The majority of inspired air is drawn through the nares, rather than the mouth. The nares can open and close readily, thus controlling to a degree what the horse inhales. In addition, the hairs on the inside of the nares help filter out debris.
Moist mucous membranes that collect foreign material line the various twists and turns of the upper airways. The design of these nasal passages maximizes airflow contact with the mucous membranes. Not only do these membranes collect debris, but they also help to regulate the inhaled air's temperature and humidity. Once debris is captured on the mucous membranes, nasal secretions (runny nose) or a sneeze will remove it.
As an important secondary protection, these membranes can call up the cellular defense system (white blood cells) to attack any invading alien. These white blood cells and other inflammatory mechanisms destroy bacteria and viruses and actually engulf small particles. While the white blood cells wage war, the membranes become swollen and irritated. When the clear nasal secretions change to a milky white, the body has enlisted the help of the white blood cells.
Sometimes an allergic reaction dust and mold can occur, resulting a chronic rhinitis (inflamed nose). The end result is a swollen, inflamed nasal passage. This is bothersome to the horse but more importantly, the combination of swelling, inflammation and thick secretions increases air flow resistance, which can lead to poor performance and other problems.
There is a limit to the filtering capabilities of these nasal passages, and any missed dust or debris will pass into the large airways leading to the lungs (tracheal and bronchial tubes). This debris and secretions will be sneezed out or swallowed. The tracheal and bronchial membranes can also call on cellular defense mechanisms to help fight airborne bacteria and viruses.
These cellular reactions are important and helpful, but like the mucous membranes in the nose, the airways will fill with fluid and mucous. This further complicates the athletic horse's ability to breath properly.
Any debris that escapes the primary airways will end up deep in the lungs, causing further problems. The lungs not only have all of the above cellular defenses, but will constrict with inflammation. This reaction can be secondary to general inflammation or a true allergic reaction. Any allergy to dust and mold can lead to Chronic Obstructive Pulmonary Disease (COPA) or "heaves". This syndrome is closely related to human asthma.
Inflammation over a long time can lead to the deposit of fibrous tissue (scar formation) in the lung. Once scar tissue is formed, the elasticity and function of the lung are compromised. This process is similar to human emphysema. Any airway obstructions will make adequate oxygen exchange difficult for the equine athlete. In fact, airway obstructions have been proposed as a component of the common disease of athletic horses, EIPH (Exercise Induced Pulmonary Hemorrhage), or "bleeders".
We do know that these horses bleed with exercise. With airway obstructions (mucus, debris, swelling, bronchial construction) the incoming air has more resistance. As a result, the horse must exert a deeper inspiration to create enough negative pressure for air to get by the obstruction and into the air sacs deep in the lungs where oxygen transfer can take place. This forced inspiration and greater negative pressure, in theory, are involved in capillary rupture and bleeding.
The key to all of these airway diseases is early detection, treatment and, best of all, prevention. The best prevention is to improve air quality by limiting exposure to dust and mold.
All "sun-cured" hay has mold and dust, no matter how good the quality appears. It is not possible to wilt and sun-dry hay without having some mold and dust. In addition, our feeding practices worsen this problem.
Horses have long necks so they can eat off the ground; in our infinite wisdom, we place hay well off the ground in nets and racks, allowing the dust and mold to be more easily inhaled. With the horse eating off the ground, the natural nasal secretion will more easily run forward with the pull of gravity.
Anyone serious about minimizing dust and mold should consider feeding a high quality hay product from the Northwest, where mold and dust is much less prevalent, is preferred. Triple Crown Premium Chopped Forages fit the bill. Kiln-dried hay cubes will also greatly decrease the amount of foreign material inhaled into the respiratory tract.