Equine Respiratory System: How Horses Breathe

By Dr. Steve Fisch

A performance horse or racehorse cannot perform to full potential without air – the respiratory system that moves oxygen through the body. 

The equine respiratory system works in conjunction with the horse’s cardiovascular system and other systems. When a horse is properly conditioned, the respiratory system has increased oxygen uptake and decreased ventilation during exercise.  

Equine Respiratory System

The main goal of the respiratory system is to transfer oxygen from the air that is breathed to the red blood cells. From there, oxygen is transported throughout the body. 

Carbon dioxide (or CO2) is a waste product of metabolism and is eliminated from the body via the lungs. If this respiratory system is compromised in any way, it will severely affect the athletic ability of a horse and possibly threaten its life.

How a Horse’s Respiratory System Works

The lower airway consists of the lungs and bronchi. 

The lungs have protective mechanisms that help prevent infection. The air horses breathe is not sterile and contains many contaminants such as dirt, dust, pollen and chemicals, as well as bacteria, viruses and fungi. The protection actually starts in the upper airway with filtering, humidifying and warming of the inspired air. 

The upper respiratory system, trachea and bronchi are lined with tissue that always is covered with a sticky mucus that traps contaminants. The tissue lining the trachea and bronchi has cells with extremely small hair-like fibers sticking out into the airway called cilia.

Cilia increase the surface area of the filtering surface and play an active role in airway protection. The debris that is collected on the surface of the airways actually is transported away from the lungs by these cilia. The small cilia fibers move in a coordinated rhythm to move the thin layer of mucus that floats on top of the cilia and the debris that is stuck to it out of the lungs and up the trachea where the horse will swallow it.     

To see a full diagram, of a horse's anatomy, download this free anatomy chart from AQHA. 

Respiratory Disease in Horses

Anything that decreases the effectiveness of the equine respiratory process increases the chance for the development of respiratory disease, such as:

• Cold air.

• Smoke. 

• Chemicals in the air, like ammonia. Ammonia fumes in poorly cleaned stalls or because of bad barn ventilation are a common cause of decreased ciliary action.

• Inflammatory conditions of the airways, such as chronic obstructive pulmonary disease (COPD) and flu. 

Respiratory Concerns When Hauling Horses

Another very important way by which the cilia clearing – or mucociliary clearance mechanism – can be overwhelmed and rendered less effective is from not allowing the horse to lower its head during transport. 

In fact, the transportation process can stress the defense mechanism of the lungs in several ways. 

1. Exhaust fumes from a poorly maintained or designed truck can have a negative effect on the cilia’s effectiveness. 

2. A trailer that has poor drainage can allow for the buildup of ammonia fumes. If the ventilation within the trailer is not good, it can make all of these negative factors regarding lung health worse.

How to Make Trailers Safe for a Horse’s Respiratory System

Take these respiratory precautions before transporting a horse:

• Allow the horse to lower its head when in the trailer. There have been several scientific papers evaluating the effects of transportation and head posture on lung health. It has been demonstrated that horses confined with their heads elevated for 24 hours developed an accumulation of inflammatory airway secretions that was associated with increased numbers of bacteria in the lower respiratory tract. These findings indicate that hauling horses in a manner that prevents them from lowering their heads during transportation and cross-tying may contribute to a lower respiratory tract disease.

• Ensure good ventilation, such as open windows or vents.

• Make sure urine can drain, so ammonia fumes do not build up.

Natural Mechanisms That Protect Equine Respiratory Health

There are other protective mechanisms in place to protect the equine respiratory system against infection: 

• Millions of special cells deep within the tissues of the lungs that can kill bacteria and inactivate viruses. 

• These cells are neutrophils and macrophages that basically eat the bacteria and viruses.

Equine Respiratory Health Exams

Besides blood work, ultrasound, radiography, endoscopy and respiratory fluid analysis, there are ways to evaluate the respiratory system, which include evaluating respiratory rate and character of breathing. A physical examination is required to adequately evaluate the respiratory system. 

During the respiratory exam of a horse, the veterinarian looks for clues such as: 

• Nasal discharge (from one or both nostrils).

• The color, clarity and thickness of that discharge (clear and thin? Thick and green or yellow? Bloody and smelly?) . 

• Amount of air flow from both nostrils.

• Swellings around the head.

• Coughing (and if so, what type).

• Fever. 

• Breathing at rest, at work, on inspiration, or expiration.

• Extra abdominal movement while breathing.

• If any other horses on the farm are showing similar signs. 

If you suspect respiratory issues in your horse, contact your veterinarian.

About the Author: Dr. Steve Fisch

Dr. Steve Fisch owns AVS Equine Hospital, a full-service hospital and reproductive center in Tallahassee, Florida, which among its offerings includes surgery, performance-related lameness treatment and referral care.