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 al