Interaction of the CNS, hypothalamus, pituitary gland, gonads, and their target organs results in finely coordinated sequences of physiologic events that lead to estrus and ovulation in females and to ejaculation of fertile semen by males. For optimal results, ovulation and deposition of semen into the female genital tract must be closely synchronized. Failure of any single functional event in either sex leads to infertility or sterility.
The ultimate manifestation of infertility is failure to produce offspring. In polytocous species, a subnormal number of offspring also constitutes infertility. In females, infertility may be due to failure to cycle, aberrations of the estrous cycle, failure to conceive, or prenatal and perinatal death of the conceptus. Major infertility problems in males are caused by disturbances of the production, transport, or storage of spermatozoa; aberration of libido; and partial or complete inability to mate.
Most (if not all) major infertility problems have a complex etiology, and several factors, singly or in combination, can cause reproductive failure. Pathogenesis may be equally complex.
Because the female bears the offspring, she reflects either success or failure of reproduction. However, the first diagnostic step (regardless of the complaint) is to establish the etiologic role of the female and the male. Additionally, each point of human involvement in the reproductive process, such as observation for estrus, preservation of semen, and insemination methodology, is a potential source of error. Such human errors can be detected or excluded by assessment of performance, with the main emphasis on techniques and procedures and their adequacy and quality.
Diagnostic methods have been developed to test the anatomic and functional soundness of both sexes. These include the signalment, a complete history, and clinical examination, supported by diagnostic aids such as endoscopy, ultrasonography, and laboratory tests (eg, hormone assays, microbiology, cytology, serology, cytogenetic examination, genetic testing, and semen evaluation). The choice of diagnostic methods is determined by the species as well as by the size and temperament of the animal. Decisions with regard to type and extent of laboratory tests are based on history and information gained during the course of clinical examination. In each case of reproductive failure, the diagnostic plan should provide evidence to establish the role of the female, the male, and the breeding management program.
Reproductive problems are seldom accompanied by alarming signs of disease. Furthermore, there is a time interval between when a failure occurs and when it becomes apparent. Examples are intervals between unsuccessful service and return to estrus or failure to give birth. This lag period may allow recovery, yielding negative results on examination. Interpretation of results also must account for species differences and, in species with a seasonal reproductive pattern, the fact that infertility may be physiologic during certain parts of the year.