Professor, Institute of Comparative Medicine, Division Companion Animal Science, University of Glasgow Veterinary Faculty
Bearsden, Glasgow, UK
The WSAVA HILL's Pet Mobility Award
1. Clinical Features
Traditionally, it has been assumed that osteoarthritis (OA) in the cat is very rare and/or the cat does not suffer with clinical disease1. However, there have been several recent publications where this has been challenged and it is now apparent that osteoarthritis is an important clinical disease in the older cat and many cats in significant chronic pain are going untreated. One of the main explanations is the fact that lameness is not the most common clinical feature of feline OA2. The most common features are a reduced level of activity and a reduced ability to jump. Since OA is more common in the older cat, these changes in lifestyle are often explained as the effects of old age rather than as an indication of chronic pain. Reduced levels of activity include less time spent outdoors, less time spent playing/interacting with the owner and less time hunting compared to previous. The reduced ability to jump includes hesitation, refusal to jump or jumping only reduced heights compared to previously. Assessment of lameness in feline patients is obviously difficult although a stiff gait and an obvious limp are sometimes reported by the owner although not necessarily confirmed at the time of clinical examination. The owner is key in diagnosing feline OA but must be asked the correct questions; the use of mobility questionnaires is very helpful.
In the prospective study reported by Clarke and Bennett (2006) the median age was 11 years2. The domestic short- hair was the most common breed represented and there was a male to female ratio of 1.6:1.0. Fourteen per cent of the cats were thought to be obese. The elbow (59%) and hip (48%) joints were the most commonly affected with only 5.5% of shoulders, 7% of stifles and 9% of hocks showing clinical disease. Periarticular joint thickening is often very subtle and quite difficult to detect on palpation. Synovial effusion is rare and very few cats show a reduced range of movement of affected joints. Crepitus is very rare. Most cases of feline OA appear to be primary or idiopathic rather than secondary. In the study by Clarke et al (2005), 63.8% of cases were primary, 14% had hip dysplasia and secondary OA and the remaining 22.2% had osteoarthritis following some traumatic event such as a luxation and/or fracture3.
2. Radiographic Features
The radiographic prevalence of degenerative joint disease (DJD) disease in cats is significantly high. Clarke and colleagues (2005) examined 218 cases which included cats of all ages (mean age 6.5 years) and found a prevalence of appendicular osteoarthritis of 16.5% and spondylosis deformans of 15%3. The total prevalence of radiographic DJD including all categories was 33.9%. The prevalence of OA increased significantly with age and the hip and elbow joints were most commonly affected. In an earlier study by Hardie et al (2002) the radiographic prevalence of DJD was found to be 90% although these authors only examined cats older than 12 years of age (mean age 15.2 years)4. They found a prevalence of 26% axial DJD, 10% appendicular DJD and 54% appendicular and axial DJD together. The elbow was most commonly affected. Godfrey (2005) examined 292 cats with an average age of 9.5 years but only assessed the appendicular skeleton5. He found a prevalence of 22% with elbows, hips and stifles most commonly affected. 27% of cats had one joint affected, 59% two, 9% four and 5% three joints.
The presence of osteophytes is the key radiographic feature of feline OA although these can sometimes be difficult to identify. Soft tissue mineralization and intra-articular mineralised bodies are more common features of feline OA, particularly in the elbow joint, compared to other species. Soft tissue thickening and synovial effusion are generally less apparent in feline OA.
Osteoarthritis of the shoulder joint is characterised by osteophyte formation on the caudal rim of the glenoid and on the caudal edge of the humeral head. The clavicle is very obvious in the feline shoulder and should not be confused with a pathological lesion. The cat also has a very prominent coracoid process of the glenoid and its appearance is influenced by radiographic positioning; it is easily mistaken as new bone formation on a caudo-cranial film.
Although very subjective, the presence of subchondral sclerosis beneath the ulnar articular notch is a key feature of feline elbow OA. Apparent subchondral sclerosis may be due to the increased thickening of the bony trabeculae beneath the articular surface, by osteophyte formation at the articular margin, by soft tissue mineralization and/or mineralised intra-articular bodies superimposed on the bone. Most osteophyte formation appears to occur on the medial side of the joint, the distal humerus and proximal ulna. The supinator sesamoid becomes more obvious in an osteoarthritic joint and is often increased in size. Soft tissue mineralization and mineralised intra-articular bodies are regularly seen in the feline elbow and can be very extensive (the two are difficult to distinguish without pathological studies.)
The carpus is the least affected joint. Soft tissue mineralization and osteophyte formation are seen.
Although hip dysplasia is well documented in the cat, it only accounts for about 22% of cases of hip OA2. This of course is influenced by how hip dysplasia is presently defined in the cat. Kellar et al (1999) reported a 6.6% prevalence of hip dysplasia in a hospital population of cats6. The Maine Coon breed is reported as having a prevalence of 18%7. Increased joint laxity within the feline hip joint in cases of dysplasia has been demonstrated8. Clarke and Bennett (2006) recorded a Norberg angle for the feline dysplastic hip of 87.5 degrees compared with 99.2 degrees for the normal hip, the cat having a shallower acetabulum than the dog2. Osteoarthritis of the hip joint is characterised by osteophyte formation, particularly on the cranial effective acetabular rim and around the femoral neck. Osteophytosis of the femoral neck is generally seen as a sclerotic line.
