Monday, March 9, 2015

2015 PNER Conference Panel Review – Tying Up in the Arabian Endurance Horse – Part II

Disclaimer: I am not a scientist studying these issues, I’m just a curious individual with enough of an equine science background to make some sense of the biology behind these topics. This post is based on notes taken during the panel given by Dr. McKenzie at the PNER Conference this year, and later research conducted online. Horse health topics fascinate me. Check out this posts internal links if you want to read more into the science behind the tying up disorder.

So what about Arabians and the endurance sport?

As a very brief review, Tying up is the broad term for muscular breakdown and the symptoms that arise from that breakdown. Exertional rhabdomyolysis (ER) is the catch-all term for this disorder – and ER has several causes including an abnormality in the muscle tissues themselves or just muscle breakdown from overwork.

Preliminary research in Arabians and part-Arabians (and specifically horses competing in endurance sports) suggests another cause for tying up other than the disorders apparent in Quarter Horses and Thoroughbreds. Tying up in Arabs often accompanies fatigue. Symptoms include firm muscles, red urine, poor heart-rate recovery, a dull appetite, and non-specific lameness. 

The research studies targeted 50-mile endurance races for their field research, drawing blood before the ride and four hours post-ride. They also took hair from the horses to prove ER in Arabians was not the specific disorder PSSM, and had the owners of the horses fill out a questionnaire about their horses. The results of this study showed that horses get muscle damage from any endurance ride. This is not necessarily a bad thing by itself, as muscle damage is an expected result of exercise (this is how we build muscle, ultimately, whether you are a human or horse athlete). However, the degree of muscle damage seemed to be a good indicator of whether the horse had ER or might be prone to episodes of tying up.

Four of the 101 horses in the initial study have severe muscle damage based on the high creatine kinase (CK) levels in their blood. Surprisingly, the owners of these horses said they had never tied up. Since only one of these horses was pulled during the ride in question, and the others were symptomatically normal, ER in Arabians may be very clinically subtle! This means that horses displaying this level of muscle damage might not be showing the standard symptoms of tying up even though they have some underlying muscle abnormality associated with ER.

Another study looked at Arabians who had a history of tying up by taking muscle biopsies at different periods. These horses’ muscle glycogen was normal – so the tying up in this case was not caused by the disorder PSSM. Based on the biopsies, one possibility is that ER in these horses is a disruption of the fibers in the muscle cells. The alignment of these fibers does not look normal – the myofilaments are disorganized and scattered where they would normally run in more or less parallel lines.

A final study did metabolic testing of endurance Arabians and part-Arabians after mild exercise. Here the researchers were interested in the blood CK levels as well – but all the horses tested after mild exercise had normal blood CK levels. This can be explained by the fact that these were endurance horses, more accustomed to much greater athletic effort! Unlike other breeds with ER, Arabians with a history of tying up will probably appear completely healthy when compared with normal horses until they have an episode.

So what we know so far about tying up in Arabians: it’s a NEW disorder and more about filament alignment in the muscle cell than how glycogen is stored (as with PSSM) or how calcium is regulated within the cells themselves (as seen in Thoroughbreds). Even in non-symptomatic horses (who owners say have never tied up recognizably) the filament disorganization is evident in muscle biopsies. Increased age seems to be a risk factor in these horses, meaning that the older a horse gets, the more likely it is to tie up.

Interestingly, many endurance horses with this muscle-filament abnormality seem to do very well when they are not tying up – even getting best condition awards. Like other kinds of ER, it’s possible that we could be selecting for the disorder in the Arabians we use for endurance.

What can you do about it if your horse displays symptoms of tying up?

Altering a horse’s diet shows a lot of promise in preventing episodes of tying up. Various studies show that a low-carbohydrate diet helps prevent symptoms for those horses that present with chronic ER. So a low starch and high fat diet is very beneficial for these horses and in fact respond rapidly to the high-fat and low-carbohydrate diet. Another benefit of this diet (particularly for breeds where nervousness is a trigger) is that a high fat diet is associated with les anxiety in the animal. (The studies did show that Thoroughbreds with ER were more intolerant to a starchy diet than other breeds.) The good news is: most endurance horses already receive the ideal diet for chronic ER to prevent episodes of tying up. Since we want our endurance horses to be fueled by fat, this helps those individuals who might have ER of this new type. For endurance athletes, amino acid supplementation may help as well. 

Regular exercise is also a good way to manage a horse that ties up, since periods of rest after a hard effort or coming back to work after time off are common triggers associated with tying up. Stall rest for 48 hours in these horses can be very detrimental for horses with any kind of ER, but working the horse daily can ward off episodes of tying altogether. Anecdotally, endurance horses that do fine and complete a race with good scores often tie up when they are brought home for a well-deserved rest when they tie up at all. Meaning, even though you want your horse to rest after a race, a horse that is prone to tying up may do better with daily short but brisk exercise after an endurance ride. In the study in Thoroughbreds, they tried a half hour of high speed exercise daily and that reduced the occurrence of tying up symptoms.

Remember that: Dr. McKenzie noted that tying up can be triggered by something in the horse’s environment (such as being overworked to the point muscle breakdown occurs). So tying up can occur in any horse, regardless of their breed or genetic makeup, if certain stressors are present. It’s best to avoid stressors like overwork and nutritional deficiencies that might trigger a sporadic episode of tying up, but if you are still having a horse tie up without these stressors, it would be a good idea to investigate whether your patterns has a genetic abnormality that might call for some additional management steps to keep them healthy and happy.

As for my closing thoughts:

Since horses without the genetic link can still tie up just from working too hard, I believe it is doubly important to be sure we don’t over-exert our equine partners. We, as equestrians, ask our horses to work with us, and it’s our responsibility to listen to them and make sure we prepare them for what we ask.

I also have to point out that these disorders are hereditary and we may have been selecting for them unintentionally (or perhaps intentionally). PSSM can be tested for now (using hair), and testing has become more common among the Quarter-type breeds. There is not yet an easy test for the Arabian disorder (muscle biopsies are not that fun), and since it appears to be so asymptomatic compared to what is seen in other breeds, it’s even harder to trace. But we should try and trace it because tying up is terribly painful and stressful for the horse in question, and just because a horse does not appear to be tying up in the traditional sense, these horses’ blood CK levels indicates abnormal damage to the muscles. We should not breed those horses that have a history of chronic tying up to prevent future generations from being in pain or needful distress.

I believe equestrians do have a responsibility to not breed horses with conditions that cause them harm in some way, even if it means they might win more races and get a higher score at a show. Over-breeding is already a huge problem in the United States, so let’s avoid this added problem and have our horses tested when tests become available.

I’ve linked to various scientific studies throughout this post, but if you are interested in exploring further, I recommend these papers and articles:

Hereditary skeletal muscle diseases in the horse. A review.

Exertional rhabdomyolysis: Does elevated blood creatine kinase foretell renal failure?

It’s interesting to know these things, even if you have been fortunate enough to avoid tying up!

Last time: What we know about tying-up in general - 2015 PNER Conference review.