161 - Improving the Diagnosis of Respiratory Tract Diseases in Captive Tortoises Using CT Scans and Radiography

Poster Board Number: 161
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Odd poster #s – first hour
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Abstract Body : Respiratory tract infections are among the most prevalent medical conditions affecting captive tortoises but attempts to identify the causal agent and underlying mechanisms are often unsuccessful. Chelonian lungs are unusual in their morphology, generally expanding along the internal surface of the carapace. Beyond that, the anatomical details of respiratory tract in tortoises are poorly documented. It is also unknown how various shell morphologies, such as kinetic hinges, impacts tortoise lung functional morphology.

Specimens from three tortoise (Testudinidae) species were studied, including a giant Aldabra (Aldabrachelys gigantea), forest hinge-back (Kinixys erosa), and African spur-thigh (Centrochelys sulcata) tortoise. K. erosa and C. sulcata were perfused with BriteVu contrast agent and scanned using an Epica GT-30 CT scanner. As A. gigantea was too large for a traditional imaging approach, it was scanned in a Siemens Somatom medical CT-scanner in sections following dissection. Resulting CT scans were studied and segmented using Aviso 2020.2 and Mimics 25. Meticulous gross dissection, including anatomical descriptions, morphometric data collection, and photodocumentation was performed for each specimen.
In all tortoise species, the lungs were a rounded, wedge shape in cross-section and tapered laterally. Lung tissue was diffuse and amorphous, composed of spongy, brown parenchyma interspersed with thin spicules of structural fibers, surrounded by a thin, translucent pleural membrane. The lungs filled all available space in the dorsal shell, following the various internal curvatures of the carapace. It extended anteriorly from the scapular girdle and axillary buttresses to posteriorly near the pelvis. In K. erosa, the caudal extent of the lung tissue was constrained posteriorly by its carapacial hinge. While the lung tissue was occasionally partitioned in all species, no clearly differentiated lobes were apparent. A thickened midline membrane separated the left and right lungs from each other.

The trachea and primary bronchi of all taxa were broad and composed of complete cartilaginous rings. Non-cartilaginous portions were thin and translucent, suggesting a permeability to gas exchange. The trachea coursed caudally through the midline of the neck before deviating dramatically to one side at the level of the fundus of the stomach. The bronchi were elongated, asymmetrical, and took a sinuous path through the lung tissue. The condition was exaggerated in K. erosa, which displayed unusual tracheobronchial anatomy in which both the trachea and primary bronchi were extremely extended and took a tortuous path through the lung tissue, heading dorsally to terminate abruptly in the dorsal carapace. To our knowledge, this morphology has not been previously described and has profound clinical implications for the treatment of respiratory infections in captive tortoises, especially K. erosa and other hinged-backed species.

Many anatomical details of the respiratory system observed in tortoises, such as tortuosity of primary bronchi, lack of subsequent bronchial subdivisions, and absence of clearly differentiated lung lobes differ from limited information available for aquatic turtles. As the only primarily terrestrial family of Testudines, the shell of tortoises has evolved to approximate a geodesic dome. Their highly domed carapace permits greater dorsal expansion of lung tissue compared to their flatter-shelled, aquatic counterparts.