What Is Osteomyelitis in Cats?
Osteomyelitis is an inflammation of bone tissue anywhere in the body, including legs, spine, and mouth (dental abscess). The inflammation may occur without infection, and this is called sterile osteomyelitis. However, in cats, infectious causes are more common.
When bone becomes inflamed, bone tissue responds by creating new bone and destroying existing bone. This causes pain, which can in turn cause limping (leg), neck or back discomfort (spinal bones), or decreased or no appetite (tooth socket). Varying degrees of swelling may also be noted over the affected area.
If a cat is acting sick—lethargic, running a fever, not eating or drinking, or not able to walk—seek emergency care ASAP. If your cat is limping but otherwise acting normally, seek care with your regular veterinarian.
Types of Osteomyelitis in Cats
Bacterial: Bacteria is the most common cause of osteomyelitis in cats. It can spread to bone through the blood (hematogenous spread) or gain access via open wounds.
Acute: The onset of lameness, pain, fever, and lethargy seems sudden.
Chronic: The cat has a longer history of limping/pain but is otherwise acting normally.
Fungal: Fungal organisms are inhaled into the lungs and get into the blood and lymphatic system, which then leads to bone infection. The prevalence of these organisms is geographical, so your veterinarian may inquire about any travel history with your cat. Veterinarians who work in fungal-endemic areas see fungal causes of osteomyelitis more often.
Sterile: Osteomyelitis without bacteria; this is rare in cats.
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Symptoms of Osteomyelitis in Cats
Early in the infection, a cat with osteomyelitis may be:
Limping or not wanting to walk
Showing painful swelling over the affected area
Not wanting to eat
Running a fever
A cat with a more chronic bone infection may no longer have a fever or be lethargic. However, the cat will likely still be limping or not walking normally, and will have muscle-wasting due to not using the sore part of the body appropriately. There also may be open draining wounds over the sites of swelling, both early on and later in infection.
Causes of Osteomyelitis in Cats
Although bacteria can gain access to bone via the bloodstream, this route is more common in kittens and immune-compromised cats. A healthy adult cat is more likely to get a bone infection through a bite or other penetrating trauma (such as a stick or wire). Open fractures, where there is a broken bone with exposure through damaged skin and soft tissues, may also lead to bacterial infection and subsequent osteomyelitis.
In addition, bacteria can gain access to bone during bone surgery if sterility is not maintained. Bacteria that enter the surgical site or that are on the implants at the surgery site can lead to infection.
One of the most common forms of osteomyelitis is in the oral cavity. Dental disease and dental abscesses are a form of osteomyelitis. Just like humans, cats can develop gum inflammation, tartar accumulation, and tooth abscesses. Bone in the jaw reacts to the oral inflammation, causing pain. This can affect a cat’s ability to eat, and the cat may drool, drop food, or have bad breath.
Some fungal organisms can cause osteomyelitis in cats. However, unlike bacteria, which is widespread, the three major fungal causes of osteomyelitis are only found in certain geographical regions. In the United States:
Coccidioides is endemic (found) in the Southwest.
Blastomyces and Histoplasma are most prevalent in the river valley region of the Midwest, especially along the Mississippi River.
Cryptococcus is a fourth fungal cause that can be found in many geographical regions.
Fungal organisms gain entrance to the body primarily by inhalation and spread to tissues of the body (including bone) via the blood and lymphatic system.
How Vets Diagnose Osteomyelitis in Cats
If your cat has a fever, lethargy, and lameness, a baseline lab-work panel is usually performed. A cat with sudden onset osteomyelitis will likely have an elevated white blood cell (WBC) count. If a cat has a more chronic or longstanding illness, the WBC count may be normal. Depending on the degree of bone inflammation present, blood calcium and phosphorus levels may be elevated.
X-rays of the affected areas will be taken to look for evidence of soft tissue and bony changes. In the early stage of inflammation, bony changes may not yet be noted; however, soft tissue swelling can be seen. As more time passes from the onset of clinical signs, changes in bone will be noted on subsequent x-rays. There will be both bone destruction and new bone formation.
Your vet may recommend x-rays two weeks after the initial set to look for changes in bone that may not have been initially present.
Usually, a diagnosis of osteomyelitis can be made based on a combination of history, physical exam, lab work, and/or radiographic findings. However, additional diagnostics may be recommended:
Advanced imaging, such as computed tomography (CT) or magnetic resonance imaging (MRI), may be pursued if x-rays are not providing a clear answer.
Fungal blood titers (antibody levels) may be recommended if you live in an endemic area or if your cat has a history of recent travel to an endemic area of the US.
Bone biopsy is a surgical procedure performed to determine the cause of osteomyelitis (bacterial vs. fungal) and to make a definitive diagnosis of osteomyelitis (as bone cancer can look similar on x-rays to fungal osteomyelitis). During this procedure, samples of abnormal bone tissue are taken and submitted to determine if the bone is inflamed or if it is bone cancer.
Treatment of Osteomyelitis in Cats
Bacterial osteomyelitis is treated with antibiotics, which are best selected by obtaining culture samples from the affected bone. Samples may be obtained via needle aspirate of the site or during surgical cleaning of the area. The type of bacteria present can be identified, and a panel of antibiotics tested to determine which option is best. For acute infections, antibiotic therapy may last 4-6 weeks, and for chronic infections, 6-8 weeks to several months.
Fungal osteomyelitis is treated with antifungal medications. Anti-inflammatory and other pain medications are likely to also be used.
Surgical intervention is needed with the presence of dead bone, debris, or implants (screws, plate, teeth, etc.). All these materials serve as a focus) for infection, and until they are removed, the infection will persist.
A complication of osteomyelitis is pathological fracture of the affected bone, which occurs when a diseased bone breaks because of damage and weakening due to infection. This is a difficult situation, because the bone cannot simply be repaired. Diseased bone is usually too weak to hold implants, and surgical failure is not uncommon. Ultimately, the treatment plan chosen will depend on the patient and the situation.
Recovery and Management of Osteomyelitis in Cats
A cat being treated for osteomyelitis will be monitored long-term for side effects of medications and response to treatment. Initially, rechecks will be more frequent. After surgery your cat may be kept in the hospital for a few days to a week or so, for wound care and monitoring.
Response to treatment is likely to be followed via x-ray imaging. Lab work may need to be rechecked with long-term use of antifungal medications.
Cats may be susceptible to recurrent episodes of osteomyelitis, especially fungal osteomyelitis. Bacterial osteomyelitis may also recur, especially if resistance to antibiotics becomes a problem.
Pet parents play an important role in treatment outcomes. They need to follow up with all recommended recheck appointments. Also, all medications must be given as directed by the care team. If in doubt, contact your veterinarian for guidance.
Prevention of Osteomyelitis in Cats
Since most cases of osteomyelitis in cats are bacterial, with bacteria most commonly gaining access to bone through open wounds, keeping cats indoors is the best way to protect them. If kept indoors, your cat will be less likely to suffer fractured bones, penetrating injuries, or cat bite or fight wounds.
Birchard S, Sherding RG. Osteomyelitis. Saunders Manual of Small Animal Practice. 2nd ed. Saunders; 2000.
Birchard S, Sherding RG. Systemic Mycoses. Saunders Manual of Small Animal Practice. 2nd ed. Saunders; 2000.
Rothrock, Kari. Veterinary Information Network. Osteomyelitis (Feline). September 2017.
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