Paget-like bone remodeling disorder in a red-tailed boa (Boa constrictor constrictor): diagnosis and management

Abstract

Importance

Paget-like bone remodeling disorder is a rare skeletal disease in reptiles, and clinical management strategies are poorly defined. Reporting clinically documented cases may improve recognition and welfare-centered care.

Case presentation

A 23-year-old male red-tailed boa (Boa constrictor constrictor) presented with anorexia, dysecdysis and spinal rigidity. Radiography and computed tomography showed diffuse vertebral deformities with irregular bone proliferation, and erosive lesions. Serum alkaline phosphatase activity was markedly increased. Blood cultures yielded Escherichia coli. Treatment with enrofloxacin and dexamethasone was followed by a transient decrease in alkaline phosphatase activity and partial improvement in appetite. Husbandry modifications, including transfer to a larger enclosure, improved mobility and short-term quality of life. The snake later developed recurrent anorexia and dysecdysis and was euthanized for welfare grounds. Necropsy and histopathology revealed extensive vertebral deformities, thickened trabeculae, mosaic lamellar bone and osteoclasts proliferation, consistent with Paget-like bone remodeling disorder.

Conclusions and Relevance

This case supports the value of imaging, clinicopathologic assessment, and histopathology in diagnosis, and suggests that supportive husbandry modification may provide temporary welfare benefit in affected snakes.

INTRODUCTION

The Boa constrictor (Boa constrictor) is a widely recognized snake species. These snakes reach an average length of 2–3 m and are known for their active terrestrial locomotion [1]. Such movements depend largely on the flexibility of the vertebral column, supported by the surrounding musculature and integumentary structures, thereby underscoring the importance of spinal mobility in overall health and locomotor function [2].

Paget’s disease of the bone (PDB) is a chronic skeletal disorder characterized by abnormal bone remodeling and increased bone turnover, resulting in skeletal deformities. In humans, this disease has been associated with specific genetic mutations; however, its etiology in animals remains poorly understood. Viral and bacterial infections have previously been proposed as possible contributing factors in some reported cases [1,3]. Excessive osteoclastic resorption, followed by disorganized new bone formation, underlies the pathogenesis of the disease, causing pain, skeletal deformity, and, as the condition progresses, complications such as osteoarthritis and pathological fractures.

Cases of this condition have been documented in several animal species, including dogs, rodents, and reptiles [4,5,6]. Reports of Paget’s-like bone pathology in reptiles are exceedingly rare, representing only a very small number of isolated case studies in the veterinary literature. Unlike mammals—where Paget’s disease has been more widely recognized—reptilian skeletal disorders involving abnormal bone remodeling remain poorly characterized and are often identified only at advanced clinical stages. The few published cases have predominantly involved Burmese pythons and golden lancehead snakes, presenting with multifocal vertebral deformities, impaired locomotion, and radiographic evidence of proliferative, structurally disorganized bone [3,7]. Even in these limited reports, diagnostic approaches, underlying etiologies, and histopathological features show considerable variability, underscoring the current lack of an understanding of this condition in reptile species. Furthermore, the scarcity of clinical data makes it challenging to establish prognostic expectations or therapeutic strategies for affected animals. In human medicine, management strategies for Paget’s disease include bisphosphonates, calcitonin, analgesics, and anti-inflammatory drugs [8]. However, reports of therapeutic interventions for affected animals remain rare.

Given the paucity of veterinary case reports and the clinical relevance of early recognition for animal welfare, the present report describes a suspected case of Paget-like bone remodeling disorder in a Boa constrictor, with the aim of contributing to the understanding and management of this condition in reptiles.

