Cryptorchidism

Cryptorchidism by Susan Lennard

DEFINITION: Cryptorchidism: Greek derivation; Hidden testicle. Cryptorchidism is a condition affecting the dog in which one or both testicles do not fully descend into the scrotum. The condition may be presented in two forms:
1.) Unilateral cryptorchidism refers to the normal descent of a singular testis.
2.) Bilateral cryptorchidism results in the retention of both testes.

Due to the thermal suppression of spermatogenesis, bilateral cryptorchids are sterile while unilateral cryptorchids are usually fertile. Cryptorchidism is estimated to exist in 6-8% of the canine population (Canine and Feline Endocrinology and Reproduction, Feldman and Nelson). Saint Bernards are noted as a breed at risk. An additional phenomenon, monorchidism, may occur in very few dogs. Monorchidism is a rare condition in which only one testis actually develops in the dog's body. Confirmation of monorchidism may only be determined via plasma testosterone analysis. The concentration of plasma testosterone is significantly lower in dogs who lack one or both testes. Unlike unilateral cryptorchidism, the testis is not ectopic but rather, does not exist in any location.

HERITABILITY: Cryptorchidism is a congenital anomaly which is described as a sex limited (occurs only in males) trait. The incidence of cryptorchidism is greatly higher in that of pure-bred dogs and specifically, in that of heavily line-bred animals. Bilateral retention is reported to be most common in in-bred animals (Vet. Clinics of North America: Small
Animal Practice-Vol. 21, No. 3, May 1991). The dam and sire of the cryptorchid pup may appear to be normal while being carriers for the defect. Given the polygenic nature of cryptorchidism, it is difficult to calculate quantitative results with respect to frequency and distribution. Simple recessives provide statistical expectations as we consider a limited number of possibilities yet polygenic modes of inheritance are far more complicated as a singular gene may be responsible for but one aspect of an entire process in which several genes dictate the completion of the event ie: descent of the testes. With this in mind, it is not difficult to understand how an affected (cryptorchid) animal may produce seemingly normal offspring.

Testicular descent involves a seemingly simple process of moving from one location within the abdomen, to another. The mechanism by which descent is achieved however (contractions of abdominal muscles as well as intra-abdominal pressure), is considerably more complicated. Central to proper development and subsequent descent of the testes, is an intricate yet undetermined interplay of androgenic hormones, non-androgenic secretions and genetic predisposition.

Current research provides that it is probable that multiple genes are responsible for the condition of cryptorchidism. Unfortunately, our knowledge of the specific nature or cause for undescended testes is limited to studies involving adult men and pigs; conclusions which are not necessarily applicable to dogs. Hormonal analysis has eliminated hormones as being the sole cause for a failure in descent. Rather, nonandrogenic factors which determine gubernacular outgrowth are thought to be contributory.

COMPLICATIONS: The ectopic testis is prone to a variety of complications including testicular torsion and neoplasm. Tumors may also occur in fully descended testes, however most develop at the site of the undescended testis. Tumors are classified by their histologic appearance.
1.) Sertoli cell tumors tend to be relatively small in size and rarely exhibit invasive behavior. Enlargement of the tumor, however, may result in hemorrhage and necrosis of local tissue.
2.) Seminomas will metastasize, spreading to surrounding lymphatic sites and the abdominal viscera. In addition, seminomas have been linked to symptoms which occur as a consequence of improper hormone levels. Ex: Alopecia, prostate disease and pendulous penile sheath.

It is not important that I detail the various tumors. It is important to understand that the majority of tumors occur in the cryptorchid male, at the site of the undescended testis. For this reason, an orchiotomy (removal of the testis) is recommended.

ESTABLISHING CARRIERS: A study involving Beagles revealed that 40 offspring from a single dam were needed in order to establish the dam as a carrier. Cryptorchidism, therefore, is extremely difficult to control once it is within a population. Establishing carriers is most tedious. Given these facts, we are inclined to throw up our hands and proclaim defeat. After all, we are seldom used to a good and challenging race; a race which, in the end, we may have to begin over again. Fortunately, we are encouraged by results which reveal substantive improvements in agriculture.

