The respiratory tract's (RT) main function is to furnish oxygen to red blood cells, which in turn deliver this oxygen to every cell and organ in the body. Of course oxygen is inhaled as air. Along with oxygen and other gasses, air also contains lots of viruses, bacteria, fungi, dust, pollen, and other particulate matter. Because of all this debris the RT also functions to eliminate this material so the system is not clogged up or hampered in any way. Because the RT is a direct opening from the outside world to the inside of the body it is also one of the easiest routes for harmful organisms to enter the body. That is why respiratory disease (colds, flu, allergies, heaves, asthma, etc.) is one of the most common maladies affecting both horse and human. Fortunately the RT also has some defensive mechanisms.

To understand the function of these mechanisms we need to understand their structure. The nostrils and mouth both lead to the back of the throat (pharynx) and to the trachea or windpipe. If one gently squeezes the front of a horse's neck the trachea with its hard cartilage rings can be felt. These rings form a rigid framework that keeps the trachea open at all times. Once the trachea enters the chest cavity it connects to the lungs where it branches into two major bronchi, one supplying both left and right lung. The major bronchi branch and re-branch hundreds of thousands of times to form continually smaller bronchioles. At the end of this respiratory tree the bronchioles terminate in the alveolus. The alveolus is a microscopically small sac, a single cell thick, where the oxygen molecule crosses from the lung tissue to the complex blood capillary bed surrounding it. All this dividing and subdividing results in a much greater surface area to transfer oxygen but also a much larger area to clear up once infection is established.

The defensive mechanisms of the RT include the mucociliary apparatus and the immune system. Mucus producing glands and cells with microscopic hair-like structures known as cilia are found throughout the system. These together are known as the mucociliary apparatus and function as a physical defensive mechanism for the RT. The sticky mucus captures most of the contaminants inhaled and the cilia beat in an outward direction expelling the mucus and its contaminants. The end result is that much of the harmful material inhaled is trapped and removed. Another respiratory defense is the immune system. The immune system supplies a local immune reaction and white blood cells. The local immune reaction helps the body recognize invading organisms and white blood cells eliminate them. Vaccines help the immune system to recognize the viruses and to mount an attack against them (for more information contact our office or our website for the handout "Vaccination and Immunization, How Vaccines Help Horses Fight Disease).

The ability of the horse to fight respiratory disease is based on two elementary concepts. First is the amount of microorganisms breathed and second are the defense mechanisms against them. Respiratory disease usually starts with a virus infection that injures the cells of the RT especially the ciliated cells. Because the cilia are damaged, bacteria can more readily infect, and any additional factor capable of damaging the mucociliary apparatus does so more easily because the damaged cells are weaker.

Factors affecting organism number The amount of microorganisms a horse breathes is directly related to how many there are in the air. This in turn is dependent on how many other horses there are in the immediate area and the effectiveness of the ventilation. Increased numbers of horses simply allows more opportunity for microorganisms to be shed in the air. Also when horses travel to events they are inevitably exposed to new horses. This results in a dramatic increase in the number of viruses in the barn when they return. Normally the horse can effectively combat the microorganisms unless overwhelmed by large numbers.

Ventilation or the lack of it has multiple effects. First, poor ventilation will allow microorganisms to accumulate, suspended in the air, hence increasing the number of organisms ready to be inhaled. A second very important effect of poor ventilation is to allow the buildup of ammonia. Dirty stalls with the accumulation of manure and urine are ripe with ammonia. Ammonia lingers in the first few feet above the ground and feeding the horse on the floor worsens the problem because it places the head directly into the ammonia. Ammonia damages the ciliated cells and slows the mucociliary apparatus.

Factors affecting the defensive mechanisms The horse's ability to resist and eliminate microorganisms is related to its general health, immunity, and the mucociliary apparatus. General health influences immunity, as does regular vaccination. Unfortunately current equine respiratory vaccines e.g. against rhinopneumonitis (Rhino) and Influenza (flu) are not very effective. This leads to the recommendation of vaccinating every 2-3 months in horses at high risk. Still, regular vaccination of high-risk horses will decrease the severity and duration of respiratory disease.

