Thumps: This rhythmic twitching of the muscles near a horse’s flanks are a warning that his internal calcium reserves are running low.
Synchronous diaphragmatic flutter, called “thumps,” is a metabolic disorder with a distinctive outward sign: The muscles along the horse’s flanks twitch rhythmically in time with his heartbeat---up to 50 or more times per minute. The affected horse is not in pain and will behave normally.
Thumps develops when a horse’s calcium reserves run low as he exerts himself. Calcium helps regulate nerve function---specifically, it acts as a gatekeeper that helps a nerve at rest remain at rest. But when serum calcium levels drop, nerves can become “excitable,” prone to firing inappropriately. The phrenic nerve, which controls the movement of the diaphragm muscles and passes over the base of the heart, is the source of thumps. When calcium levels are low, the phrenic nerve can fire in response to signals meant for the nerves that control the heartbeat---which makes the horse’s flanks twitch in synchrony with his pulse.
Thumps is most prevalent in endurance horses and others who work long hours, but it can occur in any horse who sweats continuously for extended periods without eating and drinking. It can also happen after longdistance shipping.
Treatment is basic---stop working the affected horse, and give him time to rest and recover with plenty of water plus his normal diet. If thumps do not subside within 15 minutes or so, call your veterinarian. Unless your horse has done this before, your veterinarian will want to do some bloodwork and may administer intravenous fluids.
Usually, a horse needs at least a few days to restore his internal fluid and mineral balances. Once your horse has recovered from a mild case, your veterinarian is also likely to advise you
about management strategies that can prevent another episode. These recommendations apply only to horses who have developed thumps before.
1. Condition your horse appropriately. Regular exercise does more than get a horse’s muscles fit---it prepares his metabolic pathways for the changes they’ll undergo to support strenuous effort. Talk to your veterinarian or a trusted trainer if you need help devising a fitness regimen for your horse and your intended activities.
2. Keep your horse cool. Any horse will sweat while working hard, especially in hot weather, but the steps you take to cool him by other means will help to limit his loss of electrolytes0. For example, wet him down before riding on hot days, and take breaks to splash water on him as you go.
3. Eliminate or reduce his ration of alfalfa hay. Alfalfa contains high levels of calcium, and while it may seem counterintuitive, a diet that is too high in this nutrient can lead to deficiencies when the horse works. In effect, when the horse’s body is used to having excess amounts of serum calcium available, it loses the ability to call up reserves during an extensive workout. Replacing alfalfa with grass hays, which contain less calcium, is enough to eliminate thumps episodes in some horses prone to the condition.
4. Limit bran mashes. Calcium and phosphorus are two of the most abundant minerals in a horse’s body, and he needs to consume adequate quantities of both to keep his bones strong and his cells functioning well. However, it is important that the horse consumes the minerals in a ratio of at least 1.2 parts calcium for every 1 part phosphorus---too much phosphorus in the diet inhibits a horse’s ability to absorb calcium. Wheat bran, a common base of hot mashes, is high in phosphorus. A bran mash once a week or so isn’t going to upset a healthy horse’s nutritional balance, but don’t make it a daily staple. Rice bran in its natural state is also high in phosphorus. If you choose to feed rice bran, select a product formulated for horses that includes added calcium carbonate. 5. Use electrolyte supplements judiciously. A sweating horse loses prodigious amounts of electrolytes---calcium as well as sodium, potassium, magnesium, chloride, bicarbonate and phosphate---all of which he can replenish on his own after a few days with access to grazing or hay plus a salt block. But any horse who sweats heavily for at least an hour or two without eating or drinking could benefit from an electrolyte supplement to help him recover faster. Administering electrolytes may help a horse with thumps replenish his stores, but do not “overdose” him in an attempt to speed his recovery. Always offer plenty of fresh water to any horse resting after exertion but especially after you’ve given him electrolytes. Better yet, offer the horse one bucket of plain water and one with dissolved electrolytes so he can take what he needs.
This rhythmic twitching of the muscles near a horse’s flanks is a warning that his internal calcium reserves are running low.