Thirty years of artificial selection have resulted in myriad improvements ranging from increased egg and milk production to a reduction of cryptorchidism in commercial livestock. Sheep provide an excellent example of how we might control cryptorchidism within a population of animals.

By removing all cryptorchids (affected males) from their breeding programs, sheep farmers experienced a 18-28% decrease in the incidence of cryptorchidism. Their startling results were not based on complicated permutations with regard to possible "carriers" They employed the best of all common sense by focusing on the obvious; eliminating from
their breeding programs, animals with one or no descended testicles. Frankly, it doesn't get much easier.

Commercial viability dictates that breeders of these animals place such emphasis on reproductive failures. After all, they cannot *afford* to risk infertility. Affected animals were systematically removed from the breeding population thereby reducing the incidence of cryptorchidism.

FINAL SELECTION: The frequency with which undesirable alleles appear within a population, may vary. All populations have a tendency, over time, to approach homozygosity (AA or aa). This "genetic drift" is a direct result of both natural and artificial selection. Genetic drift occurs as we select specific values for phenotypic expression thus altering genotype. Fixed characteristics, or those qualities that distinguish one pure breed from the other, are a consequence of genetic drift through artificial selection. Ex.: Coat color, size and temperament.

The high frequency of cryptorchidism within certain breeds as opposed to others, is a direct reflection of selection. A bilateral cryptorchid is infertile and thus, is unable to produce offspring. As a result, the undesirable allele is self limiting through natural and artificial selection as they are unable to bear young with the same defect. However, the unilateral cryptorchid is fertile. Through artificial selection, we may also decrease the percentage of undesirable characters by eliminating affected animals (those with one testicle but fertile) from breeding programs, thus preventing them from producing 'carrier' and similarly affected progeny.

If breeders continually introduce "mutants" into a population, that population will have a natural tendency to move toward homozygosity for the recessive allele/s; in this case, cryptorchidism. Over many generations, cryptorchidism becomes the norm rather than the exception. In other words, we "fix" the allele for cryptorchidism. Conversely, a
population will experience a decrease in cryptorchidism if we systematically eliminate affected animals from the breeding program. Where cryptorchidism may have been the norm, it now becomes the exception. Thus, results are not seen overnight but rather, are realized over a period of generations.

We cannot hope to eliminate the existence of undesirable alleles. We can, however, minimize the frequency with which they occur. Understanding that every breeder has a right to assign importance where they feel it is most deserved, each and every problem is as worthy as the next. Whether considering cryptorchidism, epilepsy, hip dysplasia heart disease, or thyroid disease, the goal should not be to eliminate a single defect but rather, to confront them all with equal fervor so to decrease their incidence within the population.

LEGACY: Responsible breeding practices dictate the future viability of all breeds, and just as we enjoy the legacy of past endeavors, breeders of purebred dogs are obliged to afford the same courtesy to future generations of fanciers. After all, compromising the reproductive capabilities of any population of animals will ultimately lead to its demise while leaving those who contributed, their rightful legacy.

Copyright 1994: Susan Lennard

References:

1.) Small Animal Medical Dictionary, Lorenz and Cornelius Canine and Feline Endocrinology and Reproduction.
Feldman and Nelson Tumors Of The Genital System And Mammary Glands
2.) Andrew S. Loar Merks Veterinary Manual,(6th edition), Management of Reproduction:
3.) Sm. Animals Genetic Analysis (5th edition)
4.) Griffiths,Miller,Suzuki, Lewontin and Gelbart: Canine Cryptorchidism
5.) Veterinary Clinics Of North America: Small Aimal Practice-Vol. 21, No. 3, May 1991.
6.) Textbook Of Small Animal Sex/Slatter, Male Reproductive System/Shirley D. Johnston

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