As mentioned before, the mucociliary apparatus is responsible for trapping and removing microorganisms and debris from the RT. Anything that damages this function enhances the chances for disease to become established. As mentioned above viruses damage the ciliated cells. Other very common influences also hinder the beating cilia and mucociliary apparatus. These factors include ammonia as mentioned before, cold air, exercise, and forcing an upright head posture by cross tying and trailering. When trailered horses have their heads maintained in an upright posture. This posture physically slows down the clearing mechanisms of the cilia and is a major contributor to shipping fever (respiratory disease common with long term hauling). So when a horse travels to a large exhibition not only is it exposed to high numbers of microorganisms but also its ability to eliminate them is hindered by holding its head up during a long trailer ride. Compounding the problem is the accumulation of urine and manure and poor ventilation. The conditions are perfect for the development of respiratory disease.

Treatment and control
The failure of the RT to avoid disease is caused by a shift in the normal balance of microorganisms and the body's ability to eliminate them. In the horse most respiratory diseases are caused by viruses and bacteria. When the mucociliary apparatus and immune system fail it inevitably results in the microorganisms going deeper and deeper into the RT instead of being expelled. If the disease settles in the upper airway (trachea, bronchi) it is known as tracheitis and bronchitis. If the disease affects the lower airways (bronchioles, alveoli) it is called bronchiolitis or pneumonia. Although any disease of the RT can be difficult to overcome, pneumonia can be the most persistent.

Treatment revolves not only around administering the appropriate antibiotic in the proper dose and interval but just as importantly rest and controlling the air quality. Horses not allowed sufficient rest following a viral infection suffer more severe symptoms (wheezing, coughing, fever, lack of appetite, and depression) and take longer to recuperate.

Once a horse contracts a respiratory disease it is very important to improve the quality of air they breathe. If pastured, air quality is usually not a problem. In stable situations it involves minimizing dust and improving ventilation. Dust can be decreased by feeding grass silage or complete pellet rations. High quality hay usually has less dust than cheaper varieties. Also feeding wet hay in a hay net and spraying water over the shed rows can help control dust. Controlling air quality can mean the difference between a short illness and a very long one. The importance of clean stalls was illustrated above.

One of the most common complications of equine respiratory disease is stopping treatment too early or irregular dosing. If the infection has progressed to pneumonia antibiotic treatment often continues for weeks. Although the disease can last a long time, continual improvement should be noticed during treatment. If improvement is not seen within 3-5 days the diagnosis and therapy should be reevaluated. When administering antibiotics it is critical that they be given as often as prescribed. If the horse is not getting all of its medication consult your veterinarian.

Controling Respiratory Problems:

1. Keep dust to a minimum! Soak and drain hay to eliminate dust, feed in hay net. Do all you can to decrease environmental dust. Sometimes turnout is best, other times confining to a stall (each barn is different).
2. Keep stalls clean & dry; feces and urine soaked bedding release ammonia. Ammonia is very damaging to respiratory systems, especially when compromised by an infection or emphysema.
3. Look for signs of improvement or worsening.
4. Improvement signs: less coughing, less nasal discharge, lower temperature (if fever was present), more energetic, greater appetite etc.                  Worsening signs: No change in coughing pattern, temperatures above 101.5, decreased appetite, increased nasal discharge, discharge changes from clear to colored and thick. If you suspect worsening CALL.
5. Administer antibiotics at the following dosage:
6. Penicillin _______ cc once / twice daily.                                                    Trimethoprim (SMZ) _________ tablets twice daily.                                        Naxcel ________ cc once / twice daily.                                        ______________ cc / tablets / once / twice daily                                         Continue antibiotics for ______ days.
7. Keep animal quiet, stall rest, or turnout (if horse does not buck, kick, or exercise), handwalking. Eliminate any strenuous activity!
8. This animal should be re-examined in ______ days.

IT IS IMPORTANT THAT THE HORSE RECEIVE ALL THE ANTIBIOTICS RECOMMENDED. IF YOU ARE HAVING DIFFICULTY ADMINISTERING ANTIBIOTICS, PLEASE CALL!