflammation is present in the lungs and involves passing a long tube through the nose and then into the trachea and then to a part of the lungs. Fluid is put into the lungs and then collected for analysis. Based on the types of white blood cells that are present, the degree of inflammation that’s present can be determined. 

One of the most important reasons to have a complete respiratory workup performed is to help differentiate between infectious and inflammatory respiratory disease, since the treatments for the two diseases are different. Pneumonia is treated with a course of antibiotics, while inflammatory respiratory disease that isn’t associated with any infection is treated with a course of steroids. Trying to treat an infectious respiratory disease with steroids could actually make the infection worse, since steroids suppress the immune system.

As discussed, respiratory disease in horses can be caused by many different things, whether infectious or non-infectious. Regardless of the cause, many of the clinical signs noticed by the owner can be similar from one disease to another. After discussing the history of the horse’s respiratory disease with the owner and performing an initial examination, the veterinarian can then suggest appropriate diagnostic tests. Based on the results of these tests, an effective treatment plan can then be designed and the horse started on the appropriate medications.  

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Janet Han, DVM, MS, DACVIM, of Great Lakes Equine Wellness Center, completed an equine internship at the Rochester Equine Clinic in Rochester, NH followed by a residency in equine internal medicine at the Marion duPont Scott Equine Medical Center in Leesburg, VA. She has a master's degree in Biomedical and Veterinary Sciences at Virginia Polytechnic Institute and State University. Dr. Han is a Diplomate of the American College of Veterinary Internal Medicine.