CHAPTER 30: UPPER RESPIRATORY TRACT DISEASE
Introduction
The primary function of the respiratory system is to deliver to the body and to expel carbon dioxide and other wastes from the body. The respiratory tract also plays a role in the acid-base balance of the body, the breakdown of certain metabolic wastes, the activation of metabolic substances such as angiotensen, olfactory sensation, thermoregulation and immune defence. In the horse the protective immune mechanisms of the respiratory tract are of special interest as respiratory tract disease is the most common reason for poor performance in horses competing in athletic endeavours. The respiratory tract protects itself by warming and humidifying inspired air in the conchae of the paranasal sinuses, and by filtering inspired air to trap foreign particles before they reach the deeper lung tissues. Large particles are trapped in the hairs of the nares or in the mucus that lines the upper airways. The then uses the cilia on the endothelial cells of the upper respiratory tract to create currents that push the particle-containing mucus towards the nasal cavity where it can be expelled by coughing or sneezing. Smaller particles that manage to enter the lungs are phagocytized by macrophages in the alveoli, removing them from the respiratory tract. Inflammation or irritation of the mucosa of the airways impairs the function of the mucocilliary elevator allowing particles that are too large to be phagocytized to enter the lungs. The contamination of the alveoli inhibits the lungs’ ability to perform its various functions. Respiratory diseases can be viral, bacterial, immune mediated (allergies), or mechanical in nature. They can be acute or chronic, serious or inconsequential. Some are highly contagious while others are limited to the affected individual horse. While some diseases can affect both the entire respiratory system, most are categorized as either upper airway disorders or lower airway disorders. Disorders of the upper airways are generally mechanical in nature while those of the lower airway are often the result of infection.
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