Neil A. Forbes, BVetMed, DECZM (Avian), FRCVS, RCVS, EU Recognised Specialist (Avian Medicine)
The larynx connects the glottis (opening of trachea), to the base of the tongue. Thyroid and epiglottic cartilages are absent. The larynx functions to open the glottis during inspiration and to close it during swallowing.
The trachea demonstrates significant interspecies variation, including the presence (hornbills, toucans and some ducks) or absence of a crista ventralis, the level of bifurcation into the primary bronchi (penguins more proximal), the length and convolutions of the trachea (55 species [mainly cranes and swans] have complex loops), inside or outside the sternum, whilst some ducks have distended syringeal bulla for voice alteration. Emu and others have a ventral slot in the distal trachea to permit dilation of a tracheal pouch. The trachea consists four layers: mucous membranes, submucosa, cartilage and adventitia. The internal lining comprises simple and pseudo-stratified, ciliated columnar epithelium, with simple alveolar mucoid glands and goblet cells. The tracheal rings are complete (in respect of which, endotracheal tubes are never cuffed, for fear of causing pressure necrosis). The trachea divides into the primary bronchi at the level of the syrinx (within the thoracic cavity), at the level of the syrinx (organ of vocalisation).
Full and thorough history collection is vital, the species, nature of husbandry and accommodation together with lifestyle, have a great bearing on predisposition to respiratory disease.
Birds with tracheal disease present with respiratory stridor; inspiratory or expiratory noise; tail bobbing; coughing (although in parrots, this is typically due to mimicry); loss or change of voice or wheezing. Any pre-handling (noninvasive) clinical pathology tests that can be carried out (e.g., faecal parasitic ova analysis), should be. If tracheal parasites are excluded, preparation should be made for more invasive diagnostic tests. Preoxygenation (30 minutes) may be appropriate prior to handling. Typically, the clinician is uncertain of pathology until tracheoscopy is undertaken; preparations in respect of equipment and staffing should be prepared.
Volatile anaesthesia, endotracheal tubes (if the trachea is inflamed, a smaller diameter tube is used to minimise iatrogenic inflammation)
Endoscope (typically 2.7 mm, 0° or 30°), Suction and appropriate sized nasogastric for removal of tracheal exudate, foreign bodies
Fine microbiology swabs, microscope slides (for cytological samples)
Air sac breathing tubes and surgical kit for aseptic insertion or tracheotomy
Perform prior to entubation. Wet cold sterilisation of the scope should be avoided, as any disinfectant that enters the trachea (on the scope) may interfere with subsequent culture. The tip of the endoscope should be dipped in sterile water prior to glottal insertion (to avoid lens condensation).
The tracheal lining normally appears pale 'silver pink', glistening and dry. If the tip of the scope brushes the tracheal lining, immediate inflammation or haemorrhage should not occur. Tracheal swab or saline flush and aspiration should be conducted if any abnormality is suspected.
The endoscope must be advanced to fully visualise the syrinx and proximal primary bronchi, the author favours restraining the bird's head in the left hand, maintaining the trachea straight and parallel with the table top and the scope advanced gently with the right hand, the head is moved, to keep the tracheal lumen directly ahead of the advancing lens.
Bacterial: Bacterial pathogens commonly comprise Streptococcus spp., Staphylococcus spp., Klebsiella spp., and Pseudomonas spp., but other Enterobacteriacae may occur.
Fungal: Candida is rarely found, but Aspergillus spp. is the commonest of all pathogens.
Aspergillosis is characterised by a white (to cream), soft to hard 'globule' of exudate at any point in the trachea, (most commonly at the syrinx). Suction or surgical removal may be required. Medical therapy with voriconazole or similar must be maintained for 2 months, with regular rechecks for reoccurrence.
Viral: Herpes tracheitis of Amazons, infectious bronchitis and other species specific viral pathogens are occasionally found.
Parasitic: Tracheal mites (Sternostoma tracheacolum), are common in finches and occasionally in canary. Gasping, wheezing, coughing, neck stretching may be evident. Tracheal transillumination (wetting feathers with alcohol), will enable visualisation of intratracheal mites. Mites are controlled by treating the whole group with topical per cutaneous avermectin therapy on two occasions 7–10 days apart.
