Monorchidism is the retention of one of the testicles within the body, or a failure for a testicle to completely descend into its proper position. When both testicles are affected, the condition is referred to as 'cryptorchidism'. The condition has varying degrees of severity, with some testes being extremely hard to locate within the abdomen. Although not regarded as a serious problem by itself, the condition can have some potentially dangerous side effects that can seriously impact on the health of the cat.

Depending on the breed of your cat, it may be difficult to notice in passing that the animal has undescended testes. However, there are several obvious visual clues that your cat has developed complications resulting from the condition:

Oftentimes, the animal will be experiencing pain due to pressure being exerted on the undescended testicle itself, or via twisting of the spermatic cord. Both of these problems have serious implications for the health of the testicle and the animal as a whole. You may notice your pet wincing, becoming grouchy in its behavior or walking with an altered gait in order to minimize the pain. 

An affected cat become uncharacteristically aggressive. This may be the result of pain and the instinctive reaction on injured animal to project an impression of force and danger in order to avoid being seen as easy prey. 

If both testicles remain undescended, it is not uncommon for the animal in question to develop bladder control issues. This in itself is cause to present the cat to a vet for diagnosis.

The exact cause of monorchidism is a subject of debate within veterinary circles, with some professionals asserting that it is a hereditary condition (as it is in dogs) and others claiming that environmental factors are to blame. On a practical level, however, no conclusive evidence has been brought to light that supports either theory. Thus, without further research it is at present impossible to predict or prevent the appearance of monorchidism in cats.

When a veterinarian examines your cat, they will most likely perform a manual inspection of the abdomen in order to locate the missing testicle via touch. If this is not successful, then an ultrasound machine or X-ray may be used to map the layout of the lower body and discern where the testicle has migrated to. Additionally, they may have questions regarding the cat's recent behavior and earlier life in order to determine exactly how long any associated problems have been developing for.

Oftentimes, the vet will advocate surgical removal of the testes in question. The main reason for this is that there is as of yet no proven method or drug that can safely induce the undescended testicle(s) to descend into the scrotum, and to leave them in place would cause undue pain to the cat. Furthermore, in the event of the development of testicular cancer (one of the main causes of pain in cats with monorchidism), failure to remove the tumors will only allow them to spread through the animal's body and cause further suffering. The surgical process itself is comparatively quick; with the targeted organs having been located via ultrasound earlier, there will be very little exploratory surgery required.

Interestingly, removal of an undescended testicle will have a negligible impact on the fertility of the cat. This is because the higher temperatures found inside the body will prevent the production of any sperm in the undescended testicle. That said, your vet will most likely recommend that you opt to completely castrate your pet. This is so, in the event that monorchidism is proven to be hereditary, your cat will not have had the opportunity to pass the condition on into the wider gene pool.

Following surgery, the vet will most likely fit your cat with an E-collar to prevent it from tearing out the stitches in its abdomen. It is important to remember to prevent your cat from leaving the house until the stitches are removed, as the environmental hazards in the outside world could result in a premature opening of the wound.

Your vet should also provide you with pain medication and antibiotics as well as specific instructions on how to administer them. They will also most likely want to book a follow-up appointment or two in order to make sure the cat is healing properly.