Case Studies
 

Bacterial Osteomyelitis in a 3 Year-Old Cat.

Bacterial Osteomyelitis: Domestic Shorthair

VETERINARIAN SPECIALTY CASE STUDY

Osteomyelitis is an inflammation of bone or bone marrow. In dogs and cats it can be caused by fungal, bacterial, or potentially viral pathogens – with bacterial being the most common cause.

By Heather McDonnell Siemon, DVM
and Jason Balara, DVM
Diplomate AVCS

1. Referral

“Nubia” a 3 year old female spayed domestic shorthair cat presented initially to the ASG oncology department for limping on the right pelvic limb for about 3 weeks. She was referred from her primary veterinarian after radiographs of the limb showed a lytic lesion in the distal femur. Thoracic and abdominal radiographs were unremarkable and CBC and biochemistry showed only an elevated globulin level.

Radiograph: Bacterial Osteomyelitis

Figure 1: Radiographs of the right stifle showing a lytic lesion in the distal femur

2. Physical Examination

Based on the examination and radiographic findings, there was concern for a neoplastic process such as multiple myeloma or osteosarcoma. Further staging with an abdominal ultrasound revealed a mildly enlarged spleen but was otherwise normal. A fine needle aspirate of the spleen was taken which was low in cellularity but did not reveal any overt neoplasia in the spleen. A protein electrophoresis was submitted to assess for the presence of a monoclonal gammopathy but this showed only an elevation in the alpha 2 globulin fraction and so was not supportive of multiple myeloma. A fine needle aspirate was taken of the right femoral lytic lesion. This returned as “blood with low numbers of mildly to moderately pleomorphic ovoid to spindloid cells and mild mixed cell inflammation” with a pathologist comment that “given the clinical description (aggressive bone lesion), the findings are more suspicious for mesenchymal neoplasia (i.e osteosarcoma).” Options for further investigation were presented to the owner including right pelvic limb amputation or biopsy of the lesion. Her owner opted for a right pelvic limb amputation for “Nubia”.

3. Procedure

“Nubia” was transferred to the ASG surgical service for right pelvic limb amputation. She was routinely prepared for surgery and anesthetized. A routine right pelvic limb amputation was performed. She did well under anesthesia and recovered over the next 2 days in the hospital with no complications. Pain relief was provided with buprenorphine and Nocita and she was discharged into her owner’s care with oral buprenorphine to keep her comfortable.

4. Diagnostics

“Nubia’s” right pelvic limb was submitted for histopathology. The microscopic findings showed chronic, focal, moderate osteomyelitis with “central osteolysis, surrounded by mature woven bone. The cortical bone is also partially lytic”. Grocott’s methanamine silver stain was applied which failed to uptake, indicating an absence of any fungal organisms. This sample was sent to another reference laboratory for a second opinion. Findings were concurrent with the original diagnosis.

5. Diagnosis

Bacterial osteomyelitis of the distal right femur

6. Discussion

Osteomyelitis can be caused by fungal, bacterial, or potentially viral pathogens. The most common type of pathogen implicated in osteomyelitis is bacteria with a variety of both anaerobic and aerobic species implicated. Gram positive and gram negative bacteria are implicated in roughly even distribution. The most commonly isolated species is Staphylococcus pseudintermedius.

Bacterial osteomyelitis can occur when the bone becomes contaminated by external bacteria, usually through a penetrating injury such as a bite wound, surgery, or fracture. It can also occur via hematogenous spread from infections within the body but distant to the bone. Bone is naturally resistant to bacterial infection and so commonly we see other factors contribute to the development of bacterial osteomyelitis. This can include immunosuppression, foreign material in the wound, fractures, or necrosis of the bone.

Surgical implants can develop bacterial biofilm which can contribute to the development of osteomyelitis in adjacent bone. When hematogenous spread is seen it is commonly found in the metaphysis of juvenile animals due to the anatomy of the vasculature in this area of bone which is rich in blood vessels. Bacterial osteomyelitis can occur acutely or can develop into a chronic infection, the latter of which is more difficult to eradicate.

In cats, infection with the bacterial species Bartonella has recently been implicated in cases of osteomyelitis. This species of bacteria is often transmitted via bites by infected fleas and is commonly found incidentally in healthy cats. One study in 2009 found recurrent osteomyelitis lesions in a cat who tested positive for Bartonella vinsonii subsp. berkhoffii via PCR. In humans, Bartonella has been implicated in osteomyelitis via transmission through cat scratches.

Treatment for bacterial osteomyelitis depends on the bacterial species, location, severity, chronicity, and many other factors. Culture and susceptibility testing are vital for appropriate antimicrobial selection and repeated culture may be necessary to assess response to therapy. Antimicrobial therapy may need to continue for several weeks for full clinical resolution, especially in chronic cases.

In severe cases of chronic or recurrent osteomyelitis more aggressive interventions such as surgical debridement or amputation can be elected. Foreign material such as surgical implants may require removal to achieve full resolution.

7. Recovery

In Nubia’s case it is unknown how she developed osteomyelitis in her distal right femur. She recovered well from her surgery and is reportedly doing well at home.

 


Image of cat is not Nubia.

REFERENCES: [1] Ettinger, Stephen J., and Edward C. Feldman. Textbook of Veterinary Internal Medicine: Diseases of the Dog and Cat. 5th ed., vol. 2 2, W.B. Saunders Co., 2000; [2] Greene, Craige E. Infectious Diseases of the Dog and Cat. 4th ed. Elsevier 2012; [3] “Bartonella Infection (Cat Scratch Disease, Trench Fever, and Carrión’s Disease).” Centers for Disease Control and Prevention, 14 Dec. 2015, www.cdc.gov/bartonella/veterinarians/index.html; [4] Varanat, M., et al. “Recurrent Osteomyelitis in a Cat Due to Infection with Bartonella Vinsonii Subsp. Berkhoffii Genotype II.” Journal of Veterinary Internal Medicine, vol. 23, no. 6, 2009, pp. 1273–1277., onlinelibrary.wiley.com/doi/abs/10.1111/j.1939-1676.2009.0372.x.

Animal Specialty Group

DVM, Diplomate American College of Veterinary Surgeons

Dr. Jason Balara attended veterinary school at Tufts University, completed an internship at Angell Animal Medical Center, and finished his residency from both institutions. He believes in constantly improving the state of veterinary medicine, and passes on his wisdom by teaching students, interns, and residents alike.