Syngamus trachea (gape worm) is most commonly found in aviary birds who consume earthworms (the latter are intermediate host for S. trachea). Adult S. trachea worms, are located in the trachea and live in permanent copulation (Y shaped as the male worm is much shorter). Infected birds may present with dyspnoea, stridor, loss or change of voice, respiratory distress, gasping, coughing. Any patient where Syngamus is possible, should undergo, faecal examination prior to other diagnostics. All birds in the same aviary should be treated (benzimidazole or avermectin) and future access to soil and earthworms be prevented.
Trichomonas gallinae is very rarely found in the trachea and should be considered as an alternative differential.
Toxic/irritant: Inhalation of smoke (e.g., household fires) is less commonly serious compared with mammals (principally as air passes through the avian pneumonic system, where as in mammals, alveoli are the end point); also, smoke particles may settle out in the air sac system prior to reaching respiratory exchange surface. Fine particulate matter inspired (e.g., plaster dust), may cause a long-term tracheitis and may be problematic to resolve.
Trauma: Internal trauma to the trachea may arise following aspiration of food or crop contents, or from pressure after cuffing or use of oversize endotracheal tubes. This is a particular risk in macaws, where the trachea is very wide at the glottis, but rapidly narrows. A correctly sized tube at the glottis will be excessive at the distal point. Tracheal stenosis typically arises 10 days later.
Air sac breathing tube: In situations where there is significant loss of functional respiratory diameter, or risk of this arising during or following treatment, then an air sac breathing tube should be inserted. This should be placed into the left (as larger) caudal thoracic air sac, gaining access either between the 7th and 8th rib (in front of the leg), or after pulling the leg forward, via the sublumbar fossa. A tube diameter 25% larger than the patient trachea, with apertures at the end and offset on the wall of the section of tube to be inserted within the air sac. Preplaced sutures, should be situated at the level which will interface with the thoracic wall. The length of tube within the chest wall should not exceed 30% of the width of the thorax at this point. At least one of the two sutures should be passed around a rib, as if in the skin alone, this may lift away from the thoracic wall and the tube withdraw from the air sac.
After air sac tube placement, tracheal lesions may be aspirated per glottis, otherwise tracheotomy or tracheotomy may be necessitated.
Tracheotomy: indicated for treatment of syringeal aspergilloma or retrieval of a foreign bodies. The bird is placed in dorsal recumbency, with the head directed towards you. The front of the bird is elevated at 45°, to facilitate inter-operative visualisation within the thoracic inlet. A skin incision is made adjacent to the thoracic inlet. The crop is identified, bluntly dissected and displaced to the right. The inter-clavicular air sac is entered, and the trachea elevated. The sternotracheals (attached bilaterally to the ventral aspect of the trachea), is transected. Stay sutures may be placed into the trachea in order to draw it in an anterior direction. In most species it is impossible to completely exteriorise the syrinx. A tracheotomy may now be performed, cutting 1/2 the tracheal circumference, (using a number 11 scalpel blade or hypodermic needle). The incision is repaired with single interrupted sutures (6/0 polydioxanone 2–3 sutures) placed to include two rings either side of the incision. If additional access is required, the superficial pectoral muscles may be elevated, and an osteotomy of the clavicle performed. On closure, the two ends of the clavicle are opposed but not joined. The muscle is replaced and sutured into position. The crop is sutured in place, creating an airtight repair over the interclavicular air sac.
Tracheotomy: In cases where a severe tracheal stenosis occurs following trauma (including recent entubation) or infection, tracheal resection and removal of the affected tissue can be performed. Depending on the site of the lesion, most species can cope with loosing up to 6 (on occasions 8) tracheal rings. In such cases close apposition of cartilages following surgery, using a suture material which elicits minimal tissue reaction (e.g., polydioxanone) is used in order to minimise the risk of intraluminal granuloma formation. Trauma to tracheal tissues during surgery is minimised. It is preferable to place sutures in the trachea at the time of resection, to facilitate apposition and anastomosis. Two to four sutures are used (depending on patient size) and are all preplaced before any are tied.