Having a dog with one or both testicles retained (cryptorchidism) is one of the most common congenital defects in all purebred dogs, especially Chihuahuas. Cryptorchidism is inherited from the parents and is a sex-limited autosomal recessive trait in dogs.
The incidences of cryptorchidism in dogs ranges from 1.2 to 10%. and in some breeds, is as high as 15%. Dogs with cryptorchidism can have reduced fertility or be infertilite and if retained testicles are kept intact in the dog (even if they are surgically relocated), they have a significant risk of eventually developing testicular cancer, or worse, the saddest of which we've seen was a 2 year dog (not a Chihuahua) where the retained testicle was left in place in the abdomen where it quietly strangled the dog's gut until it was critically ill and had to be put down.
For this reason we have - without exception - pet-homed and neutered every single cryptorchid we have ever bred, resulting in some heartbroken tears (Oh Baron (pictured)!, Oh Eddie! *sob*), a much better health outcome for the affected dogs once the retained testicle is removed and a much reduced incidence of this health issue occuring in our lines.
Until we can DNA test for cryptorchidism it might still crop up occasionally, but by being vigilant and not allowing known cryptorchids to breed (and ensuring they are neutered at 12 months), we have done our best to eliminate the danger of breeding dogs doomed to a shortened lifespan and the suffering of testicular cancer (or worse). It's also worth noting that neutering a cryptorchid dog can be considerably more expensive than a regular neuter, as it may involve abdominal surgery to locate the retained testicle, so bear that in mind if you're thinking of buying an affected puppy.
Our first two Chihuahuas were cryptorchids, sold as pets by a responsible breeder who knew she shoudln't breed from them, and were neutered at around 9 months. Loki's (pictured) surgery, even back in 2011, cost around $700. It would be double that now for a similar operation. They are both still much-loved, happy and healthy dogs though, so not for a moment do we regret bringing them into our family or the money it cost to remove the threat to their health by getting rid of their retained testicles.
The good news for all dogs is that researchers at Iowa State University have narrowed down the offending genes to 4 promising candidates including COL2A1, HOXA10, INSL3 and TIMP1, which show likely associations with cryptorchidism (albeit in Siberian Huskies, who are the focus of their study). This is just the beginning, however, and the researchers admit that it's possible different genes might be responsible for cryptorchidism in different dog breeds, or even within different families of the same breed. A wider sample and more research is needed before we can just order up the test with all the other DNA screening we do, but it's good to know we may not be that far away from being able to tick the "Cryptorchidism" box on the DNA test form, too.
In the meantime, we'll just continue to do what we do (which has been very effective so far) and if you're contemplating purchasing a dog from a breeder, checking how many testicles your prospective puppy has, might save you some heartbreak further down the line.