The following postings are from a response to a question about thumps in endurance horses.
Mike Tomlinson, DVM
"Thumps" (SDF) is a problem that we have had problems dealing with for many years. In some horses it indicates impending doom in others it is as significant as a slight cough.
We know what causes thumps, we know how to treat thumps, we can do some things to prevent thumps, but no one can tell you the severity of every case of thumps. The worst are easy to agree, the very slight most agree upon, but the vast majority in between are an enigma as to the true consequences in the middle of a ride.
The descision to pull for thumps is just as infinitely variable. Fifteen years ago everybody pulled every horse that even thought of thumping. Ten years ago the west coast decided that a little thumping was not all that worrysome and quit pulling. Five years ago it was decided for the sake of uniformity that everyone would once agian pull all thumping horses. (Dates are very approximate and gross generalities only).
I have personally known horses that thumped on every ride and did very well (during the time that thumpers were allowed to continue on). I have seen horses that eventually died start by just slightly thumping.
Veterinarians that have a thorough knowledge of physiology, anatomy and endurance may be able to evaluate the level of danger that the thumps indicate. Notice I say "may be able" not "are able".
It is my personal opinion, though, that thumping horses, due to the inability of the average person observing to evaluate the severity of the problem, should not be awarded BC and should not be allowed to continue out of any vet check if thumping.
Dane Frazier, DVM email@example.com
There have been some excellent comments about synchronous diaphragmatic flutter (thumps) posted to which I have little to add. It might be of some interest to the group if they considered what the function of calcium is in the body.
Firstly, the roles calcium plays are beyond memorization. The following is only the most important of an incomplete list.
Intuitively, we know that the bones contain calcium (and phosphorus) to mineralize the collegen matrix thereby contributing to the tensile strength of bone. What is underrecognized and mostly underaprreciated is the other important roles calcium plays.
Nerves communicate with muscles via a chemical conversation involving neurotransmitters (acetylcholine). The release of the neurotransmitter from the nerve endings is dependent upon calcium moving into the nerve. In other species (cattle), calcium deficiency causes paralysis; although man, the dog, and the horse present differently.
The endocrine system (by releasing hormones into the blood) along with the nervous system provides the control and cooridination of other body systems. SOME hormones have their effect on target tissues by activating a "second messenger" within the cell that results in the effect of the hormone. Calcium is a "second messenger" in some of these very important control cycles. (The action of insulin to cause cells to take glucose out of the blood is dependent upon calcium as a second messenger.)
The instigation of all muscle contraction is dependent upon calcium being released from its protein bound storage sites in the muscle cell into its ionic (Ca++) form. Likewise, muscle relaxation requires that ionic calcium must be rebound. Speculatively, some forms of early tying up syndrome may be the result of an aberration of this process.
The blood levels of calcium also effect the STRENGTH of the heart muscle contraction, not necessarily its rate under less than normal circumstances. Note: In the veterinary check points on an endurance ride, its the rate that sets the "gate" for the horse to be examined. We do not have any good horseside parameters on endurance rides to assess strength of contraction.
Blood calcium deficiency results in an increased irritability of neuro-muscular tissue, since calcium blocks sodium gates in normal circumstances in these tissues. (Physiologist say that the nerve is closer to threshold). Because the nerve is more likely to "fire" with calcium deficiency, irritation to the phrenic nerves that course over the base of the heart by the spontaneous depolarization of the sinoatrial node (the pacemaker of the heart) results in the "firing" of the nerve. The contraction of the abdominal muscles timed to the beating of the heart is the classical presenting clinical signs of calcium deficiency in the horse. We can see and measure this horseside at endurance rides. The other effects listed above go by without notice.
SHOULD A THUMPING HORSE BE ALLOWED TO CONTINUE IN AN ENDURANCE RIDE?
Recognize that a thumping horse is contending with more than a non painful jerking of its flanks. The horse is in the throws of a metabolic state whose effects occur at the cellular and the subcellular level and whose effects can not be measured easliy.
If the ride is being run under AERC rules, then the horse must be fit to continue which involves three parameters:
1. Not consistently lame at the trot
2. Recovery to a preset heart rate
3. Metabolically stable
At many, many rides these parameters must be met within 30 minutes of arrival at the veterinary examination point.
Therefore, a thumpimg horse that continues to thump past the 30 minutes of entry into the check should be eliminated under our rules in order to maintain an even playing field across the country.
SHOULD A THUMPING HORSE BE CONSIDERED FOR BEST CONDITION?
As synchronous diaphragmatic flutter is by definition an unstable metabolic state, they should not be considered for an award recognizing superior conditioning and performance. I agree with Dr. Tomlinson that if the thumper is the best of those available for consideration, the award should not be given at that ride.
Greg Meyer - firstname.lastname@example.org
Thumps are caused by an electrolyte imbalance. As a result of the inbalance the cells that make up the phrenic nerve (as well as other nerves) are not repolarized to the same level as in a normal animal. The result of this is the nerve cells are very near their firing treshold and can spontainsly fire or fire with very little stimulation. In electrolite inbalance, the phrenic nerve can be stimulated to fire by the physical and / or electrical activity of the heart. The phrenic nerve runs in very close proximity to the heart. When a stimulis is transmited by the phrenic nerve (the nerve fires) the diaphragm contracts, this is why the the diaphragm flutters in unison with the beating of the heart.
The electrolyte imbalance(s) seen in thumps is usually hypocalcemia,hypochloremia, hypokalemia, hypomagnesemia. This can come about from a variety of reasons one of them being prolonged exercise. Feeds high in calcium (alphala for example) may predispose a horse to thumps because these feeds renders the animal less able to liberate Ca from their bone stores. Treatment is by electrolite replacement.