Dr. Kevin Voller
The chronic coughing horse is the topic of today. Coughing can originate from either the upper respiratory tract or the lower respiratory tract. The upper respiratory tract refers to the nasal passage ways, larynx (throat) or the trachea. The lower respiratory tract refers to the lung itself. Coughing comes about from irritation of the lining of the airways or the lung itself having inflammation or increased fluid accumulation in the airways or the lung. Most, but not all, upper respiratory issues relate to some type of infectious process, such as viral or bacterial infections. These can involve inflammation or infection of the tissues lining the upper respiratory tract or structures of the airways such as the guttural pouches or sinuses that can retain fluid. The contagious causes of a cough are the infectious viral and bacterial agents such as influenza, rhinopneumonitis, rhinitis, equine viral arteritis and streptococcus equi. These infectious agents can cause issues in the lower respiratory tract from the primary infection. Problems also arise after the infection has resolved, yet the inflammation induced in the lung has not been eliminated. The non-infectious primary causes of a cough include lung worm, exercise induced pulmonary hemorrhage, inflammatory airway disease, and recurrent airway obstructive disease. Other causes such as neoplasia, summer pasture – associated obstructive pulmonary disease will not be discussed here.
Below is a diagram of the respiratory tract of the horse. For our purposes we will define the upper respiratory tract as beginning at the nostril and extending to where the trachea (windpipe) divides prior to entering the lungs; the lower tract is from this division all the way into the lungs. The analogy used to understand the lower respiratory tract would be a large oak tree. The oak tree has a trunk which rises from the ground before it divides into large branches, which extend further up and divide into smaller branches which extend further up, dividing into smaller branches before ultimately ending in the leaves. The trunk would equal the trachea, which divides into the major bronchi, which divide further into smaller bronchi which ultimate split into bronchioles which ultimately end in the alveoli (air sacs) – the alveoli being the leaves on the tree in our analogy. The alveoli are the site where the oxygen and carbon dioxide exchange take place.
Coughing may be the only symptom that is a concern for owners, but it is often not the only symptom. In some horses other symptoms may be present and would include increased respiratory rate, nasal discharge or, in some cases, nondescript poor performance or even exercise intolerance. In some cases, the problem may start just as a persistent cough, but over time progress to the other symptoms. As stated earlier, the goal of today’s discussion is only to touch on a couple of the causes of lower respiratory tract disease that cause chronic coughing.
Lungworms are a rare cause, but can have symptoms identical to some of the other problems. In my twenty plus years of practice I have only seen lungworms a couple of times. Lung worm life cycle is such that the adult develops in the bronchi and bronchioles of the airway. They lay eggs which are then coughed up, swallowed and then hatch in the intestine of the horse. The small worms called larvae are passed in the manure. These crawl out of the manure pile onto the grass. The larvae are then eaten by the horse that consumes the grass. Those larvae then migrate through the gut wall and are carried to the lungs through part of the circulatory system. The lung worms cause no obvious GI signs while migrating through the gut wall, but the adults and their larvae and eggs cause inflammation in the lung resulting in coughing, increased respiratory rate, and mucous in the lung. In almost all cases lung worm infection in the horse came from exposure to pastures that have contained infected donkeys or mules. The donkeys and mules typically show no symptoms of lungworm infection. Treatment with ivermectin or moxidectin typically resolves the parasitic infection.
Exercise induced pulmonary hemorrhage (EIPH) occurs when a horse exerts itself and bleeds within the lungs. Horsemen refer to this as a “bleeder”. This is a disease primarily of the race horse or the barrel racing horse, as it seems to occur only when the horse has to exert itself to this extent. Owners may see varying degrees of hemorrhage that come from the nostril, from marked to none. Some horses may cough following the bleeding, but others may only have poor performance, running a slower time, or start objecting to running. In some cases it has been the horse that becomes overly excited at a barrel competition. Bleeding does not typically occur during the training sessions. Diagnosis is made by seeing the blood externally, performing an endoscopic examination very shortly after a competition (within hours) or finding evidence of prior bleeding in the BAL sample. This can be found up to days afterwards. Some “bleeders” may have primarily inflammatory airway disease which may be successfully treated. If the inflammatory issue is eliminated, the “bleeder” status of the horse may resolve. The cause of EIPH is unknown at this time, and is an ongoing source of research. Treatment of EIPH involves preventing or moderating the bleeding by using Lasix (furosemide) prior to competition.