CASE PRESENTATION

A 23-year-old male red-tailed boa (Boa constrictor constrictor) presented in August 2024 with poor shedding and anorexia, following its last voluntary feeding. On physical examination (Table 1), the snake demonstrated generalized rigidity and an inability to flex its body. Radiography revealed diffuse irregularities along the vertebral column (Fig. 1A). Blood samples collected from the ventral coccygeal vein revealed marked elevations in alkaline phosphatase (ALP; 1,512 U/L) levels, exceeding previously reported reference values (ALP; 96–116 U/L) for snakes [7].

Table 1. Clinical timeline, feeding history, and major diagnostic findings in the red-tailed boa.

Time	Diet (item and size)	Body weight (kg)	Laboratory findings	Diagnostic	Outcomes
Day 0	6 mice (middle)	3.6
Day 30	5 mice (middle)	3.7
Day 90	19 mice (small)	3.6
Day 120	10 mice (small)	4.0	ALP 1,512 U/L	Radiograph performed
			Blood culture + Escherichia coli
Day 180	4 mice (middle)	3.7	ALP 473 U/L		Enclosure doubled
Day 240				CT performed
Day 270	4 mice (middle)	3.5
Day 300	2 mice (middle)	3.5		Radiograph performed
Day 420					QoL decline and euthanasia

ALP, alkaline phosphatase; CT, computed tomography; QoL, quality of life.

Fig. 1. Diagnostic imaging findings in a red-tailed boa with Paget-like bone remodeling disorder. (A) Whole-body radiograph showing diffuse vertebral osteolytic and osteoproliferative change with coarse, irregular trabeculation and indistinct corticomedullary margins. (B) Computed tomography image showing generalized vertebral hypertrophy with cortical thickening and medullary sclerosis.

CT, computed tomography.

For further evaluation, CT was performed using a 16-channel multi-detector row scanner (Philips MX 8000 IDT, Philips Medical Systems, The Netherlands). CT images demonstrated irregular osseous proliferation and multifocal erosive lesions throughout the vertebral column, consistent with the radiographic findings (Fig. 1B). Based on these findings, an initial diagnosis of Paget-like bone remodeling disorder was made. Blood culture revealed Escherichia coli, and the animal was treated with enrofloxacin 10 mg/kg intramuscular (IM) every 48 h (q48h) (30 mg) and dexamethasone 0.5 mg/kg IM q48h (1.5 mg) to control infection, inflammation, and pain for 2 months. After two months, the snake’s ALP level decreased to 473 U/L, and bacterial growth was no longer detected. Nevertheless, the vertebral deformities persisted, and the snake was unable to bend its body, maintaining a progressively extended posture. No injection-site reactions or gastrointestinal adverse effects were observed. Long-term corticosteroid risks were monitored; no opportunistic infections were detected after culture clearance.

To improve mobility, the animal was transferred to an enclosure twice as large as its previous habitat. With continued treatment, food intake gradually resumed, reaching 30% of baseline, subsequently improving to 50%–60%. Maintenance therapy consisting of weekly injections of the same regimen was continued.

Beginning in June 2025, the snake showed a progressive decline, characterized by markedly reduced movement, complete anorexia, and recurrent molting failure. Following the zoo-based quality-of-life assessment, euthanasia was performed. The zoo-based quality-of-life assessment used in this case is a standardized tool jointly completed by keepers and veterinarians. It evaluates appetite, pain, response to the zookeeper, feces/urine, grooming, and response to medication on a 1–5 scale, with 5 indicating normal status and 1 indicating marked impairment. When all parameters score 1, euthanasia is considered. Euthanasia was performed following sedation with ketamine (20 mg/kg; Yuhan, Korea) and midazolam (1 mg/kg; Bukwang pharm, Korea). After confirming a deep anesthetic plane by the absence of a tail-pinch response, potassium chloride was administered via intracardiac injection, and respiratory arrest was verified as the end point. Gross postmortem examination revealed extensive vertebral deformities involving the entire spine, with a diffusely rigid and irregular vertebral column architecture. The remaining internal organs were unremarkable.

