Heaves; Equine COPD
Heaves - equine COPD
Equine COPD (also known as heaves, recurrent airway obstruction, chronic bronchitis or small airway disease) is a disease that has been recognized for centuries. It is common in countries with cold climates such as ours, where horses are kept in barns for prolonged periods of time and where hay is often moldy. It is currently believed that COPD is caused by an allergic reaction to these molds when inhaled by horses. The disease affects primarily mature horses and is a common cause of a premature end to their athletic careers. The first clinical signs noted by an owner may be an occasional cough, after what appears to be a viral respiratory infection. The signs may then progress to paroxysmal coughing, exercise intolerance and severe impairment in lung function which can be fatal.
It is well established that heaves results from the inhalation of molds in hay and bedding, and removal of these molds allows a remission of the disease. This is best achieved by replacing the hay by dust free food, such as pelleted hay, haylage etc. However, for a number of owners, these options may not be possible for various reasons. When environment dust control is not feasible or when horses are in an acute crisis and have respiratory difficulty, medication is required.
Our research interest has been focussing on the effects of various medication on lung function of horses with COPD. Respiratory mechanics measurements is a research tool which allows an objective evaluation or measurement of the lung function in horses, and therefore is very useful to evaluate the efficacy of therapy over time.
Respiratory mechanics, as the name suggests, is based on general principles of physics, where the pressure required to move an object is based on the mass of the object, the speed of the displacement, the resistance of the system etc. Air is moved through the airways down to the lung due by changes in pressure in the thorax (chest), caused by displacement of the thoracic walls resulting from the movement of the respiratory muscles. During inspiration the thoracic cavity expands, causing a negative pressure in the thorax that attract air inwards. During expiration, the thoracic cavity contracts, resulting in outward movement of air .
The technique we use to measure respiratory mechanics requires the determination of the intrathoracic pressure and the flow of air in and out the respiratory system. A tube positioned in the esophagus (similar to nasogastric tubes used to administer mineral oil in a horse with colic for example) measures the pressure within the thorax; at the end of tube, a measuring device called pressure transducer is placed. The tube is small and its placement is well tolerated by horses. A device call a "pneumotacograph" placed on a facial mask measures the flow of air. These two signals, allow the calculation of various respiratory parameters such as tidal volume (quantity of air inspired during a breath), minute ventilation (volume of air inhaled per minute), airway resistance (the resistance offered by the respiratory tract preventing the movement of air; horses with COPD have an obstruction of the airway which impairs airflow), airway compliance (measures the elasticity of the lung; also abnormal in horses with COPD), inertance. The calculation of inertance provide information on the elasticity of the lung.
We use this tool mainly in a research setting, since the technique is not very sensitive, meaning that the mechanics of breathing is abnormal at rest only when there is already evidence of respiratory difficulties. However, it has allowed us to determine which drugs are effective in treating the disease and which ones are not. We measure the various parameters in COPD horses, often donated by our clients, before and after treatment and thus can determine which medications are most effective in alleviating symptoms of the disease. Work from our laboratory has shown that inhaled corticosteroids, as used by people with asthma are also very effective in horses.
We are also currently studying the underlying mechanisms of the disease in the hope of eventually not only improving symptoms but either preventing the disease or it's progression.