Chapter 46 Diseases of the Respiratory System
Pneumonia is a common presenting sign in the sick reptile patient. Although pneumonia is primarily a disease of the respiratory system, its cause may be multifactorial. A number of infectious agents can cause pneumonia, and if linked to poor husbandry, an inadequate diet, and poor sanitation, a serious and often life-threatening disease may result. The respiratory system of the reptile is unlike that of the mammal. The glottis of snakes is situated rostrally in the oral cavity, allowing for respiration while eating large meals. Most snakes have only one functional lung, the left one being absent. Reptiles, unlike mammals, can function using anaerobic metabolism, which allows them to compensate for increasing respiratory disease until it is well advanced. Respiration is usually controlled, not by blood carbon dioxide levels and pH, but by the temperature and the partial pressure of oxygen (PO2) in the tissues. As the temperature increases so does the oxygen demand, which the patient meets by increasing tidal volume, not respiratory rate. Environments rich in oxygen (oxygen cages) can, therefore, suppress ventilation in the sick patient. Together, the poor ability of the reptile lung to move inflammatory exudates out of the lung, the suppressed tidal volumes resulting from pulmonary infiltrates and exudates, and the ability of the reptile to shift to anaerobic metabolism can make early diagnosis of disease quite difficult.
The respiratory system of the lizard is different from that of mammals. The position of the glottis is variable in lizards, being more rostral in carnivorous lizards and at the base of the tongue in others. The tracheal rings are incomplete, and the trachea bifurcates near the base of the heart. Lizard lungs are equal in size, and gas exchange occurs in the cranial portions. The caudal portions function similar to the air sacs of birds, being poorly vascularized and not used for respiratory functions. Both inspiration and expiration are active processes in the lizard, and no diaphragm exists to facilitate respiration. As in snakes, lizards have the ability to function with anaerobic metabolism and may be able to conceal disease until it is advanced.