Nonsteroidal anti-inflammatory drugs in the cat

The recognition of pain in the cat is difficult when compared to other species of domestic animals and humans. Acute pain is probably easier to recognise than chronic pain in the cat. Animals in acute pain will often remain quiet and immobile and have a tense appearance. Occasionally they may become aggressive and resent handling. Vocalisation is relatively rare apart from the occasional growl although this may to some extent be related to the breed of cat. In fact, they may even continue to purr. During recovery from major surgery cats may demonstrate a manic reaction on emergence from anaesthesia which may be difficult to differentiate from an acute pain response. If acute limb pain is severe they may attack a dressing or even attempt to discard it. This behaviour may be accompanied by considerable vocalisation. After abdominal surgery, cats which are in pain may adopt sternal recumbent position and tense their abdominal muscles. If post-operative pain is present in the facial region cats may attempt to rub or scratch the affected area which may compromise a successful outcome to the procedure ( ). Chronic pain is often associated with a number of conditions including neoplasia, oral disease, wounds and dermatitis in addition to osteoarthritis and degenerative joint disease. The incidence of these latter conditions has probably been underestimated in cats ( ). The behavioural changes associated with these conditions may be insidious in onset and easily missed and are often attributed by the owner to the ‘ageing process’. Lameness or exercise intolerance are not common complaints by owners. There may well be behavioural changes which can generally be classified as a reduced ability to ‘care for themselves’ and an inability or reluctance to jump, particularly up onto or down from surfaces. It may well be that these changes are so subtle that they are not appreciated by owners until they observe the obvious improvement after appropriate analgesic therapy ( ). The use of nonsteroidal anti-inflammatory drugs (NSAIDs) in cats is the subject of a review in this issue of the journal ( ). These drugs are a group of weak organic acids, carboxylic acids (aspirin, carprofen) and enolic acids (meloxicam, phenylbutazone). Their main therapeutic effects of reduction of fever, pain and inflammation are due to their inhibition of prostaglandin production from arachidonic acid by the cyclooxygenase (COX) enzymes. There are two distinct forms - COX-1 and COX-2. COX-1 is involved in the regulation of gastrointestinal and renal blood flow and has a role in blood clotting. In contrast, COX-2 is an inducible enzyme which is expressed at the sites of inflammation in response to inflammatory mediators. Earlier published work had indicated that the prostaglandins that mediate inflammation were produced by COX-2 whereas those that were important in gastrointestinal and renal function were produced via COX-1. This gave rise to the belief that NSAIDs exerted their useful therapeutic effects by COX-2 inhibition, whilst the inhibition of COX-1 was considered to be responsible for some of the toxic side-effects. It is now clear that this was very much an oversimplification and that the whole subject is more complex and species differences may also be important. Most NSAIDs are metabolised in the liver and excreted via the bile duct or kidneys. As the cat has a reduced ability for glucuronidation of drugs then it follows that agents (aspirin, carprofen) which are metabolised by this route will have a longer drug elimination half-life in this species than in the dog. In contrast, drugs (meloxicam) which are cleared by oxidative enzymes have a similar or reduced half-life in cats when compared to dogs. A number of adverse effects, following the administration of NSAIDs have been reported in cats. However, renal effects appear to be relatively rare following a single dose of carprofen or meloxicam to healthy cats. It is under conditions of dehydration, hypovolaemia and hypotension, that NSAIDs may impair auto regulation and lead to produce a decrease in renal blood flow often progressing to acute renal failure and death. Whilst gastrointestinal ulceration is known to occur in a number of species following NSAID administration, the situation in the cat appears to be far from clear as to the mechanisms involved. It is well known that NSAIDs affect haemostasis by their effects on platelets and vascular epithelium. Aspirin has been used as an anticoagulant in the cat and other agents have been studied for their effects on clotting in relation to surgery and the situation is again far from clear. However, none of the NSAIDs which have licence/approval have been evaluated for their effects on clotting and no significant changes have been reported. If cats should accidentally ingest NSAIDs then an emetic should be administered within the first hour and after that activated charcoal has been recommended. Treatment is basically on a symptomatic basis with the administration of fluids and supportive therapy with H₂-receptor agonists or protein pump inhibitors to reduce the gastrointestinal effects.