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An Introduction to the Avian Respiratory System:
Dr. D. Bruce Hunter
Respiratory diseases occur commonly in all avian species kept in collections or in private homes. There are many organisms and environmental factors capable of causing pulmonary disease in birds (see Table 1). The purpose of this paper is to acquaint the aviculturalist with the normal anatomy and physiology of the avian respiratory tract and, the diagnostic approaches used by veterinarians to identify the causes of respiratory tract disorders. After the cause of the respiratory problem has been identified, it is then possible for the veterinarian to provide a prognosis for recovery and initiate the appropriate therapy.
Anatomy and Physiology
The respiratory tract of the bird demonstrates unique adaptations that have evolved to accommodate high metabolic rates and highly energetic activities such as flight. It has long been recognized that the avian respiratory tract is more efficient than that of mammals: for example, canaries were used for rapid detection of toxic gases in underground mines; geese and other birds are capable of sustained high performance flight over long distances and at extremely high altitudes.
The increased pulmonary efficiency in birds is related to differences in anatomical structure. Morphometric studies have shown that most birds examined have a smaller total lung volume on a body weight basis than mammals but, have a 15% greater surface area for gas exchange. In addition the bird has an increased pulmonary capillary blood volume and a much thinner blood/gas exchange barrier than mammals, which result in a greater pulmonary diffusing capacity for oxygen. The avian erythrocyte is also more efficient in oxygen uptake. If all these factors are included in calculations, the bird may have up to an 82% increase in efficiency of oxygen uptake compared to a mammal. The budgerigar (Melopsittacus undulatus) has the capability to increase its oxygen consumption 13-20 times above its basal metabolic requirements during periods of flight.
As in mammals, the respiratory tract of the bird fulfils several important functions including: exchange of gases (ie. intake of oxygen and elimination of carbon dioxide); regulation of acid/base balance; filtration of dust, molds and other particulate material; and temperature regulation of incoming air. The respiratory tract of the bird also plays a significant role in the control of body temperature and in the regulation of body water balance.
The lung of the bird is of a fixed volume and does not expand and contract appreciably during the respiratory cycle. There is no muscular diaphragm as present in mammals but instead, horizontal and oblique connective tissue septae serve to compartmentalize the coelomic cavity.
Both cranial and caudal to the lungs are a series of thin-walled, transparent air sacs which grossly resemble clear, thin plastic. In most birds there are 9 air sacs, four paired and one unpaired, which serve as air reservoirs. In most birds the humerus and femur and several other bones are pneumatic and communicate with local air sacs. Certain psittacines have an extensive cervicocephalic air sac communicating with the sinuses and extending down the caudal aspect of the neck. The respiratory rate of the bird is slower and deeper than that of a mammal. Respiratory muscles actively move the keel during both inspiration and expiration to effect pressure changes in the air sacs, very similar to the action of a bellows. This results in continuous air flow through the rigid lung tissue.
This unidirectional air flow, coupled with a counter current or cross current blood flow, maximizes the oxygen uptake gradient in the avian lung. The air flow pattern also influences the distribution of respiratory disease, as inhaled particles or pathogens are deposited first in caudal air sacs and the caudal dorsal portions of the lungs. This caudal-dorsal location of aspiration pneumonia in the bird is quite different from the anterio-ventral distribution usually observed in cattle and other mammals.
The upper respiratory tract is extremely important as a first line of defence against respiratory disease. Air entering the respiratory tract must pass through the external nares and through the turbinate bones where centrifugal air flow and an active mucociliary apparatus removes particulate material larger than 5 mu. The paranasal sinuses also act to trap particulate material. The large infraorbital sinus is located directly below the eyeballs with diverticula extending into the upper beak and mandible. The very active mechanical clearance of inspired foreign material is one of the most important pulmonary defence mechanisms in the bird. Localization of pathogens in the sinuses may cause rhinitis and sinusitis.
Inspired air leaves the nasal cavities via the choanal slit located in the roof of the mouth. The choana, as the point of drainage for the nasal cavity, is an excellent location to obtain samples for cytology and culture. The trachea is comprised of complete tracheal rings and has limited ability to expand. For this reason inflatable endotracheal tubes should not be used in birds. The trachea has an active mucociliary clearance mechanism, moving particulate material into the oral cavity where it is swallowed. The tracheal lumen narrows at the syrinx. This narrowing increases local air turbulence and is a common site of pathogen localization.
The Clinical Work Up
The clinical work up should begin by obtaining a complete history. The bird should be carefully observed before handling and restraint. Signs to look for include: ocular or nasal discharges; asymmetry of nares, sinuses or other parts of the head; swelling around or under the eyes or extending down the dorsal aspect of the neck; abnormal grooves worn in the beak; abnormal posture; respiratory rate and character; open mouth breathing; laboured abdominal movements (tail bobbing); increased respiratory sounds such as wheezing, sneezing, choking; voice changes etc.
