Question

I have a high-performance Arabian mare that has been doing well in endurance competition. I added alfalfa to her diet about six weeks ago. I recently competed her, and she was diagnosed with thumps after the 80-kilometer ride. Can you give me some information on thumps and any feed suggestions?

Answer

Synchronous diaphragmatic flutter, also known as thumps, is usually caused by low blood calcium, but it can be brought on by low levels of other electrolytes. The phrenic nerve, which signals the contraction of the diaphragm as the horse breathes, runs through the chest and over the base of the heart before it enters the diaphragm. When there is an electrolyte imbalance, the electrical signal for the heart to contract is also conducted through the phrenic nerve and the diaphragm contracts with each heartbeat. When looking at the horse, it is possible to see the thumps or contraction of the diaphragm with each heartbeat.

Because your mare had been doing well, this may be an isolated incident as thumps is not an uncommon finding in endurance horses. Anything that causes a decrease in blood electrolytes (calcium especially, but also potassium and magnesium) can result in thumps, such as sweating excessively to the point of dehydration, being off feed before a ride, or performing at a longer distance than usual. So begin by thinking of anything that may have been different with the horse for this particular ride and that may be the cause.

The recent feeding of alfalfa could be the cause because it is rich in calcium. It seems counterintuitive to think low calcium is usually to blame for thumps and feeding calcium can make it worse. However, when there is plenty of calcium present, it can impair the body’s ability to mobilize calcium from body stores such as bone when the horse most needs it. The same thing happens in dairy cows, and less commonly in horses, that are fed an abundance of calcium in late pregnancy and are then diagnosed with lactation tetany or milk fever.

If calcium from alfalfa is the culprit, then I would reduce the amount of alfalfa fed by 50% beginning one week before a ride and supplement with grass hay. Keep doing the other things that I am sure you already do, such as keeping the horse on feed, giving electrolytes, and paying particular attention to water intake.

Consider Race Recovery for horses, like this mare, that are prone to thumps.