Summer Pasture-Associated Obstructive Pulmonary disease Can Be Managed

Thomas L. Seahorn DVM, MS
Associate Professor of Equine Medicine
Diplomate, American College of Veterinary Internal Medicine

Ralph E. Beadle, DVM, PhD
Professor of Veterinary Medicine

Modified from a manuscript accepted for publication in the Compendium on Continuing Education for the Practicing Veterinarian.)

The first step in management of horses with chronic obstructive pulmonary disease (COPD) is to determine whether bacterial infection is a complication of the disease. Abnormal findings on physical examination and laboratory assessment that suggest the presence of bacterial infection include fever, purulent nasal discharge, lethargy, anorexia, neutrophilic leukocytosis with or without a left shift and increased fibrinogen concentration, and the presence of phagocytized bacteria in a trans-tracheal wash cytology. Any of these findings indicate the need for antibiotic therapy. Streptococcus zooepidemicus is a common isolate in these patients, and procaine penicillin (22,000 IU/kg, IM, SID) or trimethoprim sulfamethoxazole (15 mg/kg, PO, BID) are generally effective treatments.

The second and most important step in treatment is environmental management, which involves determining the origin of the offending allergen(s). The allergen(s) are often identified by evaluating the typical habitat of the affected horse. Many horses in the southern United States develop clinical signs of COPD while at pasture, thus the term summer pasture-associated obstructive pulmonary disease;1 other horses throughout the country develop clinical signs when exposed to moldy and dusty hay in the stall. Horses with summer pasture-associated obstructive pulmonary disease should be moved out of the pasture and should remain in a stall with minimal exposure to dust and no exposure to grass at least until clinical signs improve. Often, the more severely affected horses must avoid pasture exposure until weather cools and pasture molds are less abundant.2

To minimize exposure to dust, several changes must be made in the affected horse's stall environment. The preferred diet while the horse is confined to a stall is a pelleted complete feed. This type of feed significantly reduces diet-related dust exposure; however, care must be taken to avoid dust that accumulates at the bottom of the bag. If owners are unwilling to change the affected horse's diet completely, they should attempt to use only the best quality feeds. Hay that has been damaged by rain, or hay that has been stored for more than a year, is often dusty and should be avoided. If poor quality hay has already been purchased, soaking the hay in water prior to feeding will eliminate the dust. Wet hay may spoil and should be replaced with fresh hay at 12-hour intervals. Concentrates also vary in the amount of dustiness; generally grains with molasses added ('sweet-feeds') are the least dusty, followed by pelleted feeds. COPD-affected horses should be fed at shoulder height, as feeding on the ground exposes them to dust in the bedding. Hay that is fed from an elevated position will often shower dust over the horse's nose during feeding.

Bedding materials represent another important source of dust. Generally, sawdust and straw are the dustiest types of bedding materials, and should be avoided, followed by dry sand, and then shavings. Bedding materials such as paper and peat moss have minimal dust, but are not readily available and are not feasible for most horse owners. The more commonly used wood shavings can be adapted to the 'dust-free' environment by applying water prior to their use or, even more effectively, by applying a fine spray of mineral oil prior to their use.

Neither hay nor bedding materials should be stored above the affected horse's stall and preferably should be several feet/yards away. Additionally, the affected horse should not be stalled near a dusty arena, and cleaning of stalls and the barn alleyway should only commence when the affected horse has been taken outside for a walk. Some of the more severely affected horses may have clinical signs exacerbated by simply exercising in a grassy area; therefore, for best success, these horses should be exercised on paved/concreted surfaces or in dirt round-pens (if the dirt does not tend to form dust).

Medical management typically involves a combination of anti-inflammatory and bronchodilator therapy, depending on the severity of clinical signs. Patients that are in extreme respiratory distress receive glycopyrrolate (0.0022 mg/kg, IV) once or twice separated by 8 hours.3 Additionally, severely affected horses are often dehydrated and heat stressed and respond favorably to intravenous fluid administration (50-75 ml/kg/24 hrs) and cool-air fans. The initial glycopyrrolate therapy for bronchodilation is often followed by oral administration of terbutaline (0.06 mg/kg, BID) for 7-10 days. The cornerstone of anti-inflammatory therapy is dexamethasone. Therapy typically begins with 0.04 mg/kg IV twice daily for 2-3 days and then the dose is decreased incrementally over the next 7-10 days depending on the initial response. For example, if the patient's respiratory rate slows and the effort to breathe decreases during the first 2 days of dexamethasone therapy, the dose of dexamethasone will be reduced to 0.04 mg/kg IV once daily for 2-3 days. If improvement continues, the dose is reduced to 0.02 mg/kg IV once daily for 3 days followed by the same dose administered orally every other day. Typically, the response to therapy appears to occur more quickly early in the season (i.e., June-July) but as the season progresses (i.e., September-October), affected horses are slower to respond, and therefore, require therapy for a longer period. Horses that have secondary bacterial infections typically fail to improve with standard bronchodilator and anti-inflammatory therapy; the infection must be controlled before bronchodilator and anti-inflammatory therapy can be effective. An alternative anti-inflammatory agent is prednisolone (1.0-2.0 mg/kg) administered orally once or twice daily;4 the response to this agent is often less favorable than the response to dexamethasone. Inhalation therapy is an alternative that may be quite effective in some patients and may have the advantage of avoiding some systemic effects.

Chronic obstructive pulmonary disease is an allergic airway disease with a seasonal recurrence of clinical signs throughout the lifetime of the affected horse. If the clinical signs are recognized early and treated effectively, these patients can have long and productive lives. Because secondary bacterial infections occur in some horses, those with recurring episodes in subsequent years or horses that fail to respond to the initial therapy should be re-assessed by a veterinarian.

      Ventipulmin¨ Syrup has recently been approved by the U.S. Food and Drug Administration (FDA) for oral use in horses.  Clenbuterol hydrochloride is the active ingredient in this product and is the first alpha 2-adrenergic agonist approved for use in horses in the U.S.

      Ventipulmin¨ Syrup is indicated for management of horses affected with airway obstructive diseases, such as chronic obstructive pulmonary disease (COPD) and summer pasture-associated obstructive pulmonary disease (SPAOPD).  Its beneficial effects in these horses include relieving bronchospasm and improving the clearance of mucus from the airways.  Both of these features serve to decrease resistance to airflow.

      The product comes in 100 and 330 ml containers that are equipped with a special dispensing cap that facilitates drawing an appropriate volume from the container with a syringe.  An insert is provided with the product to help determine an appropriate dosage schedule.  The initial dose is 0.5 mL/100 lb. bodyweight twice daily for three days.  Increasing dosages can be used for horses that do not respond to the initial dose.  A 30-day duration of treatment is recommended.  The company also recommends that, where indicated, appropriate environmental changes be instituted as well.

      A number of adverse reactions may be observed in some horses during the first few days of treatment.  These include mild sweating, muscle tremor, restlessness, urticaria (hives) and tachycardia (increased heart rate).  The product is contraindicated in pregnant mares near term and in horses suspected of having cardiovascular impairment.

      Clenbuterol can affect lung function and heart rate in humans as well as horses.  Improper use in food animals, therefore, can be a potential health hazard for humans who consume products derived from treated food animals.  For this reason, use of clenbuterol in food animals in the U.S. is strictly prohibited and use violations are a high priority for regulatory action by the FDA.  Veterinarians are cautioned to advise clients that extra-label use of this product in food animals and/or humans is not legally permissible.