Equine Hypocalcemia (Transport Tetany)

Other Names

Transport tetany; transit tetany; lactation tetany; hypocalcaemia; hypocalcemic tetany; eclampsia (rare usage); grass tetany (overlap with hypomagnesaemia).

What is Equine Hypocalcaemia?

Equine Hypocalcaemia is a metabolic disorder in which blood calcium concentrations fall below the level required for normal neuromuscular function, causing progressive muscle weakness, tetany, synchronous diaphragmatic flutter (SDF), and in severe cases, collapse and death. The condition is most commonly associated with prolonged transport, heavy sweating, and lactation — hence the common name “transport tetany.”

Calcium is a critical mineral for muscle contraction, nerve transmission, and cardiac function. When ionised calcium levels in the blood fall acutely, muscle excitability increases abnormally — nerves fire more readily, and muscle fibres contract involuntarily or fail to relax normally. This produces the characteristic clinical signs of muscle spasm, sweating, ataxia, and in extreme cases, tetanic convulsions.

Transport tetany is most commonly seen in lactating mares and horses undergoing endurance exercise, though it can occur in any horse following prolonged transport, heavy exertion in hot conditions, or feed and water deprivation. The condition is less common in horses than in cattle, where hypocalcaemia (milk fever) is a significant production disease, but when it occurs it can be rapidly life-threatening if not treated promptly.

Causes

Hypocalcaemia in horses is almost always caused by a combination of increased losses and decreased intake or absorption of calcium:

Lactation — heavy milk production depletes calcium stores, particularly in mares with high-producing udders in early lactation
Prolonged transport — stress, feed restriction, and reduced water intake during long journeys impairs calcium absorption and mobilisation
Endurance exercise — heavy sweating causes losses of calcium in sweat; horses that complete long-distance events in hot conditions are at particular risk
Feed deprivation — calcium is constantly lost in urine and sweat; extended periods without forage reduce ongoing calcium intake
High oxalate diets — tropical grasses such as kikuyu, buffel, and setaria bind calcium in the gut and dramatically reduce absorption; this is a significant cause in some regions
Alkalosis — transport stress can cause a respiratory alkalosis that reduces ionised calcium even when total calcium remains normal

Concurrent hypomagnesaemia (low magnesium) frequently accompanies and exacerbates hypocalcaemia, as magnesium is required for normal parathyroid hormone function and calcium mobilisation from bone.

Symptoms and Signs

Signs of equine hypocalcaemia range from mild muscle tremor and ataxia to severe tetanic crisis. Common presenting signs include:

Muscle tremors and fasciculations (fine twitching), most visible over the hindquarters and flanks
Stiffness of gait and reluctance to move
Sweating — often profuse and disproportionate to the level of exertion or ambient temperature
Synchronous diaphragmatic flutter (SDF or “thumps”) — a rhythmic, audible thumping sound from the flank synchronised with the heartbeat; caused by diaphragmatic muscle excitability
Muscle cramps and tetanic spasm in severe cases
Ataxia and incoordination
Facial muscle twitching, flared nostrils
Elevated heart rate and anxious expression
Recumbency and inability to rise in severe cases

SDF (“thumps”) is a classic and highly recognisable sign — if you can see or hear the flank moving in synchrony with the horse’s pulse, hypocalcaemia should be the immediate clinical suspicion.

Diagnosis

A presumptive diagnosis can often be made on the basis of clinical signs, the horse’s history (recent transport, endurance exercise, lactation), and rapid response to calcium supplementation. Laboratory confirmation via blood calcium measurement is important — ionised calcium measurement is more clinically relevant than total calcium, as it is the ionised fraction that governs neuromuscular function. Concurrent hypomagnesaemia, hypokalaemia, and metabolic alkalosis are commonly found.

ECG monitoring may be warranted in horses with significant cardiovascular signs, as hypocalcaemia affects cardiac muscle function and can cause arrhythmias.

