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

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]

Hypoparathyroidism

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 ca