The Rig – Cryptorchid Horse

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Dr Peter Gillespie. BVSc MACVS.

A cryptorchid horse, or rig, as it is commonly known, has one or both of its testicles retained in its inguinal canal or abdominal cavity (Fig 1). Often the condition goes unnoticed until the time comes for the horse to be castrated.

During early foetal development, the testicles are located in the abdominal cavity, behind the kidneys. As the foetus develops, they start to descend towards the inguinal canal, eventually passing through it and into the scrotum during the final weeks of development.

Retention can occur anywhere along its descent. We can classify cryptorchids as being either abdominal or inguinal, depending on the location of the retained testicle. Inguinal retention is by far the more common of the two.

Let’s look at each of these seperately;

Inguinal retention is when the retained testicle is positioned in the inguinal canal. It can be further described as either a temporary or permanent retention.

Temporary retention occurs mainly in ponies and is characterised by small testicles weighing less than forty grams. This condition usually involves only one testicle and in 75% of cases, it is the right side. The retained testicle can occasionally be felt in the inguinal canal but more often than not the horse has to be sedated to allow confirmation of its position. If left alone, the retained testicle will eventually descend into the scrotum by the time the horse has reached three years of age.

Permanent inguinal retention occurs in all types of horses and is characterised by testicles that weigh more than forty grams. Horses with this type of retention are often referred to as ‘high flankers’. Again this condition usually involves only one testicle but right and left sides are equally affected. The retained testicle can be difficult to palpate even after the horse has been sedated.

Abdominal retention describes when the testicle is positioned in the abdominal cavity. This type of retention can be further described as being complete or incomplete.

Complete abdominal retention is where both the testicle and the epididymus are retained within the abdomen. These testicles are usually quite small, having failed to develop fully due to the effect of the higher abdominal temperature. This makes them difficult to locate. They are also fairly mobile within the abdomen and may become mixed up with coils of intestine.

In incomplete abdominal retention, part of the epididymus is located in the inguinal canal but the testicle itself remains in the abdomen.

Diagnosis of a cryptorchid is a job for your veterinarian. Without adequate sedation, most horses resent being handled between their back legs. They are able to quickly retract their testicles upwards especially under the influence of fear, cold weather or cold hands, making the confirmation of the type of retention, difficult.

Palpation of the scrotum of a newborn foal can be confusing. At this age the tail of the epididymus can be larger than the testicle itself. If just the epididymus is positioned in the scrotum, it can easily be mistaken for a testicle

In cases where both testicles are retained in the abdomen it can be difficult to tell whether the horse is a cryptorchid or a gelding, especially if the history of the horse is unknown. In these cases, blood tests can be used to confirm the presence of testicular tissue within the abdomen.

There are two tests available, which one is used, depends on age. In horses three years and older, a single blood sample can be taken for determination of a hormone known as oestrone sulphate. Cryptorchids have levels four to five times higher than geldings. If the horse is less than three years of age, paired blood samples are necessary to measure testosterone levels before and after an HCG injection, a drug that stimulates testosterone release from the testicles. If the level in the second blood sample (taken 60 minutes after the administration of HCG) is higher than the pre-injection sample, it confirms the presence of testicular tissue.

Once a diagnosis of cryptorchism has been made, a decision has to be made on what course of action to follow. Because the condition is inherited, the only case for not castrating a cryptorchid is when it is suspected that it is a case of temporary inguinal retention. This means allowing enough time for the retained testicle to descend. If it hasn’t appeared in the scrotum by the time the horse is three years of age, it should be treated as a case of permanent retention and castrated accordingly

The surgical procedure for castrating a cryptorchid is a lot more involved than the normal castration procedure so the length of time the horse is anaethetised for is greater. Usually there is no way of knowing where the retained testicle is positioned until the horse is fully relaxed and lying on its back. Careful dissection over the inguinal canal will allow the veterinarian to determine its location.

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