Anatomy and Physiology of the Avian Upper Gastrointestinal Tract

Beak and Oropharynx

The avian beak is composed of an upper mandible/bill (rostrum maxillare) and a lower mandible/bill (rostrum mandibulare). Specifically, the upper beak keratin layer is called the rhinotheca and the lower beak keratin layer is the gnathotheca. The combined keratin sheath of both upper and lower beaks is termed the rhamphotheca. The keratin of the beak grows continuously at a species-specific rate. In large psittacines, the maxillary keratin will be replaced every 6 months, and the mandibular keratin every 2–3 months. The total weight of the upper beak is reduced by inclusion of diverticulae from the infraorbital sinus. The maxillary rhinotheca joins the frontal bone through a kinetic joint (the craniofacial hinge joint), which has a variable degree of flexibility amongst avian species, but is highly mobile in psittacines. Movement of the upper beak is coordinated by four pairs of bones: the jugal arch, the pterygoid, palatine, and quadrate bones.⁸

The soft palate is absent in birds; therefore, the oral cavity and pharynx occupy a common cavity called the oropharynx. The choana is a normal fissure in the nasal cavity, and the infundibular cleft (common opening of the Eustachian tubes) is an additional slit-like opening found immediately caudal to the choana. Lips and teeth are absent in all birds, but birds in the genus Falco have a notch along the tomia (cutting edge) of the upper beak called the tomial tooth or tomial notch.⁷ Various adaptations of the tongue are present in birds, depending on the diet of the species. Psittacines are the only avian group in which the tongue contains its own intrinsic muscles.⁶

Esophagus and Crop

The avian esophagus has a greater diameter than mammals, and the cervical portion is located predominantly on the right side of the neck, opposite that of mammals. It has longitudinal folds that are greater in species such as raptors and cormorants, which ingest large prey. The esophagus becomes dilated just proximal to the thoracic inlet to form the crop or ingluvies. There are several groups of birds that do not contain a crop including gulls, penguins, and owls. The crop varies in size and shape: in psittacines, it is orientated transversely across the cervical spine; in pigeons, it is a large, bilobed structure with two lateral sacs. Several columbiformes species produce a substance called crop milk, consisting of desquamated cells of the crop lining, which is regurgitated to their offspring.⁶

Beak and Oropharynx Diseases

Disorders of the Beak

Subluxation of the upper beak occurs secondary to hyperextension of the nasofrontal hinge. This is clinically manifested as inability to close the beak and reluctance to eat. Palatine bone subluxation is also caused by hyperextension of the maxillary beak - the palatine bone luxates dorsally and requires surgical correction. Fractures of the beak and symphyseal fractures of the mandible have also been reported - mandibular symphyseal fractures often do not heal well in avian species. Three types of beak malformations are seen in young psittacines: lateral deviation of the maxilla (scissor beak), mandibular prognathism (under bite), and compression deformities of the mandible. These are most commonly seen in macaw and cockatoo chicks. Causes for these malformations include calcium deficiencies, improper dietary calcium:phosphorus ratios, and hyperthermia during incubation. Physical therapy and beak trimming are recommended treatments for these disorders in young birds, but after the beak becomes fully calcified, these treatments are not likely to be effective. Avulsion of the rostrum maxillare, rostrum mandibulare, or both have also been reported in a number of avian species.⁸ Anecdotally, birds tend to recover faster from avulsion of the maxillary beak versus the mandibular beak.

Avian Poxvirus Infection

Avian poxvirus is a large DNA virus, which is highly epitheliotropic. Infected cells contain large, acidophilic intracytoplasmic inclusions called Bollinger bodies. Poxviruses are unable to penetrate unbroken skin, but small abrasions are adequate for infection. The primary route of infection is mechanical transmission by blood-sucking insects with preferential localization of lesions to unfeathered areas (head, feet, etc.). Papular lesions progress to vesicles, which open spontaneously, dry, and form crusts. Inhalation or ingestion of the virus leads to the diphtheritic form of the disease, with caseous lesions on the mucus membranes of the oropharynx, tongue, and sometimes trachea. The diphtheritic form or "wet form" is the predominant form reported in psittacines. Birds recovered from avian pox infections, or that have been vaccinated, are usually immuneto reinfection with that particular strain.

Hypovitaminosis A

The most common cause of hypovitaminosis A is a predominantly seed-based diet. This deficiency will lead to squamous metaplasia of epithelial cells including those of the salivary glands and mucus membranes of the oral cavity. Blunting of the choana papillae in psittacines and white plaques in the oral cavity are signs of chronic vitamin A deficiency.

