By Lais R. R. Costa; Jill R. Johnson; Cyprianna H. Swiderski; Department of Clinical Sciences at Mississippi State University and Equine Health Studies Program at Louisiana State University 

The horse counterpart of human asthma is called equine recurrent airway obstruction (RAO).  Because of the similarities of RAO and asthma, RAO is now called severe equine asthma. Equine RAO or severe equine asthma includes diseases characterized by bronchoconstriction (airway narrowing), excessive mucus secretion and obstruction to airflow resulting in reversible breathlessness, wheezing and coughing.   

Two forms of recurrent airway obstructive diseases of horses have been recognized for decades. The first to be recognized over a century ago is the hay-associated RAO, also referred to as chronic obstructive pulmonary disease (COPD), chronic obstructive lung disease (COLD), chronic airway disease, chronic bronchiolitis, alveolar emphysema, broken wind and heaves. In this form of severe equine asthma, the airway obstruction is manifested after exposure to indoor environment containing excessive dust particles originated from dusty, moldy hay and bedding. Dr. Ralph Beadle, a Professor at Louisiana State University, described the second form of equine RAO in 1983. In this form of severe equine asthma, the signs of bronchospasm, hypersecretion and airflow obstruction are manifested after exposure to pasture environment during the warmer months of the year and is therefore called pasture-asthma, or summer pasture-associated obstructive pulmonary disease (SPAOPD) or summer pasture-associated RAO. It is also referred to as summer heaves, pasture-associated heaves, pasture-associated pulmonary disease, and summer pasture-associated heaves.

Summer pasture-associated severe equine asthma is manifested as difficulty breathing due to bronchospasm, hypersecretion and airflow obstruction after or during exposure to pasture during late spring to mid fall. Affected horses may have signs ranging from exercise intolerance and coughing to labored breathing and increased expiratory effort, as well as wheezing, and eventually weight loss. The difficulty breathing (obstruction to airflow) is a result of the thickening of the airway wall, contraction of the bronchial muscles and excessive mucus secretion plugging the airways.

Similar to asthma, recurrent airway obstruction is in fact a complex disease that involves a number of factors. These factors include genetic predisposition, ineffective lung clearance, exposure to environmental conditions and exposure to certain respirable particles (i.e., aeroallergens, such as mold spores and pollens). Nonetheless, the exposure to an excessive amount of “respirable dust particles” is an important triggering factor associated with the exacerbation of equine asthma. 

Horses affected with moldy hay-associated severe equine asthma show onset of clinical signs of respiratory effort and airway obstruction after exposure to “dust particles” in the indoor environment. Therefore, important environmental factors include poorly ventilated barns, dusty and moldy hay and dusty bedding (e.g., straw bedding). Certain mold spores commonly present in moldy hay (thermotolerant actinomyces, especially Aspergillus fumigatus, Micropolyspora faeni and Thermoactinomyces vulgaris) have been incriminated as important triggering agents of the hay-associated heaves. 

Summer pasture-associated severe equine asthma has been reported most commonly in southeastern region of the United States, especially Florida, Georgia, Louisiana and Mississippi, although, it has been described in other parts of the world, including England and Scotland. In the southeastern region of the US, affected horses are generally older than 6 years of age, and often kept at pasture conditions for more than 12 hours a day throughout the year. Predisposition of certain breeds has been suggested, but not proven. The breeds Quarter Horse, Appaloosa and Paint were over-represented in a survey done by Drs. Seahorn and Beadle in the state of Louisiana. Horses of other breeds were also reported as affected with the disease; those breeds included Thoroughbreds, Arabians, Tennessee Walking horses, pony breeds and others. The over-representation of Quarter Horse-type breeds amongst the horses affected with pasture-associated severe asthma may not be a breed predilection. Instead, there may be more horses of Quarter Horse-type breeds exposed to the environmental conditions and triggering agents associated with the disease. 

Some horses may manifest signs of airway obstruction in both environments (indoor associated with hay as well as in pasture conditions). Although this is not very common, the management of horses with hay-associated and pasture-associated severe asthma is more difficult. It requires a careful control of the environment to minimize exposure to respirable dust particles at all times. 

Summer pasture-associated asthma is a seasonal airway disease that can therefore be managed well by implementing environmental changes prior to the season of clinical exacerbation. The horse may then remain in clinical remission of the disease. The clinical exacerbation of summer pasture-associated equine asthma and manifestation of clinical signs generally occurs after exposure to pasture conditions during late spring and summer, although some horses only develop clinical exacerbation at the end of summer or early fall. The exact time varies from animal to animal. In a study evaluating a group of affected horses housed in south Louisiana, the probability of clinical exacerbation was estimated to be 50 percent in mid to late June.  