Inflammatory airway disease is a descriptive disease that is characterized by poor performance, exercise intolerance or coughing with or without excess mucous, and non-septic inflammation(NO infection present). This is a disease of the younger horse and the horse is not sick-meaning no fever, depression or inappetitence. It may originate from a viral infection that has resolved but the inflammation is still present. Poor air quality, barns with poor ventilation or chronic exposure to dust or molds or hay with the same may be an inciting cause. Some horses may have an allergic component that started or perpetuates the inflammation. Diagnosis is based on results of BAL cytology and culture from the lung or lower airway. Treatment depends on the cause. Anti-virals such as interferon or rest help resolve viral causes. Corticosteriods are beneficial for inflammation reduction and may reduce the responsiveness to the dusts, molds or allergens. Anti-histamines are of benefit for cases caused by allergy. In most cases bronchodilators are not of help.
Reactive airway obstructive (RAO) disease is the new term for what previously has been referred to as chronic obstructive pulmonary disease (COPD). Horsemen have historically referred to this condition as “heaves”. So what is the difference, and why the name change? Because the medical term COPD refers to the human respiratory condition and the horse “COPD” is very different in cause, condition and treatment. In horses, there is spasm of the small airway in addition to increased mucous and purulent material (pus) development within the airway. So why? We are still not sure. There are, however, changes that occur within the tissues that make them over respond to various chemical stimulants (histamine) as well as to dusts and molds. Most horses with RAO/heaves are housed in a barn for some period of time, are exposed to dusty/moldy forage or on dry dusty lots or paddocks. The goal for treatment is to improve air quality. This means trying to keep the horse out of doors, removing access to round bales or any dusty hay. Some horses may have to come off hay all together and be maintained on hay cubes or pellets. Various medications are used to treat the symptoms. These are typically based on corticosteriods, bronchodilators and, for some horses; the use of antihistamines may be beneficial. Another set of horses will require antibiotic treatment based BAL results, culture and sensitivity.
So how are the different conditions diagnosed? Your veterinarian will get a history; perform a physical examination including an auscultation of the respiratory and cardiovascular system. Blood work may be indicated based on the history and examination. What has helped the most in arriving at a diagnosis and treatment protocol has been the bronchoalveolar lavage (BAL). This is a “washing” of the lower airways of the horse with sterile fluid, recovering a portion of this fluid, performing a microscopic analysis and culture of this fluid. The procedure is performed at the clinic because processing of the sample is very time sensitive and requires immediate centrifugation and processing of the fluid. How is a BAL performed? The horse is lightly sedated to reduce coughing. A sterile tube that is similar in size to a stomach tube is passed up the nostril and down the trachea to the bronchi where it gently lodges. A cuff on the tube is inflated and sterile fluid is put down the tube and recovered. This fluid is then processed for analysis of the cells in the recovered sample. The types and percentage of the cells as well as presence or absence of bacteria in the cells determines the appropriate treatment. A sample for culture and antibiotic sensitivity are also submitted to determine if there is an infection as well as what antibiotic is appropriate for treatment.
When do you need to have a veterinarian out to examine a persistent cough? Most infectious respiratory conditions should resolve in 3- 4 weeks and frequently much more rapidly than this. The “sick” horse – meaning increased respiratory rate, elevated temperature, in appetence or depression – may require timely examination and treatment; consult with your veterinarian. If you have a horse that has a cough that is not resolving or a persistent cough that is developing I would suggest that you contact your veterinarian for a course of action. The course may involve management changes only, but if this does not resolve the issue further diagnostics and treatment may be required.