Infertility diagnosis in the different species, evaluation of the male for clinical management

Andrological or breeding soundness examinations are common for bulls, boars, dogs, rams and stallions. This chapter describes the comparative evaluation of anatomy and physiology for a breeding soundness examination (BSE). Further, the chapter summarizes the diagnosis of common reproductive problems.

Species-specific reproductive function
There are subtle anatomical differences in the basic structure and architecture of the testis and epididymis among species, and all, except the dog, have a full component of accessory sex glands (seminal vesicles, prostate and bulbourethral glands). The dog has only a prostate gland which, like that in man, is bulbous, surrounds the urethra, and with age often undergoes benign prostatic hypertrophy (BPH). Anatomically, there are three types of penis. In all cases, the penis remains in a sheath when non-erect. The boar, bull and ram have a fibroelastic penis which is semi-rigid even when non-erect and withdrawn into an S-shaped curve; erection is a result of engorgement of the corpus cavernosum. The stallion has an erectile penis, like the human, and undergoes substantial enlargement of length and diameter upon engorgement of the corpus cavernosum. In the dog, an os penis allows intromission prior to complete erection with vascular engorgement of the corpus cavernosum. This is accompanied by engorgement of the bulbus glandis, a specialized area of the corpus spongiosum. Muscular contractions of vaginal-vestibular muscles around the engorged bulbus glandis create a mechanical lock between the bitch and stud dog. In all species except the boar, the testes hang between the back legs. In the boar the testes are located on the caudal aspect of the back legs.

Depending on the species, spermatogenesis requires 39 to 61 days and transit through the epididymis an additional 7 to 17 days. Two to 5 days of this transit time are required for maturation of sperm within the caput and corpus epididymidis, and the remaining 4 to 14 days are used for maintenance and storage within the cauda epididymidis until ejaculation or voiding concomitant with urination. For the species being considered, each spermatozoon has a similar but distinctive paddle-shaped head with a compact acrosome over the rostral portion of the nucleus, under the plasma membrane. Other characteristics are as depicted in Fig. 1 of the chapter on semen. Numerous detailed analyses of sperm morphology have been completed and, at least for boars and bulls, a number of specific spermatozoal defects have been linked with the inability of that spermatozoon to fertilize an oocyte or to produce a normal embryo.

Daily sperm production typically is 10-19 x 106/g testis in bulls, dogs and stallions and 21-25 x 106/g testis in sheep and swine. Because weight of a single testis can range up to >700g for the pig, daily sperm production per male ranges from approximately 0.4 x 109 for dogs, 5-8 x 109 for bulls, rams and stallions, to 16 x 109 for boars. For dogs, the paired epididymides contain 2-5 x 109 sperm, whereas those of the bull or stallion contain 30-80 x 109 and those of boars and rams 100-130 x 109 sperm. Frequent ejaculation reduces the number of sperm stored in the cauda epididymidis and repetitive ejaculations in a single day could remove up to 50% of the number previously present.

Endocrine regulation of reproductive function involves gonadotropin-releasing hormone (GnRH), luteinizing hormone (LH), follicle-stimulating hormone (FSH), testosterone and estradiol (in a negative feedback control mechanism) as in other mammals. In most domestic or pet species there is diurnal and/or seasonal variation in the frequency or amplitude of pulsatile secretion of LH, and concomitant changes in the concentration of testosterone as measured in peripheral blood. This seasonal variation is especially significant in horses and sheep, where the female typically is sexually receptive only during a portion of the year. There is a distinct seasonal cycle to gonadal stimulation provided by the hypothalamic-pituitary axis, and sperm production fluctuates by 30-50% annually, although sperm are produced throughout the year. This endogenous seasonality of the reproductive cycles is synchronized by the effect of photoperiod length on animals.

Why a breeding soundness examination?
Semen collection from domestic and pet animals is relatively easy, which is a distinct advantage to the clinician. This is especially true of cattle for which there has been tremendous utilization of genetically superior sires for artificial insemination. A clinician evaluating a male before sale/purchase or before the start of a breeding season will perform a thorough clinical examination augmented by detailed examination of the reproductive organs and collection of one or more samples of semen. The question addressed is "Is this male likely to be of low fertility or, for other reasons, not useful as a breeding male at this time?'' If the answer is yes, because the male fails to pass one or more elements of the examination (Table 1), the owner would be advised not to use him for breeding at that time. The male, however, might pass a similar examination in 1-6 months. If the male passes the BSE, it is likely that the male will be of reasonable fertility when mated to normal females, but factors not detected (or subsequent changes) may intervene. In another situation, a male is presented because the owner is suspicious or knows that fertility is reduced relative to what it had been earlier or relative to normal males of that species, or that seminal quality is ``poor.'' This is an all too frequent occurrence with valuable stud dogs, stallions or bulls. The clinician attempts to address two questions: "Why is semen quality or fertility, or sexual behaviour of this male low?'' and "What is the prognosis for improvement?'' In some cases, the prognosis and treatment may be obvious, but more typically a limited knowledge of reproductive pathophysiology or economic conditions preclude an effective treatment other than allowing the passage of time.

