There are a variety of models for assessing pain in horses using either letters or numbers. Perhaps the most searching and useful method for pain assessment of MSKD involves assessing a variety of criteria: degree of lameness/weight bearing on affected limb, restlessness, head-lowering, teeth-grinding, flaring of nostrils, sweating, rigid posture, reluctance to be handled, aggression in a normally placid horse, weight loss, and changes in eating, drinking and sleeping patterns.
The management of pain associated with MSKD historically has been almost exclusively via the use of non-steroidal anti-inflammatory drugs (NSAIDs), phenylbutazone (PBZ) being the most popular of these, largely through a combination of tradition, cost and convenience.
Whereas it could be argued that pain management is an area of veterinary medicine that still requires considerable research and thence more understanding. There have been some interesting recent studies that have begun to develop this understanding, particularly in the comparative mechanisms of action of different NSAIDs.
The potential side-effects of PBZ are well documented – these effects are a result of PBZ’s properties of inhibiting both COX-1 and COX-2 in the inflammatory pathway; COX-1 is known as the “constitutive” enzyme, helping to preserve gastric mucosal integrity and renal blood perfusion. In contrast at the recommended oral dose of meloxicam there is no association with gastric ulceration.
There is ongoing research looking at the nature of pain and pain reception comparing mechanical and inflammatory pain reception. Inflammatory pain appears to be more centrally mediated whereas inflammatory pain is in part peripherally mediated. This has led to the suggestion that PBZ, owing to its property of COX-1 inhibition, may be better for mechanical-associated pain whereas meloxicam, as a preferential COX-2 inhibitor, may better for inflammatory-associated pain.
One of the well-documented but often overlooked effects of some NSAIDs is on articular cartilage. Many older NSAIDs including PBZ, naproxen and ibuprofen are known to be chondro-toxic i.e. may have a deleterious effect on cartilage healing. By contrast, recent studies have shown that meloxicam is at least chondro-neutral and may even have a positive effect on the healing of synovial structures.
Regarding clinical efficacy, studies have shown that PBZ and meloxicam are equivalent in their respective efficacies in the management of MKSD.
Finally, a word about stomach ulcers in horses. As horses have evolved to be continually feeding (“trickle” feeders), acid is continually secreted into the stomach; this means that prolonged periods without food to neutralise that acid can lead to ulceration. Studies indicate that gastric ulcers occur in up to:
- 37% of leisure horses
- 63% of performance horses
- 93% of racehorses
Foals are also at particular risk - around 50% of foals develop stomach ulcers, particularly in the first few months of life.
Clearly from these data the selection of an NSAID for pain management in horses needs to be carefully considered.
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Written by Chris Taylor MRCVS, Virbac Technical Director