When a colt is growing inside his dam’s uterus, his testicles develop inside his abdominal cavity, just behind the kidneys. At some point before his birth, or during the first couple of weeks after he’s born, the testicles normally descend into the scrotal sac. “A string-like structure called the gubernaculums guides the testicles into place,” explains Dr. Nicola Cribb, a professor at the Ontario Veterinary College. “The pressures of the abdominal contents also help force the testicles into the scrotum.”

But not always. Sometimes one or both testicles don’t make it all the way into the scrotal sac, but become lodged in the horse’s abdomen, flank or just at the edge of the scrotal sac. The horse is then called a cryptorchid, ridgling, or rig. In cryptorchid testicles that are located outside of the abdomen, it can take more than a year for the testicle to completely descend; the testicles may be visible at some point, then one or both may disappear again for a period of time. However, if after the first two weeks of life the testicle is still in the abdomen, it generally can no longer move into place.

What causes cryptorchidism?

“There’s no scientific evidence to prove that it’s hereditary, but it’s thought to be,” says Cribb. One study found that cryptorchidism was diagnosed much more frequently in Percherons, American Saddle Horses, American Quarter Horses and all breeds of ponies, and less often in Thoroughbreds, Standardbreds, Morgans, Tennessee Walking Horses and Arabians. Because of this potential for the condition to be inherited, Cribb says that gelding the horse is generally recommended.

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