Osteoarthritis in cats: What we now know about recognition and treatment

The incidence of chronic pain in cats is not well-documented but is associated with many conditions, including osteoarthritis, cancer, interstitial cystitis, dental and gum disease, and long-standing dermatitis and wounds. It is only recently that veterinarians have begun to appreciate what the true incidence of osteoarthritis, sometimes called degenerative joint disease, might be in cats. It appears to be much more common than previously thought and could be a major cause of discomfort, especially in senior (> 10 years of age) cats.

DEGENERATIVE JOINT DISEASE VS. OSTEOARTHRITIS

Although the terms degenerative joint disease and osteoarthritis are commonly used interchangeably in the veterinary arena, a distinction has been made between the two.1 Degenerative joint disease (DJD) is a general term used to describe any degenerative change in a synovial, cartilaginous, or fibrous articulation in the skeleton. Osteoarthritis, however, is a pathologic change of a diarthrodial synovial articulation and includes deterioration of articular cartilage, osteophyte formation, bone remodeling, soft tissue changes, and low-grade nonpurulent inflammation. Not all radiographic changes are correlated with a clinical problem. As veterinarians, we are most interested in changes that have a negative impact on a cat's life because of pain and discomfort or because the cat can no longer perform normal functions such as jumping.

PREVALENCE

In one of the first studies designed to determine the prevalence of DJD in cats, radiographs of 100 cats more than 12 years old (taken as part of a diagnostic work-up for multiple reasons) were retrospectively reviewed,2 and 90% of them showed radiographic evidence of DJD. When the medical records of these cats were examined, only four contained any mention of DJD, but severe DJD of the vertebral column (termed spondylosis deformans) was sometimes associated with neurologic disease. The key question from this study was "Did the failure to observe clinical signs truly represent a lack of clinical signs in the presence of radiographic findings or a failure to recognize signs?"

Obesity and osteoarthritis: Is there a link in cats?

As in dogs,3 the radiographic findings of osteoarthritis do not always correlate well with clinical function. In a radiological study involving cats of all ages, 22% showed evidence of radiographic osteoarthritis, and when patient records were consulted, 33% of these cats also had clinical signs.4 Affected cats were significantly older than the control population. These authors also suggested that there may be little correlation between radiographic and clinical findings or that clinical signs of osteoarthritis in cats are not easily recognized.

In a recent study at a university referral hospital, the prevalence of radiographic signs of DJD was 33.9%, and the prevalence of clinical signs was 16.5%, with most affected cats being 10 years of age or older.1 These authors further classified their findings into DJD and osteoarthritis, the second being less common.

The elbow joint was the most commonly affected joint in some published studies.1,2,4,5 However, when osteoarthritis (rather than DJD) was specifically described, the hip joint was most frequently affected.1 Many cats have multiple affected joints, and bilateral involvement is common.

PATHOPHYSIOLOGY

Osteoarthritis may be categorized as primary or secondary. In primary osteoarthritis, no clear underlying cause is present. With secondary osteoarthritis, an underlying cause can be implied, such as hip dysplasia or previous trauma. In one study, half of the cats with osteoarthritis had an identifiable or historical cause, including radiographically identified hip dysplasia.1 In another study by the same researchers, 71% of cats had no obvious cause and were classified as having primary, or idiopathic, osteoarthritis.5

CLINICAL SIGNS

Compared with the radiographic features of feline osteoarthritis, the clinical signs of feline osteoarthritis are not well-documented. Unlike in dogs, lameness is not common in cats with DJD or osteoarthritis.1,5 Because of cats' small size and their innate agility, they can often cope with severe orthopedic disease. In addition, bilateral involvement is common, which makes lameness difficult to detect. It is also notoriously difficult to elicit pain on clinical examination in some cats.

In one clinical study, 28 cats that had historical or clinical evidence of osteoarthritis, or both, together with radiographic evidence of osteoarthritis were recruited for a prospective study.5 Their median age was 11 years. In addition to the cats' undergoing a clinical examination by a veterinarian, the owners filled out a questionnaire on their cats' general demeanor, food intake, and lameness. On physical examination, periarticular thickening was a common finding but decreased range of motion was not. Crepitus was not detected, and synovial effusions were seldom present. Cats were treated with oral meloxicam for one month and reevaluated. Most owners thought their cats improved, and the most common clinical sign that improved was the willingness to jump and the height of the jump. Improvement of the stiff gait seen in many cats at the beginning of the study was also significant.5

Clinical experience suggests that the behavioral changes that accompany osteoarthritis may be either insidious and easily missed or assumed to be inevitable with advancing age, so the owner does not seek veterinary advice. Because of a pet cat's lifestyle, lameness or exercise intolerance is not a common owner complaint. Changes in behavior such as decreased grooming, a reluctance to jump up on favorite places, an inability to jump as high as before, and soiling outside the litter box should prompt the veterinarian to look for sources of chronic pain (Table 1). Other changes that owners report are altered sleeping habits (an increase or decrease), withdrawing from human interaction, hiding, and a dislike of being stroked or brushed.

