What is it?
Cryptorchidism, or retained testicles, is one of the most common developmental disorders in male puppies. It is important because once puberty has started, testicles are supposed to be outside the body where they can be kept cool. An overheated testicle inside the body will be infertile (sperm cannot be made at body temperature) and is nearly ten times more likely to become cancerous with a nasty disease called a Sertoli Cell Tumour.
What causes it?
When the puppy is in his mother's womb, his testicles develop next to the kidneys (just like in all mammals, including humans). Once his body decides it is male, a ligament called the gubernaculum pulls the testicles down through the body, through the inguinal canal and into the scrotum - they are supposed to have arrived by a couple of weeks after birth. If this system fails for any reason (usually due to a failure of the gubernaculum to contract fully), the dog will be cryptorchid. It is not uncommon for testicles to be "hidden away" in the inguinal canal before puberty; however, when they grow to adult size at about 6-9 months old, they should drop to and stay in the scrotum - if they're inside the abdomen at this point, they will be too big to fit through the canal.
What dogs are at risk?
This is a genetic condition, so a dog whose father had the condition will probably be cryptorchid, too.
What are the symptoms?
Simple - affected dogs will usually only have one testicle visible! Occasionally, both testicles are retained, but this is uncommon. If your dog does seem to have only one testicle, it is very unlikely that they genuinely don't have a second one (monorchidism), it's far more probable that the second is hidden away internally.
How is it diagnosed?
Physical examination makes it very clear that the scrotum isn't fully occupied! If the second one cannot be located during surgery, it is possible to remove the present testicle and then do hormone tests to see if there is another one hidden away somewhere.
How can it be treated or managed?
Because of the cancer risk, the retained testicle MUST be removed, and because the condition is genetic, so must the "normal" one. In humans, a procedure called an orchiopexy is used to fix a retained testicle into the proper place in the scrotum; however, because of the heritability of the condition, it is considered unethical to do this in dogs.
Can it be prevented?
Only by not breeding from existing cryptorchid dogs, and preventing them from breeding by castration.