Thumps in horses is a condition that produces irregular contractions of the diaphragm, resulting in a thumping noise similar to a human hiccup.

Scientifically referred to as Synchronous Diaphragmatic Flutter (SDF), thumps causes the diaphragm to spasm in the same rhythm as the heartbeat. This can occur when the concentration of calcium in the blood is too low (hypocalcemia). [1]

Thumps can occur in any horse but commonly results from electrolyte imbalance due to excessive sweating following heavy work. Thumps can also occur with health conditions that cause hypocalcemia. [1]

Most horses recover from thumps when their blood calcium levels are restored to normal. Horses with underlying health conditions may require additional treatment. [1]

You can help to prevent thumps in your horse by feeding a balanced diet, improving their fitness level, ensuring hydration and replenishing lost electrolytes after exercise or in hot weather.

What is Thumps in Horses?

First identified in the 19th century, Synchronous Diaphragmatic Flutter (SDF) arises from metabolic disturbance involving a low concentration of calcium in the serum or blood. [2]

Thumps is characterized by the contraction of the diaphragm in synchronization with each beat of the heart. The abnormal rhythm of this contraction is stimulated by the phrenic nerve which passes over the heart. [1]

Two phrenic nerves (left and right) carry a signal from the spinal cord to the diaphragm. This nerve controls when the diaphragm contracts during respiration. These nerves run from the cervical vertebrae in the neck to the diaphragm, sitting along both sides of the heart in close contact with the pericardium surrounding the heart.

Several ions (elements that carry an electrical charge), such as calcium and sodium, are involved in transmitting neural signals to muscle to control contractions. During exercise, ions are lost in sweat leading to low levels in the blood and extracellular fluid.

Depletion of ions, especially calcium, affects how nerves transmit signals. Hypocalcemia affects sodium channels on neurons causing over-sensitivity or hyperirritability in nerves meaning they have a lower threshold for activation.

Normally, the phrenic nerve carries a signal from the central nervous system (spinal cord) to the diaphragm. In thumps, the phrenic nerve is instead activated by electrical activity in the heart. Specifically, it fires in synchrony with atrial depolarization causing the diaphragm to contract with each heart beat. [3]

The contraction may produce a thumping sound that is audible and sounds like the horse is hiccoughing.

SDF may also involve twitching in the muscles in the flank area of affected horses. In some cases, twitching may also occur in leg muscles. [4]

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Risk Factors

Although thumps is more frequently reported in horses that compete in endurance competitions, the condition can occur in the presence of any health conditions that result in low blood calcium concentrations. [2]

Conditions associated with hypocalcemia that can produce thumps include excessive sweat loss, tetany in lactating mares, transport tetany, sepsis, blister beetle toxicosis, and primary hypoparathyroidism. [1]

A diagnosis of SDF is based on a clinical examination and blood tests. Laboratory tests reveal hypocalcemia often in combination with other electrolyte abnormalities in the blood. [5]

Signs of Thumps

The clinical signs of SDF in horses include: [3]

  • Audible thumping or hiccoughing noise that occurs as the diaphragm contracts
  • Bi-lateral or unilateral movement of the flanks of the horse at the same frequency as the heart rate
  • Twitching in hind legs at the same frequency of the heart rate
  • Dehydration
  • Lethargy

Horses with a health condition that causes thumps may exhibit additional clinical signs indicative of their underlying illness.

Causes of Thumps

Any condition that contributes to low levels of calcium in the body can lead to thumps. Some of the conditions that can promote hypocalcemia include:

Excessive Sweat Loss and Electrolyte Imbalance

Sweating is the main way for horses to regulate their body temperature (thermoregulation) when it is hot outside. Intense and sustained exercise also promotes sweating as a mechanism for the body to cool down. [6]

Sustained endurance exercise can result in significant electrolyte loss from sweating. [6][7] Your horse would need to produce approximately 11 L of sweat to dissipate the heat generated from one hour of submaximal exercise. [4]

Horses that exercise for an extended period can develop a state of metabolic alkalosis. This occurs when excessive loss of chloride ions in sweat causes the kidneys to retain bicarbonate ions. This affects the acid-base balance in blood and increases blood pH. [8][9]

Alkalosis also increases the amount of calcium that binds to the main protein in blood plasma (albumin). This decreases levels of free calcium ions in the blood. [8]

Too Litle Calcium in the Diet

A lack of calcium in the diet can result in low concentrations of this mineral in the blood. Excess phosphorus in the diet can also cause a secondary calcium deficiency by interfering with calcium absorption from the digestive tract.

