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Thumps in Horses: Exhaustion, Lactation Tetany, and Other Causes of Hypocalcemia in Horses
  by Robert N. Oglesby DVM

Introduction

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Perhaps it has been a harder than a normal trail ride, maybe you have a heavily lactating mare, maybe your horse has had diarrhea for awhile, but all of a sudden you realize your horse seems to be hiccuping. His whole body is being racked with mild rhythmic spasms. There may be also signs of stiffness and depression. The condition is called thumps and is due to changes in blood ph and/or blood calcium concentrations. It may be a mild transient problem but it may indicate a life threatening electrolyte imbalance in the blood. This report discusses causes, diagnosis, treatment, and prevention of hypocalcemia in horses.

Symptoms

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Thumps look like a hiccuping horse. The diaphragmatic (phrenic) nerve is discharging abnormally resulting in the diaphragmatic muscle jumping in concert with each heart beat. The condition is most frequently seen in exhausted horses and indicates stress on the electrolyte system of a horse. It is seen less frequently in heavily lactating mares usually around birthing.

If the hypocalcemia is severe enough, you may also see: a stilted gait from muscle stiffness, muscle fasiculations, rapid heart rates, depression, and colic may be seen.

Causes

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In the exhausted horse thumps is related to a condition of the blood called metabolic alkalosis. The blood's ph has become too alkaline. The sequence of events is:
  1. excessive sweating
  2. loss of electrolytes sodium, potassium, chloride, and to a lesser amount magnesium and calcium
  3. the low chloride results in a build up of bicarbonate (an alkali)
  4. the increase of bicarb makes the blood alkaline
  5. the alkaline blood lowers the available calcium
  6. the low calcium results in sensitization of the nervous system
  7. the phrenic nerve (close to the heart) depolarizes when the heart beats the phrenic nerve makes the diaphragm jump
It may also be seen in hypocalcemic conditions like during heavy milk making (lactation). In the lactating mare, calcium loss through milk production exceeds intake and more importantly calcium resorption from bone. Usually this occurs in the first or second week of lactation but may be as late as six months. Heavy supplementation of calcium prior to birthing can actually make the condition more likely as it impedes the mares ability to rapidly mobilize her calcium from her bones.

Other causes of thumps is:
  • diarrhea
  • transport stress
  • blister beetle toxicosis
  • oxalate toxicity
  • and rarely hypoparathyroidism

Treatment

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Mild Exhaustion

If the horse is still interested in food and water and only a little tired usually cooling out the horse and allowing proper access to water and food will correct the problem. Calcium rich legumes are a good choice if the horses digestive system is use to them.

For more information on exhausted horses.

Severe Exhaustion and Other Causes

If the horse has severe electrolyte abnormalities and dehydration he will be depressed, not interested in food, and though obviously in need of water will not drink. He may also be over heated, so needs to have his temperature taken. Horses in this severe condition are at risk of shock and death and need the support of large amounts of balanced IV fluids. Correction of fluid and electrolyte balances is essential. If electrolyte concentrations are not available, 100 to 300 mls of 20% calcium gluconate per 1000 lbs can be given slow IV till the thumps resolve. Also IV preparations used to treat milk fever in cattle, containing calcium, magnesium, and glucose, can be used.

Monitoring During Treatment is Important

It is important that while administering calcium rich fluids that the heart is monitored by auscultation. Increase volume of heart sounds is a good indiction of a favorable response as is urination. Increased heart rate or arrhythmias developing are a sign treatment should be immediately discontinued as a fatal hypercalcemia can occur.

Prevention

Introduction » Symptoms » Causes » Treatment » Prevention » More Info 

For exercise induced thumps better conditioning is certainly the first step to prevention. Supplementing electrolytes daily actually weakens the systems ability to retain them. Electrolyte supplementation should be reserved for times of maximal stress. Start about 12 hours before the riding is to begin, then continue until two days after the ride is done.

Just like the other electrolytes regular calcium supplementation weakens the body's ability to conserve and mobilize calcium. Horses prone to this problem should, paradoxically, be on a low-normal calcium diet during non-stressful periods. This translates into no leguminous pasture, hay, or feed. This would include clover and alfalfa products. This keeps the bodies mechanisims to mobilize calcium from stores working at peak performance. Then during the times of exercise, lactation, or other stress the calcium in the diet should be boosted to high normal levels, but only during the stressful periods and a short time afterward. A good way to do this is with alfalfa pellets or cubes substituting for about one third the concentrate portions of the diet. If it has been awhile since the horse has seen alfalfa, you need to spend a few days introducing it to the horse.

You may have to teach your horse to drink water away from home. In an attempt to teach your horse to eat on the trail when you stop to water have a quart of molasses based sweet feed with three level tablespoons of a 50/50 mixture of table salt and Lite salt (available at the grocery store for low sodium diets) shaken in with it. While the others drink, feed your horse the food and salt. It may stimulate drinking. Also a pasture with a stream in it as the only source of water encourages horses to remember that they can drink from it.

For More Information on this Topic:

Introduction » Symptoms » Causes » Treatment » Prevention » More Info 

      ~Word Count: 1014 words (The average magazine page contains about 600 words);
      ~Last Updated: June 01, 2016;
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Discussions on Hypocalcemia, Thumps & Tetany
  Titles Subtopics Posts Updated
iconBloodwork and a history of Thumps with a Kidney EventFeb 20, 12
iconHorse has hiccups for second time todayMar 27, 08
iconCan horses get this on stall rest?12 Feb 6, 03
iconHiccups?Thumps?HELP!10 Apr 28, 15
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