Horse Electrolyte Imbalance and Thumps

Understanding synchronous diaphragmatic flutter (thumps) in horses, its electrolyte causes and the welfare-critical urgency of treatment.

Key Facts

Thumps is caused by severe hypocalcaemia (low blood calcium) often combined with hypomagnesaemia or hypochloraemia
Endurance horses are most at risk due to electrolyte loss through prolonged sweating
The visible, audible thumping is caused by the phrenic nerve firing synchronously with the heartbeat due to calcium-driven hypersensitivity
Untreated severe electrolyte imbalance can progress to muscle weakness, ataxia and collapse
Intravenous calcium borogluconate is the definitive treatment — oral electrolytes are insufficient in acute cases
Proper pre-, during- and post-event electrolyte management prevents most cases in endurance and eventing horses

Welfare Considerations

Thumps is a visible manifestation of a systemic electrolyte crisis that carries significant welfare urgency. The horse experiencing hypocalcaemia-driven thumps is likely simultaneously experiencing generalised muscle cramping, fatigue and anxiety from the paradoxical diaphragmatic contractions — all adding to the physical distress of an already exhausted endurance athlete. The welfare risk is compounded by the competitive environment in which it typically occurs: riders or handlers may not recognise the severity, or may be reluctant to withdraw from competition. Ride veterinarians must have authority and be supported by rules to mandate withdrawal and treatment without compromise.

What You Can Do

Learn to recognise thumps — visible or audible rhythmic flank thumping synchronous with the heartbeat in a working horse
Report thumps to the ride veterinarian immediately — this is a veterinary emergency requiring IV treatment
Develop a pre-event electrolyte management plan with your vet for any horse competing in endurance or 3DE events
Do not rely solely on commercial electrolyte pastes in hot conditions — ensure free access to water and loose salt
Practise recognising early signs of electrolyte depletion: fatigue earlier than expected, muscle fasciculations, reduced thirst
Support regulatory changes that give ride veterinarians binding authority to withdraw horses showing electrolyte crises