Looking through a 1923 edition of "Diseases Of The Horse," we
came upon a section dedicated to a condition which is similar to
human hiccoughs. Ninety-seven years later, we refer to this human
affliction as hic-ups, but the horse condition is still referred
to as thumps.
A horse hic-up is different from a human hic-up in the location
where the sound is made. The sound of a human hic-up is created
in the throat. In the horse, hic-ups are produced when the
diaphragm contracts violently enough to cause an audible thumping
sound in the area of the chest. This is why equine hic-ups are
referred to as thumps or to be more specific, Synchronous
Diaphramatic Flutter (SDF).
Long before we ever heard of SDF we saw horses with hic-ups.
Growing up riding ranch horses, it was not unusual to leave the
horses out in the pasture all week and catch them up for a full
day of energetic riding on the weekend. It was at the end of one
of these long days of strenuous exercise that we first saw a
horse's flank jerk with the rhythmic contraction of equine
Today, this same phenomenon is frequently seen in endurance
horses and racehorses. This fits with the prophetic statement
issued in that 1923 veterinary text that thumps "is generally
occasioned by extreme and prolonged speeding on the race track or
In these cases, the most likely cause of thumps is that the
horses were physically tired but not exhausted. This caused their
blood pH to rise.
Remember your high school chemistry. The pH of a liquid refers to
whether a solution is acid, basic or neutral. A liquid with a pH
of 7 is considered to be neutral. The range of normal blood pH
values for the horse is 7.32 - 7.44. When the pH of the blood
rises above 7.4, the blood becomes more basic and the condition
is referred to as alkalosis.
Athletic stress is not the only condition that can cause thumps.
Other conditions may include traumatic pressure on the phrenic
nerve of the diaphragm, electrolyte imbalance, low levels of
blood calcium such as milk tetany, diarrhea, colic, or a hormonal
imbalance creating a prolonged period when a mare fails to cycle.
It has also been seen in horses given lasix, a drug with diuretic
properties that changes the normal blood chemistry.
What all these conditions basically have in common is that they
contribute to an electrolyte imbalance, low blood calcium levels
and elevated blood pH. These conditions increase the irritability
of the phrenic nerve, the nerve which controls the contraction of
The current theory is that once the phrenic nerve, which passes
over the heart on its way to the diaphram, becomes sensitized, it
is stimulated by the electrical activity of the heart. The
heartbeat causes the nerve to fire, which, in turn, causes the
diaphragm to contract.
Treatment and prevention of this condition has to do with
maintaining good hydration. If adequate water is provided at
frequent intervals throughout a ride, such as small drinks at
regular intervals, better hydration for the horse can be
maintained. Electrolytes added to the water may also be
advantageous. High calcium diets such as alfalfa supplementation
should help prevent thumps. In fact, intravenous administration
of calcium is recommended for prompt alleviation of thumps.
SDF can be a frequently recurring problem for some horses. These
animals may have an injury to the phrenic nerve or, perhaps, they
just tend to overeat to the point of exciting the diaphragmatic
But before practicing some old wives' tale treatment on them,
remember that some horses may come unglued if you sneak up on
them and say, "BOO!"
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