Semen collection and analysis
There are three standard methods for collection of semen. An artificial vagina (AV) can be used with a male of each species, and semen collected by deflecting the penis into the artificial vagina as the male mounts a teaser animal or phantom. The AV is a latex-lined cylinder which contains warm water to provide heat and/or pressure. This approach produces a very physiological condition, apparently satisfying to the male, and usually provides an ejaculum most representative of that particular male. Stallions and bulls are the prime candidates for the AV and the primary limitation to its use is that the male must be accustomed to the procedure. With boars and dogs, the male is allowed to mount an estrous female or phantom and the collector simply grasps the free end of the protruded penis to mimic the spiral interior of the cervix of the female pig or the encircling vaginal vestibular muscles of the female dog, and masturbates the male. The alternative method of seminal collection, useful with bulls and rams, is electroejaculation (EE). This is achieved by placing a probe transrectally over the innervation dorsal to the intrapelvic accessory sex glands, and applying a mild electrical stimulation in a rhythmic manner. Appropriate restraint of the male is required. The resulting ejaculum typically is more dilute than that obtained by an artificial vagina, but the motility and morphology of sperm is not altered by the procedure. It is important to recognize that collection of semen, especially with an AV, can be potentially dangerous to the collector or the stud male. More importantly, although the number and quality of spermatozoa collected may be diagnostic, especially if multiple samples are collected over the course of 2 to 4 hours or at a uniform interval over several days, absence of spermatozoa in a particular ejaculum does not mean the male is permanently abnormal or infertile. Occasionally a normal male will provide seminal fluid devoid of sperm, although frequent occurrence of this will be symptmatic of a disease state.

In bulls and rams, ejaculation is essentially instantaneous, occurring over a few seconds with the semen discharged in a single jet. For boars, dogs and stallions, the ejaculatory process includes a series of emissions and ejaculations, and it is possible to collect the semen as a pre-sperm fraction, a sperm-rich fraction, and a post-sperm fraction.

Evaluating infertility in stud animals
In order to diagnose pathology in the male, a thorough history, clinical examination and semen examination are essential. The history of a subfertile male typically reflects multiple breedings and no offspring. If clinical examination reveals gross congenital or acquired lesions then the diagnosis of infertility is simple. However, a complete BSE must usually be performed to pinpoint a problem. The normal values of BSE parameters for several species are listed in Table 1.

Clinical problems in our domestic animals can be grouped into a number of primary causes.

Environmental effects might be the most common cause of reproductive problems in domestic animals. Heat is probably the greatest offender, especially in stallions, bulls, and boars. Rams are less affected because they are fall breeders. Stud dogs are usually kennelled in cooler areas; however, veterinarians see more cases of heat- and stress-induced oligospermia and increased numbers of morphologically abnormal cells. Excessively cold temperature can also limit a male's fertility, and freezing of testicular tissue results in sterility. Temperature-induced changes are diagnosed on history and BSE results. Therapy involves reversing the initial insult.

Infectious agents are common problems, especially venereal diseases. Brucellosis is a venereal disease that causes epididymitis, orchitis and azoospermia. Brucellosis is primarily a problem in cattle, sheep and dogs and usually requires euthanizing affected animals. Other bacterial infections are common in stallions and stud dogs. Diseases are diagnosed by the presence of white blood cells, culture/sensitivities and BSE. Therapy is as indicated by the culture and sensitivity.

Neoplasms are rare in domestic animals except in the dog where Sertoli cell tumors and seminomas are seen. In older stud dogs, prostatic tumors are fairly common. Non-neoplastic prostatic disease (hyperplasia or prostatitis) is also common in the stud dog. Therapy of prostatitis, prostatic hyperplasia and prostatitic neoplasia are antibiotics, castration, and ablation, respectively. Trauma is a common problem, especially in the herd situation. This is easy to diagnose clinically and therapy involves surgical intervention or medical therapy. Parasitism, metabolic problems and congenital problems are less common. These are usually diagnosed on history and clinical presentation which then guide medical treatment. Congenital problems are usually heritable and these animals should be removed from the breeding program. Finally, lack of libido can be a significant factor that is difficult to diagnose and treat.

Summary
Diagnosis of fertility problems in domestic animals is fairly simple if approached correctly. Clinical problems of domestic and pet animals can be grouped by primary cause, such as environmental effects, infections, neoplasms and trauma. Idiopathic azoospermia or infertility are also common and abnormalities secondary to congenital, metabolic or parasite problems also occur. Decreased semen quality due to high environmental temperature or humidity during the summer is a frequent problem in bulls, boars, dogs and stallions. Venereal diseases, including brucellosis, are common in dogs, cattle and sheep; epididymitis, orchitis and azoospermia typically result. Dogs and stallions commonly present other bacterial infections. The diagnosis of autoimmune and endocrinological problems is rare; however, we must be ever vigilant for their presence. The acquisition of a thorough history and completion of a comprehensive BSE will usually pinpoint the problem.

Failure to produce offspring is the most common manifestation of a fertility problem and can lead to substantial financial loss for the owner. Potential problems can be minimized by use of a breeding soundness examination prior to purchase of a breeding male; diagnosis of a fertility problem can often be relatively simple. Factors such as human-animal bonding and economics, as well as the underlying etiology of the infertility problem, dictate the appropriate course of action.

Suggested Reading

Barth AD, Oko RJ. Abnormal Morphology of Bovine Spermatozoa. Ames, IA: Iowa State University Press; 1989.

Cupps PT. Reproduction in Domestic Animals (4th Ed.). New York, NY: Academic Press Inc.; 1991.

McKinnon A, Voss J, eds. Equine Reproduction. Philadelphia: Lea and Febiger; 1993.
Morrow DA. Current Therapy in Theriogenology. Philadelphia, PA: W.B. Saunders Company; 1986.

Roberts SJ. Veterinary Obstetrics and Genital Diseases (Theriogenology). MA: Roberts Woodstock; 1986.