OA of the stifle joint is characterised by osteophyte formation on the patella, around the trochlear margin and on the caudal edge of the tibia. Soft tissue mineralization is also often seen in the arthritic stifle joint. It is important to remember that the cat generally only has one fabella (lateral) visible on the radiograph and this can be a site of new bone deposition in the arthritic joint. It is common in the cat to see mineralization within the cranial pole of the medial meniscus and this may represent a degenerative calcification within the meniscus or possibly the presence of a meniscal sesamoid bone (also called a lunula). In most cases mineralization of the medial meniscus is an incidental finding of no clinical significance. Mineralisation within the cranial cruciate ligament is also occasionally seen. Enthesiophyte formation at the attachment of the patellar ligament on the tibial tuberosity is commonly seen. This may occur as part of stifle OA but again is often seen as a solitary lesion which appears to be of no clinical significance.
OA of the hock is characterised by osteophyte formation soft tissue mineralization, and mineralised intra-articular bodies. As with the elbow, abnormal mineralization can be extensive.
Meloxicam is a very effective non-steroidal anti-inflammatory drug for treating chronic pain in osteoarthritic cats. In the study by Clarke and Bennett (2006) there was a marked improvement in over 60% of cases2. Many cats showed significant improvements in their activity levels, ability to jump and improvement in lameness, when present. Meloxicam is easy to administer, mixed with the food and the palatability is excellent. Side-effects are rare although occasional vomiting and diarrhoea are reported. The dose is 0.1 mg/kg orally on day one and then 0.05 mg/kg once daily; it is recommended to always give a full course of treatment i.e., daily for 3-4 weeks. If continuous therapy is required, then the dose can be reduced based on clinical response. Meloxicam is thought to be metabolised in the liver by oxidative pathways and thus the relative lack of glucuronyl transferase which is a feature of cats, is not a concern as far as toxicity is concerned. Since it is mainly old cats which suffer OA routine blood analyses are advisable to assess liver and kidney status. It is important to encourage adequate fluid intake in cats receiving NSAIDs and thus feeding moist (tinned or sachet) foods is important. Cats which are overweight should be placed on a reducing diet since weight loss will help in improving the clinical signs and may negate the use of NSAIDs or allow a lower effective dose. However, achieving weight loss in the feline is often a challenge. Obviously, alterations to the animal's environment can be made such as reducing the necessity for jumping, using litter trays which are readily accessible and providing a comfortable bedding. Environmental 'enrichment' is also an important consideration with the painful feline patient. This ranges from simple 'touch' therapy and grooming to ensuring that the cat feels as secure as possible in its 'core' and surrounding territories. Pheromonatherapy may also help to reduce 'stress' situations and help the cat cope with its chronic pain. Physical therapies are also now being introduced to the feline patient with encouraging results. Acupuncture is described for relieving OA pain in the cat.
Special diets are also available which can increase the levels of essential fatty acids and the matrix supplements, such as glucosamine and chondroitin sulphate. The Hill's feline j/d diet is rich in the essential fatty acids docosahexaenoic acid and α-linolenic acid, which have been shown to be those most relevant to the feline species. Increasing the levels of these omega-3 fatty acids will theoretically reduce the level of inflammation within the joints. Studies so far, although restricted, have shown a beneficial effect of this diet in reducing the clinical signs of OA in the cat. The diet can be used in combination with NSAIDs and will often result in a lower dose of the anti-inflammatory drug being effective; the moist form of the diet should always be used when NSAIDs are given. The matrix supplements and also the essential fatty acids can be given as direct supplements rather than as part of a complete diet.
Currently the feline population is an ageing one9 and thus OA is more and more likely to be diagnosed. In the last 10 years the percentage of cats over 6 years of age in the USA has increased from 24% to 47%; cats over 10 years have increased by 15% and over 15 years have increased from 5% to 14%. The average age of cats in Europe has increased from 4.7 to 5.3 years over a similar period. There are now 20 million senior cats in Europe (30% of the total cat population) and the majority of these will be suffering clinical OA.
All owners of mature/senior/geriatric cats should be asked to complete a mobility/lifestyle/behaviour questionnaire at least once a year to help identify those animals which might be suffering chronic arthritic pain. This should include all cats presenting for vaccination or other problems. The practice nurse is ideal for organising and supervising this.
1. Ness MG, Abercromby RH, May C, Turner BM, Carmichael SC. A survey of orthopaedic conditions in small animal veterinary practice in Britain. Vet Comp Orthop Traumat.,1996; 9; 43-52
2. Clarke, SP, Bennett D. Feline osteoarthritis: a prospective study. Journal of Small Animal Practice. 2006;47: 439-445
3. Clarke SP, Mellor D, Clements DN, Gemmill T, Farrell M, Carmichael S, Bennett D. (2005) Radiographic Prevalence of Degenerative Joint Disease in a Hospital Population of Cats. Veterinary Record. 2005; 157: 793-799.
4. Hardie EM, Roe SC, Martin FR. Radiographic evidence of degenerative joint disease in geriatric cats: 100 cases (1994-1997) Journal of the American Veterinary Medical Association. 2002; 220: 628-632
5. Godfrey DR. Osteoarthritis in cats: a retrospective study radiological study. Journal of Small Animal Practice. 2005; 46: 425-429
6. Keller GG, Reed AL, Lattimer JC, Corley EA. Hip Dysplasia: A feline population study. Veterinary Radiology & Ultrasound. 1999; 40: 460-464
7. Mills DL. Feline Joint Problems. Proceedings of the 5th American College of Veterinary Surgeons 1995; 533-535
8. Langenbach, A., Giger, U., Green, P., Rhodes, H., Gregor, T.P., LaFond, E. Smith, G. Relationship between degenerative joint disease and hip joint laxity by use of distraction index and Norberg angle measurement in a group of cats. Journal of the American Veterinary Medical Association.1998; 213: 1439-1443
9. Gunn-Moore D. Editorial. Considering old cats. J. Small Animal Practice, 2006; 47: 430-431.