Diagnostic Challenge: Cutaneous Nodules in a Captive Spur-Thighed Tortoise (Centrochelys sulcata)
Signalment
Approximately 3.5-month-old captive spur-thighed tortoise (Centrochelys sulcata) noted to have yellow nodules on its skin (Fig. 1). Some nodules were noted to dry up and fall off, but new ones would develop. Other conspecifics of the same age showed similar lesions. The animal died and was submitted for necropsy.
Citation: Journal of Herpetological Medicine and Surgery 35, 4; 10.5818/Diagnostic_challenge_Sulcata_Austwickia
Interpret the images, formulate differential diagnoses, and continue reading
On necropsy, multifocal well-demarcated, firm, yellow plaques effaced the epidermis and dermis, extending to the muscle (Fig. 2). Microscopically, the plaques comprised sheets of necrotic epidermis, dermis, and muscle, sharply demarcated from the underlying viable tissue. Present throughout the necrotic nodules were exceptionally high numbers of less than 1-micron diameter, slender, filamentous, gram-positive bacteria. The PCR and Sanger sequencing of formalin-fixed paraffin-embedded (FFPE) tissue identified the bacteria as Austwickia chelonae.
Citation: Journal of Herpetological Medicine and Surgery 35, 4; 10.5818/Diagnostic_challenge_Sulcata_Austwickia
Summary
Austwickia chelonae, historically classified in the genus Dermatophilus, is a bacterial infection of reptiles. The bacterium was first described from unreported chelonian species in Australia but has since been reported in multiple chelonian and squamate species (Masters et al., 1995; Jiang et al., 2019). The incidence of reptile austwickiosis appears to be increasing, particularly in captive tortoises in North America (Rostad et al., 2019). Gross lesions are typically first evident in the skin as firm, yellow plaques. As a number of reptile fungal skin diseases can appear similar, appropriate diagnostics (i.e., cytology, biopsy, PCR) to identify the source of the lesions are imperative. Microscopically, the lesions comprise plaques of necrotic cells with high numbers of gram-positive filamentous bacteria. Similar lesions also can be found in the oral and aural cavities. The demarcation between affected and unaffected tissue is often sharp, and adjacent viable tissue may exhibit minimal inflammation. For this reason, particularly in cases with some degree of postmortem autolysis, the pathogen may be inadvertently interpreted as opportunistic overgrowth. Austwickia chelonae encodes toxins similar to diphtheria toxin (Gill et al., 2024); these exotoxins may contribute to the nature of the lesions and limited inflammatory response in infected reptiles. Studies of A. chelonae antimicrobial sensitivity have identified multiple therapeutic options in vitro, but in vivo treatment appears to be challenging. Concerningly, A. chelonae has also been identified as a cause of lesions in free-ranging squamates and chelonians in Asia and North America, though the overall risk to wild reptile populations is unknown (Jiang et al., 2019; Liguori et al., 2022).
Disclaimer
The authors have no conflicts to disclose.
Gross image, captive, hatchling spur-thighed tortoise (Centrochelys sulcata). Multiple hatchlings were observed with raised, yellow bumps of the skin of the limbs and head. This individual died and was submitted for necropsy.
Photomicrographs of hematoxylin and eosin and Gram stains of the cutaneous lesions. (A) Microscopically at low magnification, the yellow skin nodules comprised sheets of necrotic material (N) with sharp demarcation (marked with asterisks) from underlying viable muscle (V). (B) On higher magnification, necrotic muscle cells, fibroblasts, and leukocytes made up the plaques in association with high numbers of less than 1-micron diameter, slender, filamentous, gram-positive (inset) bacteria. Minimal inflammatory cells were present in the viable muscle tissue underlying the tidemark of necrosis (asterisks).
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