Hypocalcemia is an abnormally low concentration of ionized calcium in the blood. Calcium is the most abundant mineral in the equine body, and its ionized (free) fraction in the bloodstream is maintained within a narrow range by parathyroid hormone, calcitonin, and vitamin D acting on bone, kidney, and intestine. Normal ionized calcium in horses is approximately 1.4 to 1.8 mmol/L; values below 1.2 mmol/L produce measurable clinical signs, and severe hypocalcemia below 1.0 mmol/L constitutes a medical emergency because calcium is essential for normal neuromuscular transmission, cardiac conduction, and smooth muscle contractility.
The most clinically recognized form is transport tetany (also called transit tetany or transit hypocalcemia), which develops in mares that have been transported for extended periods without adequate feed, water, or rest. Prolonged alkalosis from hyperventilation during transport reduces the ionized fraction of calcium even when total calcium is normal, because alkalotic conditions shift calcium from the free ionized form to protein-bound forms. Lactating lactating mares at peak milk production are at heightened risk because colostrum and milk production impose a sustained calcium drain; the postpartum period in heavy-producing mares is analogous to milk fever (parturient paresis) in dairy cattle. Blister beetles contaminating alfalfa hay cause hypocalcemia as part of cantharidin toxicosis, in which kidney damage impairs calcium regulation.
Clinical signs reflect failure of the calcium-dependent neuromuscular system: synchronous diaphragmatic flutter (thumps)—in which the diaphragm contracts in time with each heartbeat, producing a visible flank flicker—is the hallmark finding in horses. Muscle trembling, stiff gait, sweating, facial twitching, and tetanic spasms of the jaw and neck muscles may accompany or precede thumps. Treatment is intravenous calcium gluconate or calcium borogluconate administered slowly while cardiac rhythm is monitored; rapid infusion can cause cardiac dysrhythmia and arrest. Dietary calcium adequacy, particularly in horses fed grain-heavy rations low in roughage, is assessed against NRC requirements as a preventive measure. Correct dietary calcium-to-phosphorus ratio (at minimum 1:1, ideally 1.5:1 to 2:1) is a standard part of calcium-to-phosphorus ratio in ration planning.
Further Reading
- hypocalcemia (Wikipedia)
- For clinical detail on calcium disorders and treatment: disorders of calcium metabolism in horses — MSD Veterinary Manual