Too Much Calcium in the Diet

A diet that is consistently high in calcium may actually lead to excessive calcium loss during exercise and stimulate increased sweating.

When blood calcium levels are high, the parathyroid glands do not produce parathyroid hormone. This hormone is responsible for mobilizing calcium stores from the bones to ensure proper levels in the blood. [10]

During strenuous exercise, blood calcium levels decrease quickly. If the parathyroid glands are not releasing parathyroid hormone, the body cannot adequately respond to low calcium levels by mobilizing this mineral from the bones. [11]

For this reason, a diet that provides excess calcium can impair the action of the parathyroid hormone to regulate calcium levels during exercise.

Avoid feeding your horse high calcium feeds such as alfalfa hay before strenuous exercise. This will ensure the parathyroid glands can respond to low blood calcium levels following exercise by increasing their production of parathyroid hormone.

Tetany in Lactating Mares

Producing nutritious milk to nurse a foal puts significant demands on the mare’s body. Hypocalcemia can occur during lactation due to insufficient intake of dietary calcium.

If there is a significant drop in blood calcium levels in the lactating mare, a condition called tetany can occur. [1][12] Tetany is characterized by involuntary muscles spasms and it can result in thumps in severe cases.

Transport Tetany

When horses are transported long distances, depletion of electrolyte levels can occur and result in low blood calcium concentrations. [1]

Transportation usually involves restricted access to feed and water. If calcium levels fall too low, tetany and thumps can occur.


Sepsis refers to a widespread response to infection or injury that results in damage to tissues and organs. Any condition that involves a severe infection or injury can lead to a significant inflammatory response that alters calcium balance in the body. In horses, this has been documented in enterocolitis (inflammation of the small intestine and colon) and endotoxemia. [13][14][15]

Blister Beetle Toxicosis

Blister beetles contain cantharidin – a toxin that causes mucosal irritation and damage in the gastrointestinal and urinary tract when ingested by horses.

Hypocalcemia is a common clinical finding in horses that have been exposed tocantharidin by consuming blister beetles. [16][17][18]


Hypocalcemia can result if the parathyroid glands fail to secrete parathyroid hormone in response to low levels of calcium in the blood. [19][20]

Kidney Dysfunction

Low calcium is a common finding in horses with kidney problems. Kidney dysfunction can lead to excessive loss of calcium into urine. [1]

The Role of Calcium in Thumps

Calcium is a mineral that is necessary for muscle contraction, cell membrane function, blood coagulation, and the regulation of various enzymes in the body.

Most calcium in the body plays a structural role and is stored in the bones and teeth. However, calcium also plays a critical role as an electrolyte.

Electrolytes are ions that carry a positive or negative charge. Calcium, sodium and potassium ions are important for transmitting electrical signals through neurons.

Sodium is a positively charged ion that enters neurons through special channels that open and close along the neuron. The change in electrical charge along the axon of the neuron is what transmits a neural signal. Calcium ions can block the movement of sodium ions through voltage-gated sodium channels. [21]

During intense or prolonged exercise, electrolyte imbalances can affect how nerves function. A low level of calcium ions in blood (hypocalcemia) and in fluid surrounding nerves means there is less inhibition on sodium channels. This makes the neurons hyper-excitable causing them to fire inappropriately. [21]

In thumps, the neurons controlling the diaphragm are activated by electrical signals from the heart instead of the brain resulting in the diaphragm contracting in synchrony with the heartbeat.

