The respiratory system consists of the large and small airways and the lungs. When a horse inhales, the air travels down the trachea, which divides into the tubes known as the right and left bronchi, then into the smaller airways called bronchioles in the lungs. The bronchioles end in the small sacs called alveoli, where the barrier between the air and the blood is a thin membrane.
The most important function of the respiratory system is to deliver oxygen to the blood and remove carbon dioxide. The blood distributes oxygen to the body and collects carbon dioxide, the waste product of cell metabolism. The exchange of oxygen and carbon dioxide occurs in the alveoli. When this exchange fails or becomes inefficient because of disease, the animal can become seriously ill. The respiratory system protects its own delicate airways by warming and humidifying inhaled air and by filtering out particles. Large airborne particles usually land on the mucous lining of the nasal passages, after which they are carried to the throat to be either swallowed or coughed up. Small particles and microorganisms are destroyed by the body’s immune system.
Although the basic functions are the same, the anatomy of the respiratory tract varies among species. For example, the respiratory systems of dogs and cats are somewhat similar to each other, but differ from the respiratory systems of horses and humans. These differences explain in part why some diseases affect only certain species of animals. Horses are nasal breathers, and are not able to breathe through their mouths. Because of this, the horse’s nasal passages are large and can expand somewhat during strenuous exercise in order to increase the intake of air.
When the level of oxygen in the blood is too low (called hypoxia or anoxia), the animal will show signs of respiratory distress. Low oxygen levels can be caused by reduced oxygen-carrying capacity of the red blood cells, insufficient movement of gases in and out of the lungs, or inability of tissues to use available oxygen (a condition caused by some poisons). The animal’s body attempts to compensate for low oxygen in the blood by increasing the depth and rate of breathing, contracting the spleen (to force more red blood cells into circulation), and increasing blood flow and heart rate. If the brain suffers from lack of oxygen, respiratory function may be reduced even further due to depression of nervous system activity. In addition, heart, kidney, and liver functions may be reduced, as well as the normal movement and secretions of the intestine. If the body is not able to compensate for the reduced oxygen level, all body tissues function less efficiently and may be damaged.
Lung and airway disorders are often caused by direct infection with viruses, bacteria, fungi, or parasites, as well as by immune-mediated reactions or inhalation of irritants or toxic substances. Trauma (such as crashing into a fence) may lead to the collapse of a lung or airway.
Viral respiratory infections are common in horses. Most bacterial respiratory infections (with the exception of strangles) occur after an attack of a viral disease. This is because viral respiratory infections impair and/or destroy respiratory defense mechanisms, making the horse more susceptible to additional infections. The most common organisms associated with pneumonia in horses are bacteria normally present in the upper respiratory tract that take advantage of the horse’s weakened state (called opportunistic bacteria). Secondary bacterial disease may result in bacterial infections of the mucous membranes (rhinitis and tracheitis), or may produce more serious invasive disease such as pneumonia and pleuropneumonia. Streptococcus equi equi, the organism that causes strangles, grows in the upper respiratory tract and is capable of causing infection without predisposing factors such as a previous viral disease. Rhodococcus equi is an organism that causes disease of the lower respiratory tract of foals less than 5 months of age, which is slowly progressive and difficult to detect until the foal is quite sick.
Noninfectious respiratory disease (disease not caused by a bacterium or other microorganism) may occur in adult horses of various ages and can limit their performance. Inflammatory airway disease is characterized by excessive mucus in the airways and poor exercise performance in young horses. The cause is unclear, but viral respiratory infection, allergy, and environmental factors may play a role. Reactive airway disease (heaves) is triggered by exposure to organic dusts in older horses with a genetic susceptibility to allergic airway disease. The small airways in the lungs are obstructed by constriction and excessive mucus production. The severity of signs ranges from exercise intolerance to labored breathing at rest.
Diagnosing Lung and Airway Disorders in Horses
Your horse’s history and the veterinarian’s physical examination will help to determine the possible cause and site of respiratory disease. Other techniques used for diagnosis include x‑rays of the chest or neck; the use of an endoscope (a small camera and light mounted on the end of flexible tubing) to view the nose, throat, and airways; and examination of the contents of the lung and airways, which can be obtained through aspiration or retrieval of fluid used to “bathe” the airways (see Table: Diagnosing Lung and Airway Disorders in Horses).
Sudden dietary changes, weaning, cold, drafts, dampness, dust, poor ventilation, transporting, and the mixing of different age groups all play a role in respiratory disease in groups of animals. Stress and mixing of animals from several sources should be avoided or minimized if possible. Immunization can help control respiratory infection; however, it is not a substitute for proper environmental conditions and animal care.
Immunization does not always prevent respiratory infections in horses, but the duration and severity of illness is usually lessened in vaccinated horses depending on the specific disease and the vaccine used. The veterinarian will help weigh the cost, effectiveness, and risks of each vaccination against the probability of the horse’s exposure and potential for developing the disease. Vaccination recommendations and schedules will vary according to the use of the horse and its potential for exposure to contagious animals.
Respiratory disorders often involve the production of excess secretions in the respiratory system (for example, in the nose and lungs) that the affected animal may not be able to remove without assistance. One goal of veterinary treatment is to reduce the volume and thickness of the secretions and to make their removal easier. This can be accomplished by controlling infection, thinning the secretions, and when possible, improving drainage and mechanically removing the material.
Therapies can include altering the inhaled air (for example, reducing dust or adding humidity) as well as the use of expectorants (which help an animal to cough up the secretions), cough suppressants, bronchodilators (to help open airways), antibiotics, diuretics (to reduce fluid buildup), and other drugs.
Regardless of the type of respiratory disease, environmental factors and supportive care are important to aid recovery. A dust- and ammonia-free stable environment prevents further damage to the respiratory system. Highly palatable feeds help to prevent weight loss and weakness during the treatment and recovery period. Adequate water intake will decrease the thickness of respiratory secretions, making their removal from the lower respiratory tract easier. A comfortable, dry, temperature-appropriate environment will allow the horse to rest and will lessen the need for the respiratory tract to work at regulating the horse’s body temperature.
Also see professional content regarding an introduction to respiratory diseases of horses.