The environmental conditions associated with the clinical exacerbation of the disease and onset of clinical signs includes increases in temperature (measure of heat) and increases in dewpoint temperature (measure of humidity and heat). In a study in south Louisiana, the cut off daily maximum temperature greater than 86°F and the daily minimum dewpoint temperature greater than 63°F were estimated as associated with 50 percent probability of clinical exacerbation of summer pasture-associated equine asthma. Evaluation of the temporal distribution of aeroallergen counts and the onset of clinical exacerbation of summer pasture-associated equine asthma revealed certain mold spores (e.g., Curvularia, Basidiospore, Nigrospora, Alternaria, Cladosporium, Cercospora) and grass pollens to be the likely triggering agents.

Horses affected with summer pasture-associated asthma should be allowed in pasture only during the times of disease remission, which is generally in the winter. The pasture should be kept short to decrease exposure to pasture aeroallergens. Pastures with certain grasses cause more problem than others. For example, ryegrass is generally less likely to be associated with clinical exacerbation of the disease, in part because it grows best in colder climate. The owners of affected horses are encouraged to investigate the best grass for the affected horse. It is important to identify the conditions preceding the onset and at the time the horse began showing signs of clinical exacerbation in previous years. This way, the affected horse can be removed from pasture prior to onset of clinical exacerbation. During the “summer season”, the horse must be kept off pasture and ideally in a low dust environment.

Although the airway disease may be clearly associated with exposure to specific triggering agents present in pasture environment, horses affected with summer pasture-associated severe asthma appear to have a poor pulmonary clearance and therefore do not handle well an environment with excessive dust particles. For this reason, minimizing the dust while the horse is kept confined in a stall is very beneficial for these horses. The barn and stall should be well ventilated. The horse should be taken out of the stall while cleaning it. The bedding is an important source of dust particle when the horse is confined to a stall. The amount of dust particles is highest with straw and sawdust bedding. Whereas, bedding with wood shavings, particularly good quality shavings, provides much less dust particles than most other types of bedding. Other options include no bedding and rubber mats, shredded cardboard and shredded newspaper. It is very important to evaluate the response of each horse to the different types of bedding. What is good for one horse may not be good for another horse. Keeping the stall clean to decrease toxic fumes such as urea and ammonia is beneficial. 

Additional management measures to keep the affected horse away from excessive dust include keeping the horse away while unloading shavings and hay, not accumulating old bedding piles close to where the horse is kept, and not storing hay in a barn loft.

It is important to eliminate the exposure to specific triggering agents while the horse is eating, as well as to decrease the exposure to dust particles that can impair lung clearance. There are a number of options that will allow you to provide your horse the adequate nutrition with the correct amount of digestible fiber, while minimizing exposure to aeroallergens.   

One option is feeding a complete pelleted diet (which has at least 25 percent crude fiber), to avoid feeding dry hay. If hay is given, it must be well-cured and good quality hay. Soaking or “washing” the hay is preferable to simply wetting the hay. In fact, wetting of the hay has been shown to make no difference in decreasing the exposure to dust particle triggering airway disease.  Good quality alfalfa hay is often associated with less respiratory problems than grass hay. However, each case must be evaluated individually. If hay appears to be detrimental to the affected horse, there are alternative forages, which include soaked beet pulp flakes and soaked hay cubes. 

It is very important to remember that dietary changes for horses must be made gradually. The horse's gut has to adapt to the new diet over the course of several days in order to avoid a bout of colic.

It is very important to be observant in order to detect early signs of the disease. Once the horse starts showing signs of asthma, the medical treatment is aimed to improve the airflow into the lungs and decrease the inflammation associated with exposure to the triggering agents. Therefore, the horse must be removed from the environment that contains the triggering agents. 

In order to improve the airflow to the lungs, a number of medications that promote relaxation of the airway muscles (bronchodilators) and elimination of the excessive mucus with expectoration may be necessary. In addition, anti-inflammatory drugs are often given to decrease the allergic response and allow the airway to be cleared normally. 

“To help horses affected with summer pasture-associated severe asthma, it is important to eliminate or decrease the exposure to the triggering agents” 

• Summer pasture-associated recurrent airway obstruction or severe equine asthma is a seasonal respiratory disease of adult horses; 

• Clinical exacerbation is often associated with high environmental temperature and humidity; 

• Therapeutic intervention aimed at altering the inflammatory/allergic response is likely to fail if exposure to the triggering agents is not eliminated; 

• Successful management of the disease requires exploring all possible options and designing an effective plan for modifying the environment for the affected horse.

 

Reviewed by original authors in 2016.