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Feed For Thought: The unsound horse: Is it joint pain or muscle pain?

Published: 2013-12-06

With modern veterinary medicine, it is quite easy to get into an inject-a-thon trying to pinpoint the source of your equine’s pain using the common assumption that arthritis is the culprit. But what if it’s actually your horse’s muscles that are causing them pain? Just like in people, horses often suffer from delayed onset muscle soreness (DOMS) which is muscle soreness or stiffness that can occur 1-3 days after the original exertion. While it is a common misconception that DOMS is caused by lactic acid build up in the muscles, it is actually the result of damage to the muscle cell membranes caused by the free radicals (highly reactive compounds that want to combine with everything) left over from the energy generative processes which fuel the working muscles (Schwane et. al. 1983). The open muscle cell will then spill its contents of proteins and enzymes into the surrounding areas resulting in inflammation, limb lameness & reduced performance.

Fortunately, unlike the expensive treatments for arthritis, treating muscle soreness may be minimized simply by ensuring that your horse is consuming a ration appropriate for its level of work & life stage combined with a sensible training schedule. Some horses will experience chronic muscle soreness, particularly over their back: these horses in particular seem to benefit greatly from a re-evaluation of their feeding program, paying particular attention to the nutrients critical for muscle health. For horses experiencing either chronic or acute muscle soreness, the nutrients most involved in muscle function and efficiency are vitamin E, thiamine, selenium, and magnesium (NRC 2007). Next week we'll look into each of these nutrients in more detail.

But, how can you tell if the horse has a muscle issue or a joint issue? There is actually a clear cut way to figure out the cause of your horse's pain: two simple blood tests. If you recall the enzymes mentioned earlier spilling from the broken muscle cells into the surrounding tissues, two of those enzymes are creatine kinase (CK), lactate dehydrogenase (LDH) and aspartate aminotransferase (AST). Ideally, one would test for the LDH- 5 varient since that isoenzyme is only in the liver and skeletal muscle, both of which are most affected by muscle issues. While CK is skeletal muscle specific, AST and LDH is also used as a tool for liver function, though it is still useful in evaluating the severity of muscle issues. It is important to note that often times only the "catch-all" LDH test is available, which may indicate a rise of the enzyme's activity in a variety of other areas as well, such as the heart or lungs.

One of the two blood tests would be taken in a pre-exercise state, often after a day of rest, while the second blood test should be taken 2-6 hours post-exercise (Harris et. al. 1990). A normal horse will see a slight rise in CK, LDH5 & AST levels. When a moderate rise in CK, LDH 5 & AST is seen, it is possible that the horse is experiencing some muscle soreness & may even be prone to rhabdomyolysis (tying up) if not addressed. When CK, LDH5 & AST rise dramatically post-exercise, then the horse may be currently tying up (often times the resting values are also slightly increased). If resting levels are significantly raised, then there is the possibility that the horse had some acute muscle injury that needs to be addressed. If the horse is showing stiffness or lameness & no abnormal increases are seen in CK, LDH5 & AST levels post-exercise, then one should look at non-muscular causes.

REFERENCES: Harris PA, Snow DH, Greet TR, Rossdale PD. Some factors influencing plasm AST/CK activities in thoroughbred racehorses. Equine Vet. J. Suppl. 9:66-71. NRC. 2007. Nutrient Requirements of Horses, ed. 6th. Washington DC: National Academy Press. Schwane JA, Johnson SR, Vandenakker CB, Armstrong RB. 1983. Delayed-onset muscular soreness and plasma CPK and LDH activities after downhill running. Medicine and Science in Sports and Exercise. 15: 51-56.