Calcium Requirements for Horses

According to the National Research Council (NRC), the calcium requirement for a horse at maintenance is 0.04 g of Ca per kg of body weight (BW) per day. [22]

For a 500 kg (1100 lb) horse, this is equivalent to 20 grams of calcium per day.

Horses that are exercising require higher levels of this mineral in their diet to account for calcium ions lost in sweat. A horse that is in light exercise will require 30 grams per day while a horse in heavy work requires 40 grams of calcium per day.

Status Ca Requirement 500 KG Horse
Maintenance 0.04 / kg BW 20 g per day
Light Exercise 0.06 / kg BW 30 g per day
Moderate Exercise 0.07 / kg BW 35 g per day
Heavy Exercise 0.08 / kg BW 40 g per day


Growing horses and mares in late pregnancy and during lactation also require additional dietary calcium. [22]

Forms of Calcium

Outside of the skeleton and teeth, calcium exists in the horse’s body in various forms including a protein-bound, complexed, and free (ionized) state. [23]

Trace amounts of calcium are present within the cells and are essential for cellular function, including muscle contraction. However, ionized calcium is primarily found outside of cells in the extracellular fluid. [23]

Measuring Calcium in the Blood

In horses, total calcium in the blood ranges from 1.43–1.75 mmol/l. [24]

There are two different ways to measure calcium levels in the blood: ionized calcium and serum calcium.

Serum calcium measures all of the forms of this mineral in the blood, including calcium bound to anions and proteins as well as free calcium that is not bound to any other molecules.

Ionized or free calcium is the biologically active form of the mineral in the blood. Normally, ionized calcium ranges from 50 to 52.7% of the total amount of calcium in the blood of horses. [24]

Calcium Regulation in the Blood

Levels of calcium in the blood must be closely regulated for cells in the body to function properly. This is referred to as calcium homeostasis.

Extracellular concentrations of ionized calcium are carefully maintained by two hormones known as the parathyroid hormone and calcitonin.

Parathyroid hormone (PTH):

Produced by the parathyroid glands (two sets of organs associated with the thyroid gland), the secretion of this hormone stimulates absorption of calcium into the circulatory system. It activates release of calcium from bone and resorption of calcium by the kidneys. [25]

PTH also promotes the conversion of vitamin D from the inactive form of the vitamin to its active form (calcitriol). Increased calcitriol production in the body subsequently promotes the absorption of calcium from the digestive system, which raises the concentration of the mineral in the blood. [25]


Produced and released by the thyroid gland, this hormone decreases the amount of calcium in the blood. Calcitonin stimulates calcium loss via the kidneys and promotes the conservation of calcium in the skeleton. [25]

Diagnosis of Thumps

If your horse is experiencing an episode of thumps that does not subside within 15 minutes, contact your veterinarian. Your horse should be assessed to determine the cause of Synchronous Diaphragmatic Flutter.

Your veterinarian will diagnose SDF based on the horse’s history, clinical signs, and laboratory test results.

A blood sample is typically taken to assess the electrolyte levels and hydration status of horses suspected of having SDF.

Laboratory findings

Horses with SDF have a low serum concentration of ionized calcium on lab tests. Other findings may include abnormal levels of other electrolyte minerals and indicators of dehydration, such as:

  • Low sodium level (Hyponatraemia)
  • Low chloride level (Hypochloraemia)
  • Low potassium level (Hypokalaemia)
  • Low magnesium level (Hypomagnesaemia)
  • Low blood sugar (Hypoglycaemia)
  • Elevated phosphate level (Hyperphosphataemia)
  • Elevated lipid peroxide level [26]
  • Elevated blood lactate level
  • Elevated creatine kinase
  • If dehydration is present, increased packed cell volume, red blood cell count, hemoglobin, and total plasma protein [4]


Thumps often resolve after giving your horse electrolytes. [1] In horses with underlying health problems, further intervention may be necessary.

Restoring Electrolyte Levels

Horses affected by SDF are treated by replenishing calcium and other electrolytes via oral supplementation or IV administration.

