Published:  The Boxer Review - August 1999 Canine Cryptorchidism – an update By Daniel Buchwald, DVM and Norra Hansen A rather common condition among male dogs is the undescending of either one or both testicles.  This condition has been recognized in dogs for a long time, and even with very selective breeding of dogs with normally descended testicles, the trait will still show itself with relative frequency.  The correct term to define undescending testicles is cryptorchidism, which means cryptic or hidden testicle.  If only one testicle is noted in the scrotum the condition is defined as unilateral cryptorchid or monocryptorchid, and when neither testicle can be palpated in the scrotum, the condition is bilateral cryptorchidism. Many dog breeders and judges use the term monorchid to refer to dogs with only one testicle in the scrotum but such practice is incorrect and often leads to confusion.  Monorchid is the dog that only developed one testicle in the body, during the embrionary stages of development as opposed to “cryptic” which is hidden – most likely in the abdominal cavity- and the much more common of the conditions.  Monorchidism and anorchidism (absence of one or both testicles in the body) are extremely rare in dogs. For those individuals interested in embryology, I will mention that the testicles in the fetus develop in a site immediately behind the kidneys and are “dragged” toward the scrotum by the shrinkage of a fibro- gelatinous cord known as the gubernaculum testis. This cord extends from the testicles to the scrotal region in the male fetus.  Recent studies show that, at birth, the dog’s testicles are located midway between the kidneys and the inguinal ring, and at two weeks after birth they usually are midway between the inguinal ring and the scrotum.  The final scrotal position of the testicles is usually reached around 7-8 weeks of age. There are specific lines and families of dogs where the testicles may take up to 6 months to reach the normal scrotal position.  While some research indicates this time frame as within a “normal” range, others suggest that such delay can more often be seen in lines that have a high incidence of cryptorchidism. Genetic research has shown the likelihood of cryptorchidism to be inherited as a single autosomal recessive gene and to be sex-limited.  Autosomal refers to a chromosome other than “X” and “Y” (which are the ones determining sex, i.e., XX= female, XY= male).  Recessive means that for the trait to express itself it has to be inherited in “double dose”, one from the sire and one from the dam. If only one parent passes the trait to the puppy, it will be a “carrier” but the dominant counterpart (inherited from the other parent) will prevail, and the dog will appear as normal. Sex-limited means that if the proper genetic make up happens in a male, the trait will be expressed. That is, the dog will be a cryptorchid. But, the same genetic make up in a female will cause no abnormalities. In other words, only males will show the expression of this genetic make up. These considerations are of significant relevance because they shine the spotlight on the female just as much as the male due to their ability to be “carriers” of the trait.  This also suggests that there can be skipping of generations (sometimes several) before the trait shows up again, and also explains why breeding with apparently normal dogs for several generations still proved ineffective at eliminating the trait. Some books on canine genetics suggest a polygenic inheritance, but current thought strongly challenges this conclusion, leaning instead toward the single autosomal recessive theory for most breeds. The prevalence of cryptorchidism is believed to vary widely - from .8% up to 67% in male dogs- depending on breed and study.  Toys, or breeds where miniaturization has occurred, seem to show the trait more often that their larger counterparts. For example, Toy Poodles may produce a higher percentage of cryptorchids than Miniature Poodles, and Miniatures may produce a higher percentage than the Standards.  Boxers have been reported to have the highest risk for all large breeds studied. A very interesting study tries to correlate cryptorchidism with other inheritable defects, especially patellar subluxation and hip displaysia.  We should keep our attention toward the further studies of this theory.  The cryptorchid dog should be neutered because the retained abdominal testicle may be a site for future tumoral growth or testicular torsion.  The chance for tumors may be 10 times higher on retained testicles than on normally descended testicles. It is also wise to remember that if left intact, monocryptorchid dogs will possess the ability to breed, and in light of the likelihood of a recessive trait, all offspring will be at best carriers.  The neutering of cryptorchid males that otherwise would have been show/breeding potential should be done after 6 months of age to give consideration to the exception for delayed descent in some lines, but an attempt should be made to correlate late descent with an increased incidence of cryptorchidism. References:                                             . 1. Cox, VS: Cryptorchidism in the Dog 2. Cox VS, Wallace LJ, Jessen CR: An anatomic and genetic study of canine cryptorchidism 3. Baumans V, Dijkstra G, Wensing CJB: Testicular descent in the dog 4. Ashdown RR: The diagnosis of cryptorchidism in young dogs, a review of the problem