After necropsy, the heart, lungs, kidneys, liver, spleen, and vertebrae were fixed in 10% buffered formalin. The vertebrae were transferred to nitric acid for decalcification. Upon confirmation of complete decalcification, the vertebrae were processed routinely, embedded in paraffin, and sectioned at 4 μm for histopathological evaluation. All sections were stained with hematoxylin and eosin.

Histological examination revealed a generalized thickening of the bony trabeculae. Further, the inter-trabecular spaces in the severely affected regions contained blood vessels and fibrous tissue but lacked bone marrow (Fig. 2A). The thickened trabeculae were confluent and displayed irregular patches of lamellar bone outlined by prominent mosaic patterns of basophilic cement lines (Fig. 2B), indicating retarded osteocytic osteolysis. Both the cortical and trabecular bone quantities increased, with the formation of osteons and, in places, loss of distinction between the trabecular and compact bone. Osteoclasts were observed in some areas of the sections (Fig. 2C).

Fig. 2. Histopathological section of affected bone. (A) Representative image of generalized thickening of bony trabeculae with intertrabecular spaces containing blood vessels and fibrous tissue but lacking marrow. H&E, 300×, Bar = 150 µm. (B) Confluent trabeculae display irregular lamellar bone with mosaic-like basophilic cement lines (arrows). H&E, 400×, Bar = 100 µm. (C) Highlighting osteolysis driven by osteoclasts (arrowheads). H&E, 800×, Bar = 50 µm.

Clinical, laboratory, radiographic, and histopathological findings supported the diagnosis of osteitis deformans (Paget-like bone remodeling disorder). This chronic focal disorder of bone remodeling is characterized by an initial increase in osteoclast-mediated bone resorption, followed by compensatory new bone formation, resulting in the growth of a disorganized mosaic of woven and lamellar bone [8].

Definitive diagnosis relies on the integration of clinical history, laboratory testing, imaging, necropsy, and histopathological examination. However, the precise etiology remains unclear and is likely multifactorial.

The diagnostic evaluation, treatment, euthanasia, necropsy, and publication of this case were authorized by the zoo institution. Animal care and clinical management were conducted under the oversight of the zoo’s animal care and welfare committee.

DISCUSSION

The cause of Paget’s disease in humans and animals currently remains unclear, although it is known to occur in individuals with a genetic predisposition or following infection with paramyxovirus and canine distemper virus [1,6,9]. In animals, cases of Paget’s disease have primarily been documented in snakes [8,10]. Although bacterial infections have been reported to be associated with some cases [3,11], the precise etiology underlying this disease remains unclear. Moreover, effective therapeutic and long-term management strategies are yet to be established. Consequently, Paget’s disease in reptiles continues to be regarded as a poorly understood disorder, highlighting the need for further research into its underlying causes and the development of new approaches to supportive care [12].

In human patients with PDB, treatment is generally indicated for those with symptomatic disease or biochemical evidence of metabolic activity, most assessed through serum or bone-specific ALP levels [13]. Although ALP isoenzyme analysis was not available, the normal values for gamma-glutamyl transpeptidase and glutamic oxaloacetic transaminase in this case supported the interpretation that the elevation represented bone-specific ALP. Current clinical guidelines recommend providing bisphosphonate therapy as the first-line approach, with a single intravenous infusion of zoledronic acid considered as the most effective option, given its superior efficacy in inducing rapid, complete, and sustained remission compared with oral risedronate sodium [13,14]. Calcitonin remains a secondary option, reserved for cases in which bisphosphonates are contraindicated or poorly tolerated, or where urgent preoperative suppression of bone turnover is required [15]. Although antiresorptive protocols used in human patients (e.g., bisphosphonates, calcitonin) were discussed for comparative purposes, these treatments were not implemented in this case because their safety and efficacy have not been established in reptiles; instead, therapy was directed toward controlling concurrent bacteremia and inflammation and improving welfare through supportive husbandry modifications. Furthermore, adverse effects have been well-documented in humans, including medication-related osteonecrosis and atypical femoral fractures, thus highlighting the need for careful monitoring and risk–benefit assessment if such therapies are to be considered in exotic species [13,14,16]. No drug-related adverse events were observed.

