Malignant osteochondromatosis in two related ridgeback monitors

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malignant osteochondromatosis in ridgeback monitors

Two related ridgeback monitors (Varanus acanthurus) were each presented for consultation, respectively, with severe swelling of their limbs and thoraco-lumbar spinal region. Following post-mortem histopathology, a diagnosis of malignant osteochondromatosis was eventually made in both lizards. There is a paucity of reports of osseus neoplasia in reptiles. In humans, benign hereditary osteochondromatosis has been associated with malignant transformation.  

Both lizards were nine months old, hatched from the same clutch and had been housed in a large tank at an average temperature of 28oC for eight months. They were fed turkey mince and crickets. The masses had grown rapidly over the three weeks prior to presentation. Both lizards had been healthy prior to development of clinical signs.  

Lizard 1 had three swellings around the thoracolumbar spine. The largest of the masses was on the right side, with two smaller masses on the left of the spine. This lizard did not show neurological compromise and ambulated freely.  

Lizard 2 had impaired ambulation, with evidence of multiple peripheral nerve compressions. There was marked swelling of the left antebrachium, right metacarpal region and both left and right tarsometatarsal regions. The initial differential diagnosis list was headed by metabolic bone disease, however bacterial and fungal infections, gout, and neoplasia were also considered.

Whole body radiographs were performed on Lizard 1 (with paraspinal lesions) under anaesthesia. Bone density was considered normal and no lytic lesions of the vertebrae or appendicular skeleton were found. The paraspinal masses appeared slightly radio-dense relative to soft tissue. Radiographs were also performed on Lizard 2 (leg masses), which showed complete loss of bone detail in the right hind limb with focal areas of bone lysis. No intra-articular tophi were seen. A fine needle aspirate of the right hind limb swelling was non-diagnostic.  

Due to cost constraints, the client only allowed antemortem biopsies from Lizard 2. Punch biopsies were obtained from the swelling near the right tarsometatarsal joint. The firm peri-synovial tissue was abnormally thickened, relatively avascular, and fibrous yet friable. Histopathology suggested a Schwannoma (neurofibroma) with chondroid metaplasia (as commonly occurs with this tumour type). The clinical description was considered to be an unusual form of neurofibromatosis. Due to the poor prognosis associated with neurofibromatosis, the client opted to euthanase both lizards. 

Necropsy was offered free of charge (due to clinical interest because of the unusual nature of the cases) for more extensive histopathology. In Lizard 1, the tumours were found to be arising from either side of the vertebrae and growing into the coelomic cavity. Samples were collected from all three paraspinal masses from Lizard 1, and from all of the affected limbs of Lizard 2 for a more comprehensive microscopic review.

At this point, the initial diagnosis of neurofibromatosis was revised to malignant osteochondromatosis by a pathologist at IDEXX. This was because after multiple segments of bone and overlying soft tissue were examined, there was invasive destruction of bone, infiltration of joint spaces and massive extraperiosteal expansive infiltration by tumour arising from proliferation of neoplastic pleomorphic small spindle cells with a low mitotic index.  

The tumours contained prominent focal deposits of chondroid tissue and, significantly, these were also directly associated with the production of osteoid. The more comprehensive microscopic review showed that the neoplastic process was more intimately associated with bone rather than being of neural origin. The production of chondro-osseus tissue was an intrinsic feature of the tumour rather than representing a metaplastic change.  

The final diagnosis of malignant osteochondromatosis took into account the multicentric nature of the neoplastic process as well as the production of both chondroid and osteoid tissue. There is precedent for this as multiple cartilaginous exotoses (osteochondromatosis), which is hereditary in humans, is sometimes complicated by malignant transformation.

There are isolated case reports of multiple cartilaginous exotoses (Frye & Dutra, 1973) and osteoid chondrosarcoma (Schonbauer et al, 1982), and chondroblastic osteosarcoma in two related spiny-tailed monitors (Needle et al., 2013) in monitor lizards. Whether they (Varanidae) are predisposed to hereditary multiple cartilaginous exotoses (MCE) and subsequent malignant transformation is unknown. However, the present case together with these previous reports suggests that, while rare, the full spectrum of disease from benign MCE, to solitary osteochondromas, to malignant transformation of MCE in multiple locations in situ, is possible in the Varanidae. 

In humans, MCE are considered to be heterozygous, autosomal dominant hereditary defects. Juvenile patients (<10 years old) have fewer exotoses, as the condition is known to have an age-related penetrance. Hereditary MCE is characterised by pain and development of bony protuberances capped by cartilage (particularly in the metaphyses of the long bones). Multiple hereditary osteochondromatosis has a greater tendency towards malignant transformation than solitary osteochondroma, with rates of 5-25% versus 1-2% reported.

Because both lizards were brothers, it is quite possible that a high in-breeding coefficient resulted in phenotypic expression of an autosomal dominant defect (as in humans). However, as they were housed together, a viral aetiology cannot be ruled out.


Dr Alex Harrison BSC BVMS (Hons) 

malignant osteochondromatosis in ridgeback monitors

MANZCVS Small Animal Medicine, Small Animal Surgery

Dr Alex Harrison graduated from Murdoch University in 2000 and initially worked at the Melbourne University Veterinary School. He then spent 13 years working in a busy group of veterinary hospitals in Adelaide’s south, including 10 years as a practice partner, honing his surgical skills and accepting second opinion cases from colleagues. 

Dr Harrison has completed Memberships in Small Animal Medicine and Small Animal Surgery with the Australian and New Zealand College of Veterinary Scientists. 

His areas of interest include oncology, advanced medical imaging and reconstructive and orthopaedic surgery.

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