Table 1: Signs of Osteoarthritis Clients Should Watch for in Their Senior Cats

Inactivity, which may result from chronic joint pain, is much more difficult to determine in cats than in dogs since cats naturally sleep a lot and are often solitary. In many cases, the owners are not home all day to monitor their cats' activity levels and may not know whether they have changed. Activity monitors attached to cat collars and harnesses have been used to monitor daily movement in cats,6,7 including in cats with osteoarthritis during treatment with oral meloxicam or placebo.7 In that study, the activity counts increased with five days of nonsteroidal anti-inflammatory drug (NSAID) treatment, suggesting alleviation of musculoskeletal discomfort.

It is common for owners and veterinarians not to realize how affected a cat was until they see dramatic improvements after treatment, emphasizing that analgesic trials may sometimes be the only way to confirm that pain was present.

TREATING OSTEOARTHRITIS

Osteoarthritis treatment falls into three categories: lifestyle changes, pharmacological intervention, and nonpharmacological intervention.

Lifestyle changes

Much of the supportive management recommended for osteoarthritis in dogs, such as weight loss and controlled exercise, is applicable in cats but is often more difficult to implement. Overweight cats should be placed on a carefully monitored weight-loss program; several good commercial diets are available to achieve set goals. With creative use of toys and owner participation, exercise can be encouraged in cats.

Chronic pain may prevent some cats from grooming, and owners must perform this task, although the cats may resent this until pain is reduced to a tolerable level. Environmental modifications can simplify a cat's daily routine, such as placing food bowls and litter boxes in positions that do not require leaping or jumping. If cats cannot leap onto favorite resting places such as window ledges and cat towers, it impacts their quality of life. Owners can construct ramps and steps so cats may still reach these spots.

Pharmacological intervention

Any drug prescribed to treat chronic pain must be in a formulation that is simple to administer and well-accepted by cats to ensure optimal owner compliance.

NSAIDs. The mainstay of drug therapy for osteoarthritis in most species is an NSAID. These drugs pose a challenge in cats because of cats' susceptibility to NSAID toxicosis, especially with long-term dosing. Pharmacokinetic data are only readily available for single doses of NSAIDs in cats.* Most NSAIDs have a relatively long half-life in cats, and repeated dosing must be administered and monitored carefully to avoid toxicosis. For example, gastrointestinal toxicosis has been reported with carprofen,8 and based on clinical experience, toxicosis is generally associated with concurrent disease and prolonged administration of the oral formulation.Problems with repeated dosing are likely a result of individual variation in pharmacokinetics because the half-life of carprofen in cats can vary from nine to more than 40 hours.9,10

One NSAID sometimes used to treat the pain and inflammation associated with osteoarthritis in cats is meloxicam, a COX-2 selective NSAID available as an injectable and oral formulation. The injectable formulation is licensed in the United States for use in cats to treat postoperative pain and inflammation but only as a single 0.3-mg/kg dose administered subcutaneously before surgery. The honey-flavored oral liquid meloxicam marketed for dogs is widely used (extralabel) in cats in the United States because it is palatable and small doses are easily measured. An oral formulation of meloxicam is now labeled for long-term use in cats with musculoskeletal disease in many other countries; the registered dose is 0.05 mg/kg once daily.

One published study evaluated the use of short-term NSAIDs for musculoskeletal pain in cats. Sixty-nine cats with acute or chronic locomotor disorders were randomly assigned to receive oral meloxicam or oral ketoprofen.11 Both drugs were equally effective based on general attitude, appetite, weight-bearing, lameness, and pain on manipulation, but meloxicam was more palatable and easier to administer. In a prospective study of 28 cats, many cats with osteoarthritis improved within one month with meloxicam intervention; 61% showed a marked improvement, 14% showed a moderate improvement, and 25% showed a slight improvement.5

In a recently published study of 40 cats with osteoarthritis that were treated with meloxicam (0.01 to 0.03 mg/kg once daily orally) for 5.8 ± 3.4 months,12 85% of owners scored the efficacy of meloxicam as good or excellent and thought that palatability was excellent. The 40 cats with osteoarthritis were paired with 40 control cats of similar age (12 or 13 years). No difference was noted in illness events (type or number) between the two groups. No differences were found in serum creatinine concentrations between enrolled and control cats before and after treatment. Three cats with preexisting renal disease were enrolled in the meloxicam group, and the renal function of these cats did not deteriorate faster than that of the three cats in the control group that had similar blood urea nitrogen and serum creatinine concentrations; however, no statistical analysis was possible because of the small number of cats in this category.