Treatment

Treatment is directed at restoring normal circulating calcium levels as rapidly as is safe:

Intravenous calcium gluconate — the mainstay of acute treatment; administered slowly via IV drip with cardiac monitoring, as too rapid administration can cause fatal cardiac arrhythmias. The veterinarian will use a large volume dilute solution to reduce the risk of cardiac side effects
Oral calcium supplementation can be used as a follow-up measure in horses that are conscious and able to drink
Magnesium supplementation — if concurrent hypomagnesaemia is suspected or confirmed, magnesium sulphate is added to the drip
Rest — the horse should not be worked until fully recovered and electrolytes have normalised
Dietary calcium normalisation — horses on high-oxalate pastures must be removed and provided with calcium-supplemented feed

Most horses with transport tetany respond dramatically to IV calcium within 15 to 30 minutes of treatment, which can itself serve as a diagnostic confirmation.

Prevention

Prevention focuses on minimising the risk factors that precipitate hypocalcaemia:

For transport: plan journeys with regular breaks, ensure access to hay and water throughout, avoid prolonged fasting before departure, and consider oral calcium and magnesium supplementation for long journeys
For endurance horses: use electrolyte supplementation that includes calcium and magnesium during events; feed high-calcium forage before long-distance work
For lactating mares: monitor calcium intake closely in early lactation and supplement if required
Avoid feeding high-oxalate tropical grasses without calcium supplementation to compensate for the reduced absorption
Ensure adequate forage intake before, during, and after stressful events

Prognosis

Horses with mild to moderate transport tetany treated promptly with IV calcium have an excellent prognosis and typically recover fully within hours of treatment. Severe cases with tetanic crisis, cardiac arrhythmias, or recumbency carry a more guarded prognosis but many still respond well to aggressive management. Horses with high-oxalate nutritional hypocalcaemia require long-term dietary management and supplementation to prevent recurrence and chronic skeletal effects.

When to Call the Vet

Call your vet immediately if your horse develops “thumps” (synchronous diaphragmatic flutter), muscle tremors or spasms, unexplained profuse sweating, or ataxia following transport, heavy exercise, or in a lactating mare. Intravenous calcium must be administered by a vet — oral supplementation alone is insufficient in acute cases and administration rate matters for cardiac safety. This is a medical emergency.

Frequently Asked Questions

What are “thumps” in horses?

Thumps is the colloquial term for synchronous diaphragmatic flutter (SDF) — a condition in which the diaphragm contracts in synchrony with each heartbeat, producing a visible and audible thumping of the flank that matches the pulse. It is caused by hypocalcaemia-induced increase in neuromuscular excitability affecting the phrenic nerve. It is a classic sign of transport tetany and an important prompt to call the vet immediately.

Can horses with transport tetany be given oral calcium?

In mild cases or as a preventive measure, oral calcium preparations can be useful. However, in acute clinical tetany, IV calcium administration is required for rapid effect. By the time clinical signs are apparent, gut absorption is insufficient to normalise blood calcium quickly enough. Oral supplementation is appropriate for prevention and follow-up, not acute emergency treatment.

Is transport tetany the same as grass tetany in cattle?

They are similar metabolic emergencies but with some differences. Grass tetany in cattle is primarily caused by hypomagnesaemia (low magnesium) from grazing magnesium-deficient spring pastures. Transport tetany in horses is primarily hypocalcaemia, though concurrent hypomagnesaemia is common. Both cause tetanic clinical signs and both require electrolyte replacement therapy, but the predominant electrolyte abnormality and the triggering context differ.

How can I prevent thumps in my endurance horse?

Ensure your horse is consuming adequate hay and forage before and during events. Use electrolyte supplements that include calcium and magnesium rather than sodium-and-potassium-only products. Avoid sodium bicarbonate supplementation before events as this can worsen alkalosis and reduce ionised calcium. Work with a vet experienced in endurance medicine to develop an appropriate electrolyte protocol for your horse.

For further information on equine metabolic conditions and transport management, the British Equine Veterinary Association (BEVA) is an authoritative resource.