By Brian S. Burks, DVM, Diplomate ABVP, Board Certified Equine Specialist
Horses are comfortable in temperatures much lower than humans prefer. In the absence of wind and moisture, horses tolerate temperatures at or slightly below 0° F. If horses have access to a shelter, they can tolerate temperatures as low as -40° F. But horses are most comfortable at temperatures between 18° and 59° F, depending on their hair coat.
Horses evolved as plains animals, well-equipped to deal with wind, cold, and snow.
Nonetheless, horse owners like to protect their charges from the elements, often building barns to keep them sheltered and warm. In times past, horses were only in barns at night, after being outside working during the days. Now, horses may be in a barn for 23 hours, with only a single hour of exercise.
Barns are built for warmth and protection more than for air flow and ventilation. When evaluating air quality, airborne particles in numbers greater than 2.4 mg/cubic meter (M3) of air have been shown to increase the incidence of airway disease. Most barns measure 40–60 mg/M3. The breathing zone of the horse during feeding time is often 30–40 times higher. Particles include dust, endotoxin, mold spores, ammonia, and silica from arena dust. Hay has been measured to contain 19.3 mg/M3 and bedding, especially straw bedding, can be in that range or higher, making hay and bedding major contributors. This may result in destruction of the epithelial lining of the airways and contribute to the development of equine asthma.
Horses have a respiratory system that functions extremely well during exercise. A horse’s respiratory rate can increase from 12 breaths per minute to more than 150 bpm. Tidal volume (TV), the volume of air that is inhaled and exhaled with a normal breath, ranges from 4–7 liters per breath at rest but increases to 12 liters during strenuous exercise. Minute volume (MV) is the total volume of air inhaled and exhaled per minute (MV = TV X RR). Horses at rest have MV averaging 100 liters per minute, but during very hard work, MV averages an astounding 1,500 liters per minute. Even at rest, this is a tremendous amount of air flow into and out of the lungs. When the air being inhaled contains high numbers of respirable organic particles, the potential for irritation is high. Add exercise on top of that, such as training in an indoor arena during the winter, and the increased respiration rate causes deeper penetration of particulate matter.
The frigid air of winter can cause additional problems. Exercise during cold weather has been shown to cause airway diseases like asthma; repeated exercise causes chronic airway inflammation. Some studies suggest that 25-80 percent of stabled horses suffer from equine asthma. Clinical signs include coughing and snorting, decreased performance, labored breathing, nasal discharge, and abnormal lung sounds. Due to the size of the equine lung, clinical signs are not apparent until many airways are involved; many more horses are affected than can be detected. Foreign material causes airway inflammation, thickening the bronchial walls and reducing blood oxygen. Symptoms can be treated, but the underlying poor air quality must be changed, or the lung capacity may be dramatically reduced, taking weeks to months to recover.
A horse’s respiratory system is designed to warm and humidify air as it enters the nose before it reaches the lungs. A horse asked to exert energy and work must deepen his breaths, which does not allow the air enough time to warm up before it reaches the lungs. Horses can warm the air in temperatures as low as 20 degrees F, but below this temperature, damage to the epithelium occurs. Airways that have undergone repeated challenge with unconditioned air have incomplete repair and squamous metaplasia of these airways.
When the exercise intensity is increased, the volume of air passing through the upper airway becomes too great to adequately condition. When this unconditioned air reaches the lungs, there is a loss of heat and water from the surface of the lower airways. The result is a cold and dehydrated lower airway prone to cell injury. Loss of water from the airway is a poorly recognized contributor to dehydration in the horse and dehydration is a major cause of fatigue and poor performance.
Improve air quality in the barn by using the following recommendations:
Cold weather and poor air quality can affect the ability of your horse to perform to potential. While the former cannot be controlled, the latter can be greatly affected by leaving horses outside, especially in dry weather. Keeping airflow and clean air in the barn during wet weather when horses must be inside will help to prevent long term lung problems.