With care, oral meloxicam can be used long-term in cats, and doses lower than those approved for use in cats in other countries can markedly improve their comfort. Most owners are able to gradually decrease the dose to that which keeps their cats comfortable; the key to successful long-term NSAID administration in cats is to use the lowest effective dose.

The most common side effect of orally administered meloxicam in cats is gastrointestinal upset. In one study, 18% of cats showed intermittent signs of gastrointestinal upset (vomiting or diarrhea),5 but the signs were not severe enough to terminate treatment in any cat. In the largest clinical study to date,12 four out of 46 cats vomited during meloxicam treatment, and two of these cats were withdrawn from the study.

*For a comprehensive review of NSAIDs in cats see Lascelles BD, Court MH, Hardie EM, et al. Nonsteroidal anti-inflammatory drugs in cats: a review. Vet Anaesth Analg 2007;34(4):228-250.

As with dogs, monitor cats receiving NSAIDs for chronic pain for side effects related to renal and hepatic function and gastrointestinal erosions. No monitoring guidelines have been established, but a serum chemistry profile, packed cell volume, and total protein measurement are recommended before you initiate treatment and should be repeated at one and four weeks. After this, the timing of follow-up blood work is arbitrary and will vary in each patient. As with dogs, it is important to discuss the possible side effects of these drugs with the owner, and if vomiting or anorexia occurs, the client should stop administration and seek veterinary advice. It is also imperative to inform the owner that under no circumstances should any over-the-counter anti-inflammatory drugs such as aspirin be given in conjunction with an NSAID. Continual reassessment of the patient both by the veterinarian and owner is important so that the dose can be tapered to the smallest effective amount.

Opioids. In my experience, because of their side effects such as euphoria, inappetence, and constipation after long-term use (days or weeks), opioids are best reserved for terminal or hospice care in cats or for occasional breakthrough pain while the cat is receiving other analgesics or undergoing other treatment modalities.

Other analgesic drugs. The role of other drugs in alleviating chronic osteoarthritis-related pain has not been well-studied in cats. Several drugs have, however, been suggested.

Tricyclic antidepressants including amitriptyline, clomipramine, and imipramine can provide relief in people with chronic neuropathic pain and are thought to alter the actions of serotonin and norepinephrine both centrally and peripherally.13 Anecdotal reports suggest that amitriptyline (between 2.5 and 12.5 mg for an adult cat orally once a day) may be effective for chronic pain syndromes in cats including osteoarthritis.

The anticonvulsant gabapentin is clinically effective in relieving some types of chronic pain in people, although the mechanism of action is not clear.14 Individual case reports demonstrate that gabapentin shows promise in cats,15 and doses starting at 10 mg/kg given orally twice a day have been suggested.16 Some animals become sedate and occasionally ataxic when given gabapentin. These signs may decrease as a cat acclimates to the drug or can be managed by decreasing the dose. In my experience, the tablet and capsule formulations of gabapentin are more palatable and easier to administer to cats than the liquid oral formulation, and they don't contain xylitol.

Although not classified as an opioid, tramadol has weak binding affinity at mu-receptors. It is thought to activate monoaminergic spinal inhibition of pain, although this may not apply to nonprimate species. It can be administered by multiple routes, including orally, is effective for relieving moderately severe acute and chronic pain in people, and seems remarkably devoid of the usual undesirable side effects of opioids such as respiratory depression and tolerance.17 The pharmacokinetics of tramadol and its major metabolite has recently been reported in cats18 ; doses in cats would likely be less than those in dogs (on a mg/kg basis) and the dosing interval may be longer. As of yet, no published studies have reported tramadol's efficacy for relieving osteoarthritis pain in cats. The chief problem with tramadol is its bitter and unpleasant taste, which makes administering it to cats difficult, even after compounding with fish or chicken flavors.

Nonpharmacologic intervention

Rehabilitation therapy (underwater treadmill, passive range of motion, creative exercise techniques), nutraceuticals (e.g. chondroitin sulfate), and acupuncture are also used to treat osteoarthritis in cats with varying success. Many cats are more tolerant of these interventions than you might think.

Editors' note: Dr. Robertson has received financial support from Boehringer-Ingelheim Vetmedica to conduct workshops for Boehringer-Ingelheim veterinarians, write one review article, and speak at veterinary continuing education meetings.

Sheilah A. Robertson, BVMS (Hons), PhD, DACVA, DECVA, CVA, MRCVS

Section of Anesthesia and Pain Management

College of Veterinary Medicine

University of Florida

Gainesville, FL 32610

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