Degenerative Osteoarthropathy in Laboratory Housed Xenopus (Silurana) tropicalis

In this case study, 15 adult laboratory Xenopus (Silurana) tropicalis (7 adult males and 8 adult females) were examined for nodular enlargements of the clawed digits (digits 0, I, II, and III) on the hind feet. Radiographs showed smoothly margined, rounded, peripherally mineralized lesions arising from the distal phalanges of digits 0-III with osteoproductive and osteolytic components in all frogs. Micro computed tomography (microCT) scans further revealed interphalangeal (IP), metacarpophalangeal (MCP), and metatarsophalangeal (MTP) joint osteoarthritis characterized by periarticular new bone formation, rounded mineral foci both peripherally and centrally within the joints, and more rarely, linear mineralization palmar/plantar to the joints in the flexor tendons. In the nonclawed digits, the shape of the distal phalanx was variably distorted and both subluxation and malangulation of IP joints were identified. Histologically, nodules corresponded to a peripheral rim of mature cortical bone surrounding central adipose tissue, scattered hematopoietic elements, and residual bone of the distal phalanx. Occasionally, the peripheral rim of cortical bone extended proximally to encompass the distal aspect of adjacent phalanx. MCP, MTP and IP joint spaces of most digits exhibited widespread osteoarthritis characterized by periarticular cartilaginous or osseous metaplasia, bony remodeling, and less frequently, granulomatous osteomyelitis. Nutritional analyses of the feed did not indicate imbalances nor were the lesions consistent with metabolic bone disease. The exact etiopathogenesis of these lesions is unknown; however, we hypothesize that the osteoarthritic changes are due to a combination of the frogs’ mature age, the unique structure of the Xenopus spp. claw, genetics and biomechanical forces on the digits and distal phalanges of the hind feet.

Xenopus spp. are a well-established model in biomedical research and are commonly used in the fields of embryology and vertebrate developmental biology, endocrinology, toxicology, cancer, and genomics.^{1-3,5,10,11,13,16,17,19,20,22} In research laboratory environments, X. tropicalis are typically housed in either static or recirculating aquatic systems in water maintained at 25 to 28 °C, with a pH between 6.8 and 8.5, and a hardness between 100 and 300 mg/L CaCo₃.⁷ Housing tanks range in size from 1 L or more, depending on the needs of the laboratory and the type and configuration of the housing system. Stocking density ranges from approximately 1-5 adult frogs/L per tank.⁷ Laboratory X. tropicalis are commonly fed commercially available complete and balanced pelleted diets 3 times per week, or as appropriate for their different life stages.^7,18,21 With stringent husbandry and disease surveillance protocols in place, laboratory-reared and housed Xenopus spp. generally remain healthy well through adulthood and their prime egg-producing years and can live 10 y or longer in captivity.⁷To date, most reports describing disease in laboratory or wild Xenopus spp. are attributed to infectious agents (bacterial, viral, or parasitic) and/or conditions related to water quality aberrations or pollutants.⁷ Naturally occurring bone diseases due to nutritional appear to be rare in laboratory Xenopus spp., although they have been described in other frogs and amphibians.^6,12,23 Reports of other spontaneous skeletal diseases in laboratory Xenopus are particularly rare. One group²⁵ recently described axial skeletal deformities in genetically engineered and wild type laboratory Xenopus spp. Here we report spontaneously occurring appendicular skeletal degenerative osteoarthropathy with hindlimb distal phalangeal nodules (digits 0-III) in mature, laboratory X. tropicalis, as confirmed by radiography, computed tomography and histopathology.