Trauma

Trauma to the oral cavity can occur through chewing on toys, accidental electrocution from biting electrical cords, ingestion of hot or caustic substances, conspecific trauma, and a number of other ways. In the author's experience, beak trauma is much more common than oropharyngeal trauma. A recent report described an adult female Chinese goose that presented for evaluation of abnormal ventral intermandibular distension causing difficulties with prehension and deglutition of food and water. Ultimately, a submandibular lingual entrapment with fibrous debris was diagnosed and successfully surgically repaired with an extraoral approach.²

Candidiasis

Candida albicans infection in the oral cavity is characterized by white plaques, which are most commonly seen in young animals secondary to crop stasis or immunosuppression. Disease can be seen in adult animals secondary to other illness, malnutrition, or prolonged systemic antibiotic therapy. A diagnosis can be made via oral cytology or fungal culture. Unlike white plaques due to hypovitaminosis A, these white oral plaques are difficult to physically remove. Nystatin or amphotericin B have been used for treatment. Ultimately, treatment of the underlying illness is often required for complete resolution.³ Another differential for white oral plaques, especially in wild birds such as birds of prey, is infection with Capillaria or trichomoniasis (see below).

Neoplasia

Previously reported oropharyngeal neoplasms in psittacines include fibrosarcoma, lymphoma, and squamous cell carcinoma. Biopsy or fine needle aspirates are usually required for definitive diagnosis. Treatment includes surgical excision (if possible) or radiation therapy (external beam and/or strontium therapy), but ultimately, the prognosis is poor for these malignant neoplasms.³

Esophageal and Crop Diseases

Esophageal/Crop Stasis and Impaction

Crop impactions can occur in younger birds secondary to foreign material ingestion, inappropriate hand-feeding formula (prepared with inadequate amounts of water), or secondary to crop infections. Crop stasis can occur in all ages of birds for a variety of reasons including infection (bacterial, fungal, parasitic), crop inflammation, crop atony secondary to overstretching, heavy metal toxicosis (lead toxicity), foreign bodies, food impaction, goiter (budgerigars and cockatiels secondary to seed-based diets), or neuropathic gastric dilatation (proventricular dilation disease [PDD], see below).³ Care should be taken when examining birds with distended crops because of the risk of regurgitation and possible aspiration. Ingluviotomy tubes have been used successfully in a variety of species, most recently described in several birds of prey, for treatment of crop stasis (secondary to heavy metal toxicosis) and for general nutritional support.^1,4

Trichomoniasis

The causative agent, Trichomonas gallinae, is a flagellated protozoan mainly carried by rock pigeons and other columbiformes. Young birds and debilitated, immunosuppressed adult birds (most commonly raptors) are most susceptible. There are several studies which have proven raptor species that live in urban environments have a higher prevalence of T. gallinae infections. Lesions include caseous plaques and granulomas of the oropharynx, crop, and esophagus, but can also involve the sinuses and trachea. In severe cases, obstructions of the proximal GI tract or respiratory tract can occur. Infected raptors are in poor body condition, and often have visible white plaques in their oral cavity/choana. Diagnosis is made via wet mounts of oral lesions - motile protozoa can be observed best with 40x magnification. Recently, a bald eagle with no clinical signs and a negative oral wet-mount tested positive for trichomoniasis via commercially available InPouch TF (BioMed Diagnostics Inc., White City, OR, USA), and infection was confirmed via PCR techniques.⁵ The most common treatment for trichomoniasis is metronidazole 30–50 mg/kg orally every 24 hours for 3–5 days, +/- surgical debridement.¹

Crop Lacerations/Fistulas

Crop fistulas most commonly occur secondary to thermal damage to the crop from feeding overheated formula in young birds. These burns are often full thickness through the skin overlying the crop. Recent thermal burns may require a waiting period of 3–5 days prior to surgical correction to allow the tissue to "declare" itself. In the meantime, the wound should be bandaged (with a "crop bra") to allow the patient to eat normally. During surgical repair, the crop should be closed separately from the overlying skin. Crop lacerations can also occur due to sharp feeding cannulas, trauma, or ingestion of sharp objects.³

References

1.  Aguilar RF, Yoshicedo JN, Parish CN. Ingluviotomy tube placement for lead-induced crop stasis in the California condor (Gymnogyps californianus). J Avian Med Surg. 2012;26:176–181.

2.  Grosset C, Guzman DSM, Waymire A, Puchalski SM, Steffey MA. Extraoral surgical correction of lingual entrapment in a Chinese Goose (Anser cygnoides). J Avian Med Surg. 2013;27:301–308.

3.  Hadley TL. Disorders of the psittacine gastrointestinal tract. Vet Clin North Am Exot Anim Pract. 2005;8:329–349.

4.  Huynh M, Sabater M, Brandao Jo, Forbes NA. Use of an esophagostomy tube as a method of nutritional management in raptors: a case series. J Avian Med Surg. 2014;28:24–30.

5.  Kelly-Clark WK, McBurney S, Forzan MJ, Desmarchelier M, Greenwood SJ. Detection and characterization of a Trichomonas isolate from a rehabilitated bald eagle (Haliaeetus leucocephalus). J Zoo Wildl Med. 2013;44:1123–1126.

6.  King A, McLelland J. Digestive system. In: King A, McLelland J, eds. Birds, Their Structure and Function. East Sussex, UK: Billiere Tindall; 1984: 84–109.

7.  Murray M. Raptor gastroenterology. Vet Clin North Am Exot Anim Pract.2014;17:211–234.

8.  Olsen GH. Oral biology and beak disorders of birds. Vet Clin North Am Exot Anim Pract. 2003;6:505–521.