Oral electrolyte administration:

Electrolyte supplements that primarily consists of sodium chloride (salt) and additional electrolytes including calcium, potassium chloride, magnesium, and other trace minerals may be administered in feed or water to replenish the level of calcium in the blood. [1]

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Intravenous (IV) electrolyte administration:

In horses with severe cases of SDF, parenteral administration of calcium via IV fluids may be necessary to restore concentrations in the blood to a normal level. [1]

Some horses may require repeated treatments administered over several days to recover from hypocalcemia. [1]

Intravenous calcium gluconate is effective for increasing blood calcium levels. [27] Additional electrolytes may be delivered by IV.

Laboratory testing is useful to determine if calcium levels have been sufficiently restored after treatment.


Horses affected by thumps require a break from exercise so they can recover. It may take several days for internal fluid and electrolyte levels to normalize in horses affected by SDF.

Treatment of Underlying Health Conditions

Veterinary intervention may be required to determine the cause of hypocalcemia that resulted in SDF. Horses that have health conditions including sepsis, blister beetle toxicosis, or hypoparathyroidism require treatment to resolve hypocalcemia. [1]

How to Prevent Thumps

If you are participating in endurance competitions or training for high-performance events with your horse, follow these strategies to prevent your horse from developing thumps or to lessen the likelihood of recurrence:

Improve Your Horse’s Fitness Level

Ensure your horse’s stamina and strength are well-matched to their level of work by following a consistent exercise plan designed to gradually improve their fitness level.

Exercise has been shown to increase the volume of plasma in the blood – the protein-rich liquid that makes up 55% of our blood. [28] The other 45% of blood consists of red and white blood cells and platelets.

Increased plasma volume means that there is a higher amount of circulating fluid available for sweat production. Horses with improved fitness levels sweat earlier during exercise and produce sweat that is less concentrated in electrolytes. [28]

Avoid Exercise in Hot Weather

Choose to exercise your horse during cooler hours of the day rather than when it’s hot to reduce the amount your horse sweats and slow the rate of electrolyte loss.

Keep Your Horse Cool

Horses typically sweat when working hard, especially in hot weather. Take the following steps to cool your horse to limit a loss of electrolytes during work:

  • Wet your horse down before riding on hot days
  • Take breaks during work
  • Ride during cooler times of the day

Provide Electrolytes

Add electrolytes to your horse’s food or water for prior to, during and following competition to replenish these important minerals that are lost in sweat due to intense exercise.

Providing electrolytes regularly ensures mineral balance and supports hydration status. [1]

Ensure Adequate Hydration

Provide your horse access to fresh water regularly and during competition to prevent dehydration. Dehydration decreases performance by impeding muscle and brain function, causes electrolyte imbalance, and may be fatal. [1]

Electrolytes can be added to water to stimulate a normal thirst response and should be given to horses during and after competition to replenish electrolyte losses.

Feeding your horse a fibre-rich diet from sources such as beet pulp enables more water content to be held in the hindgut. This can act as a water reservoir during endurance competitions.

Provide Adequate Calcium

Ensure your horse’s dietary intake of calcium is sufficient to meet their needs, but avoid feeding excessive calcium.

If blood calcium levels are high prior to strenuous work, the body may not be prepared to mobilize calcium reserves from the bones into the blood. [11]

Limit Bran Mashes

Avoid feeding bran mashes more than once a week. Bran contains a high amount of phosphorus, which inhibits the absorption of calcium in the body. [29]

The ideal ratio of calcium to phosphorus in the equine diet is 1.5-2:1. Calcium to phosphorus ratios of up to 9:1 can be tolerated if the minimum daily phosphorus requirement is met. [22]


You can help your horse avoid thumps by providing a balanced diet, maintaining adequate hydration, and supplementing your horse with electrolytes during intense exercise.

Follow a consistent conditioning program that improves your horse’s fitness level and gradually increases the intensity of work to avoid overexertion.

If you are participating in endurance competitions or other high-performance equine disciplines, work with a nutritionist to formulate an optimal feeding program for your horse. You can submit your horses information online for a free consultation.

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