A routine and complete physical examination should be performed. If the bird is in severe respiratory distress it should be placed in an oxygen rich environment immediately, and the physical exam may have to wait until the bird is more stable, or performed over a period of several hours to avoid critically stressing the bird. Occasional respiratory emergencies are presented where the trachea is obstructed by aspiration of foreign material, localized tissue swelling etc., necessitating placing a catheter or breathing tube in the abdominal air sac to provide ventilatory support. The choanal slit, oral pharynx and glottis should be examined and all parts of the respiratory system auscultated carefully with a stethoscope. The purpose of the examination is not only to identify the presence of respiratory tract disease, but to attempt to localize the disease to the upper or lower airway.
If respiratory disease is suspected, a routine CBC should be done. Occasionally, diseases localized to the respiratory tract do not induce marked changes in the hemogram. Radiology may provide valuable diagnostic information.
Two radiographic views of the body are necessary to assess the lower respiratory tract. Radiographic lesions may be subtle and a sound knowledge of the anatomy is essential. Cloudiness, increased radiographic density to the air sac, or the ability to clearly identify air sac membranes are indications of air sac disease. Increased abdominal fat or a full digestive tract will complicate interpretation.
In most disease conditions it will be necessary to identify the pathogen in order to decide upon an appropriate treatment. Based on the findings from the physical examination and the location of radiographic lesions, further samples for cytology or culture can be collected.
In diseases affecting the sinuses or nasal cavity, samples can be collected from direct aspirates from the large infraorbital sinus; by swabbing the choanal slit; or by performing a nasal flush and collecting the sample from the choana. Normally, cytologic preparations, bacterial cultures and smears for gram stains are taken.
Techniques such as tracheal lavage and air sac washes are used routinely to obtain samples from the lower respiratory tract. The placement of the catheter for air sac lavage should be determined by the radiographic distribution of lesions. Cytologic evaluation of the fluid and bacterial culture and antibiotic sensitivities should be done routinely in cases of lower respiratory tract disease. Often the pathogenic agent can also be identified in cytologic preparations.
The endoscope is also valuable in diagnosing respiratory disease. A small diameter rigid endoscope can be inserted down the tracheal lumen of parrot-sized and larger birds, often reaching the tracheal bifurcation. In larger waterfowl and ratites we routinely use a flexible pediatric bronchoscope to examine the trachea. Examination of the caudal air sacs can also be done using the endoscope but care must be taken to prevent mechanical spread of localized air sac infection to other parts of the coelomic cavity.
Techniques for biopsy of lung tissue are being developed. Lung biopsies can provide diagnostic samples but there is risk associated with the procedure related to localized pulmonary hemorrhage.
There are a large number of pathogens capable of causing respiratory tract disease in birds. Table 1 provides an partial list of some of these agents. Treatment will depend on pathogen identification and the results of culture and antibiotic sensitivity testing. Antibiotics and other medications are usually given by the normal routes, ie. IV, IM or oral and occasionally by direct instillation into affected sinuses, trachea or into an air sac.
The poor blood supply to air sacs makes it difficult to achieve high antibiotic levels at the site of the air sac infection. Nebulization is very valuable, as the continuous air flow pattern will carry nebulized particles deep into the air sacs and lung parenchyma. Nebulization is valuable not only as a vehicle for antibiotic therapy, but also to assist in rehydrating the bird, improving the action of the mucociliary escalator and possibly loosening secretions.
Respiratory diseases in birds are common and must be diagnosed and treated aggressively for optimum results. The unique anatomy and physiology allows the veterinarian to obtain diagnostic samples from most parts of the respiratory system.
Table 1. List of Respiratory Diseases and Pathogens of Birds
The list includes diseases commonly found in pet birds, poultry, game birds (such as pheasant and quail), waterfowl and birds of prey. Those diseases marked with an * (astrict) are common pathogens in pet birds.
*avian pox virus
infectious bronchitis virus (corona)
(laryngotracheitis virus, parrot tracheitis)
newcastle disease (PMV1)
*tracheal mites (Sternostoma spp; Cytodites sp.)
air sac nematodes (Serratospiculum sp.)
*scaly face and leg mites (Cnemidocoptes sp.)
Candidia sp. (systemic)
NUTRITIONAL and TOXIC:
*vitamin A deficiency
*iodine deficiency (goiter often presents with respira- tory signs)
teflon inhalation (polytetrafluro-ethylene)
quaternary ammonium toxicity
chlorinated biphenyl toxicity
carbon monoxide toxicity
high ammonia levels
vaccine reactions (hypersensitivity)
*reactions to feather dust and dander
neoplasia of skull, beak or soft tissue etc.
ascites secondary to liver disease or other conditions
obstruction from foreign material