In addition to pharmacological interventions, husbandry plays a central role in maintaining the musculoskeletal health and welfare of snakes. Welfare assessments emphasize that enclosures should be sufficiently large to permit full body extension, as inadequate housing restricts natural locomotive behaviors, such as rectilinear and concertina movements, which may contribute to abnormal mechanical loading of the axial skeleton [17,18]. As part of the welfare-centered management, the enclosure environment was modified to provide more appropriate husbandry conditions. The temperature gradient was maintained with a hot zone at 30°C and a cool zone at 25°C. Because the snake exhibited predominantly linear movement and was unable to climb due to marked spinal rigidity, vertical climbing structures were excluded. Instead, the floor was leveled and lined with soft, moisture-retaining substrates, such as damp towels and newspaper, to enhance comfort and support shedding. The enclosure measured approximately 2 m in length, allowing the snake to fully extend its body and move with reduced mechanical strain.

Environmental enrichment that stimulates locomotion, including varied substrates, climbing opportunities, and horizontal space, is essential to reduce stress and mitigate secondary complications associated with chronic inactivity. In line with these recommendations, the enclosure in the present case was modified to be twice as long as that used before the diagnosis of Paget-like bone remodeling disorder. Following this adjustment, the snake demonstrated an improved ability to stretch and move, and gradually regained partial feeding behavior, indicating a measurable welfare benefit and temporary clinical improvement [19,20]. After diagnosis, the patient’s institutional Zoo Quality-of-Life Monitoring Record declined gradually rather than precipitously, and maintenance of food intake for approximately 6 months indicates that environmental adjustments, together with adjunct dexamethasone, contributed to interim quality-of-life gains.

The pathophysiology underlying Paget’s diseases in reptiles remains unclear. The proposed mechanisms include abnormal bone remodeling cycles caused by an imbalance between osteoclastic resorption and osteoblastic deposition, and a potential infectious contribution, supported by occasional reports of bacterial growth or histological inflammation. Although E. coli was detected in the initial blood culture, the absence of bacterial growth following antimicrobial treatment, negative vertebral cultures, and lack of histological evidence of osteomyelitis indicate that the bacteremia was transient and unlikely to be related to the chronic Paget-like bone remodeling disorder. While bacterial infection has been proposed as a potential contributing factor in Paget-like disease, the clinical and histopathological findings in this case do not support a causal association. Importantly, similar to previous reports, we were unable to identify a definitive etiology in the present case. Although temporary improvements in feeding and locomotion were achieved with husbandry changes, the animal eventually declined, losing its ability to ingest food, and exhibiting severe locomotor restrictions, necessitating euthanasia on welfare grounds. This outcome underscores the fact that the disease could not be fully treated, and that the absence of adjunct pharmacological therapies represented an additional limitation. Nevertheless, this case highlights the potential benefits of early detection using diagnostic imaging modalities, such as radiography or CT. Imaging studies play an important role in the early diagnosis of this condition, and can guide subsequent management strategies. If similar cases occur, quality of life may be substantially improved by combining pharmacological interventions with environmental modifications. Given the distinct biological characteristics of snakes, restricted movement can predispose individuals to secondary complications, such as dysecdysis, further underscoring the value of early diagnostic evaluation [21].

Although curative treatment was not possible in the present case, supportive management strategies, particularly those involving enclosure design and welfare-focused husbandry modifications, proved capable of enhancing quality of life, even if only transiently. Thus, this report underscores both the uncertainty of the disease’s etiology and the importance of early recognition and management strategies aimed at improving the welfare of snakes affected by Paget-like bone remodeling disorder.