Avian Respiratory Diseases

Anatomy and Physiology The avian respiratory tract is substantially different and much more efficient compared to mammals. The upper respiratory tract in birds is characterized by an extensive infraorbital sinus, a choana, and a larynx that is lacking an epiglottis. In addition, the larynx is not used for voice production in birds; instead, this occurs in the syrinx. The trachea in birds is comparatively longer and wider compared to mammals, increasing ventilatory dead space. The tracheal rings are closed and the trachea bifurcates at the syrinx into the two primary bronchi. Air movement within the lower respiratory tract is complex, and involves movement between various air sacs and the lungs. During both inspiration and expiration fresh oxygenated air is moved through the parabronchi of the lungs, the site of gas exchange. This unique mechanism makes the avian respiratory system much more efficient, but also makes birds much more susceptible to airborne toxins. The air sacs are thin-walled poorly vascularized membranes, which outline the entire coelom and cover the coelomic organs. The lung volume does not change during inspiration or expiration, like in mammals, and a diaphragm is lacking. Therefore, air is moved in and out of the lungs by changes in air sac volume, secondary to movement of the keel and body wall. Since the air sacs are extensive and are critical in the ventilation of the lungs, any process which leads to compression of the air sacs, such as organ enlargement, intracoelomic fluid accumulation, obesity or an intracoelomic mass can lead to respiratory problems.