CHRONIC degenerative osteoarthritis is a common condition in the older rabbit but, because they are prey animals in the wild dependent on quick response to threat, they instinctively try to hide and suppress signs of illness. This makes it difficult to recognise in the early stages.
Domestic rabbits live much longer than their wild counterparts and are becoming increasingly popular as house pets in the UK. Because of this it is not unusual to see animals of eight years and older as patients in the modern veterinary practice. Unfortunately, as they age they become prone to degenerative disease processes affecting the integrity and function of synovial joints.
The structure of a synovial joint is similar in all species. It is composed of bone, articular (hyaline) cartilage, synovial fluid and membrane as shown in Figure 1. Its stability is preserved by the surrounding joint capsule, ligaments and muscles. In the healthy animal, normal joint function allows adjacent bones to articulate freely against each other, permitting easy pain-free fluid motion.
The highly enervated subchondral bone of the joint is protected by a covering of articular cartilage, which provides a smooth surface for minimal friction during movement. Cartilaginous tissue is highly differentiated and resilient. It is able to withstand load bearing, impact absorption and shearing forces without pain.
It can be seen in Figure 2, the x-ray showing a normal stifle joint of a rabbit, that all joint surfaces are smooth to allow natural function. However, because it is avascular, contains no nerves or lymphatic tissue, and relies on the intermittent compression/decompression associated with movement to obtain nutrients, it is slow to heal following injury.
In osteoarthritis, metabolic imbalance leads to cartilage degradation, exposure of underlying bone, creation of subchondral cysts, and osteophyte formation around joint edges as illustrated in Figures 3 and 4. Often this latter condition precedes cartilage alteration 1 . These modifications lead to pain and reduced mobility.
Despite this extreme pathology, the first change noticed by the owner is often a sudden intolerance to handling. The rabbit will be presented for consultation because it has become aggressive and may have lost its appetite. Gross examination will reveal the animal is moving oddly, shuffling, and unable to hop. There may also be obvious flaky patches on the skin because the rabbit is no longer able to clean itself properly.
At this stage, several differential diagnoses may be considered relating to the animal’s altered behaviour, including, inter alia, ulcerative pododermatitis and Encephalitozoon cuniculi. Physical assessment will demonstrate pain, swelling, crepitus and possible joint instability, whilst an x-ray will confirmadiagnosis of osteoarthritis in most cases.
As the condition progresses, affected animals become increasingly reluctant to move and consequently develop muscle atrophy, sore lesions on the hocks, and soiling of the perineal area.
Reduced quality of life
They may grind their teeth, lose their appetite, experience difficulty with squatting, and have problems using the litter tray if litter trained. This greatly reduces their quality of life and consideration of the various alternative treatments available must be made.
Veterinary surgeons are in a unique position to help prevent or delay the onset of osteoarthritis in the domestic pet. Annual vaccinations against myxomatosis and viral haemorrhagic disease provide the ideal opportunity for this.
The importance of a dry draft-free environment and weight control can be focused on, whilst more general advice on management and care can also be given at this time. In the wild animal it is essential for body weight to remain low as extra mass slows reaction time. It is equally important to the domestic pet, as obesity is a risk factor in the initiation of chronic degenerative osteoarthritis in all species.
There is apocryphal evidence suggesting that a significant number of domestic rabbits in the UK are obese. Because domestic rabbits are kept under artificial conditions, their natural behaviour is limited. They tend to eat more than the wild rabbit and live a sedentary lifestyle; all of which increase the risk of obesity and its attendant problems.
The main aim of treatment is a reduction in pain coupled with preservation of mobility for the affected animal. Treatment with non-steroidal antiinflammatory drugs or analgesics is useful in the short term but is only palliative in action.
In the long term, danger of severe side effects limits their usefulness. At present there are no licensed products available to restrict disease progression, although increasing use of nutraceuticals as food supplements within the veterinary industry may help.
Glucosamine, a product that occurs naturally in the body, is a key element in the formation of glycosaminoglycans (GAGs), components necessary for the synthesis of synovial fluid and articular cartilage. The pivotal stages in this process are shown in Table 1.
As animals age, their ability to formulate glucosamine, using glucose and amino acids, diminishes. This increases their susceptibility to degenerative joint disease because they are no longer able to keep the formation and destruction of cartilage in a state of equilibrium.
Glucosamine, when given orally, is taken up preferentially and used for cartilage production. Because of this it might be expected to assist with cartilage maintenance in the older animal. It may help cartilage renewal and increase mobility in affected rabbits. As diagnosis is difficult in the early stages, use of the diet as a prophylactic measure is recommended.
Commercially available diets for rabbits, containing glucosamine hydrochloride, have recently become available. The quantity of glucosamine added to these products removes the need for supplementary dosing. As the diets are produced in the form of nuggets, which are identical in shape and colour, they avoid the danger of selective feeding, making compliance easier.
It is important, however, that the products are given in combination with explicit management regimens designed to preserve quality of life, including optimal diet and exercise control. Limited exercise protocols combined with regular physiotherapy are necessary because both prolonged immobility and excessive exercise can aggravate symptoms. The provision of thick padding to wick away moisture, non-slip flooring and low-sided litter boxes is advised.
It is inevitable that the ageing process in any animal will be accompanied by reduced mobility but, with good husbandry, this need not be debilitating.
A combination of good management practice, with attention to diet as a significant dynamic, can help maintain good health throughout life. The addition of glucosamine to the food may be a major contributory factor in maintaining vitality and well-being.
1. Yoshioka, M. et al. (1996) Osteoarthritis and Cartilage 4:87-98.
