Prior to a colt foal being born his testicles are "stored" within the abdominal cavity. Shortly before birth they descend through the inguinal ring, and into the scrotum. Sometimes for a variety of reasons one or both (more commonly one) do not descend as they should. The vaginal rings close during the first two weeks of the foal's life, and this will prevent a testicle that has not fully descended from the abdomen at that point from subsequent descent.
There are several types of cryptorchidism, or testicular retention.
The hardest type to deal with is a complete abdominal retention. With this, the testicle is fully retained in the abdomen, and is mobile within the abdominal cavity. Typically this testicle will be small and flabby. It is extremely unlikely that it will produce sperm, although there has been one case reported. The sperm in such a case would not be fertile. The testicle is not externally palpable.
Incomplete abdominal retention is only marginally easier to deal with, in that the retained testicle is not mobile within the abdominal cavity, but is usually located close to the deep inguinal ring, with portions of the testicles attached tissue passed through the vaginal ring, where they can sometimes be palpated externally with the horse standing, and often when the horse is laid down under anesthesia.
Permanent inguinal retention is where the testicle has descended through the deep inguinal ring, but is trapped within the inguinal canal. It is difficult or impossible to palpate, and may be palpable upon the horse being laid down under anesthesia. Caution should be used in that it is possible to mistake the results of this palpation for those of an incomplete abdominal retention. In both instances the tail of the epididymis within a vaginal process may be felt. Either testicle can be retained, with the other testicle also sometimes being retained abdominally. An animal with this condition is sometimes known as "high flankers".
Temporary inguinal retention is the condition that holds the most hope for the horse's owner. The retained testicle may be palpable within the inguinal canal with the horse standing, but will certainly be palpable with the horse laid down under anesthesia. The right testicle is the one most commonly affected (in more than three-quarters of cases). With this type of retention the testicle will usually descend of it's own volition by the time the colt is three years of age, although cases of it not descending until the horse is six have been recorded. In this type of retention, the testicle can sometimes be aided in it's descent by treating the animal with hCG (human chorionic gonadotropin), which will artificially stimulate the natural testosterone production, thereby encouraging descent.
Except as otherwise noted, and in some cases of temporary inguinal retention sperm production is nil, and any sperm produced will be infertile. This is as a result of the lack of temperature control afforded by the ability of the raising and lowering of the testicle to achieve an even body temperature. Occasionally sperm may be produced in the temporary inguinally retained testicle if it is located close to the scrotum.
The cryptorchid may not be capable of producing viable sperm, but testosterone production will not be affected. This means that once the animal reaches puberty (usually between 12 and 18 months), he will start to act in a stallion-like manner, which can be dangerous for those not familiar with it, and inconvenient for those that are. Additionally, at this stage the stallion will be capable of breeding a mare, although he may not be fertile depending upon whether one or both testes are retained. In the event that only one testicle is retained the second testicle will probably be fully functioning, and therefore any mare bred may become pregnant. As cryptorchidism is possibly a heritable trait (this is a hotly debated topic, with "proof" being shown both for and against the theory), this situation is not desirable. From an ethical point of view, perpetuation of such a trait should not be considered. Increased castration costs will result for male offspring that inherit the condition, and lower total numbers of sperm produced will be inclined to make any affected animal less fertile. Replication is therefore damaging to a Breed generally, and to individual owners financially, if not physically as a result of unexpected stallion-like behaviours.
Castration costs are increased depending upon the degree of retention, and therefore what level of surgery is required. Obviously full abdominal surgery is required for the completely abdominally retained testicle, as that surgery would be more extensive than for a temporary inguinally retained testicle should that be performed. Full anesthesia is recommended in all instances. Pricing will vary from surgery to surgery and surgeon to surgeon, but costs in excess of US$1,000 would not be unlikely for the more complicated surgeries.