Bitches may be bred naturally, or artificially inseminated using fresh, chilled, or frozen-thawed semen. The practice of ovulation timing has become increasingly desirable to breeders. Popular stud dogs’ owners commonly permit a limited number of breedings (usually 2), and may need to prioritize bitches based on their timing. Owners of bitches wish to minimize travel time to the stud dog facility. Boarding of bitches in season can be reduced with recognition of their fertile period. The use of extended and chilled semen and frozen semen, or subfertile stud dogs, necessitates ovulation timing for optimal conception. Proper ovulation timing permits accurate evaluation of gestational length and is essential in the evaluation of apparent infertility in the bitch. In addition, litter size is optimal with properly timed breedings.
Sound knowledge of the bitch reproductive cycle is essential. Individual bitches may vary from normal, be presented at variable times during their estrous cycle for evaluation, and sometimes exhibit pathologic variations in cycles. Each of these scenarios requires veterinary interpretation. The normal canine reproductive cycle can be divided into 4 phases, each having characteristic behavioral, physical, and endocrinologic patterns, although considerable variation exists. Bitches with normal estrous cycles but unexpected patterns must be differentiated from those with true abnormalities. Detection of individual variation within the normal range of events in a fertile bitch can be crucial to breeding management. Evaluation of the estrous cycle for true abnormalities is an important part of the evaluation of an apparently infertile bitch.
The interestrous interval is normally 4-13 mo, with 7 mo the average. The anestrus phase of the estrous cycle is marked by ovarian inactivity, uterine involution, and endometrial repair. An anestrous bitch is not attractive or receptive to male dogs. No overt vulvar discharge is present, and the vulva is small. Vaginal cytology is predominated by small parabasal cells, with occasional neutrophils and small numbers of mixed bacteria. The endoscopic appearance of vaginal mucosal folds is flat, thin, and red. The physiologic controls terminating anestrus are not well understood, but the deterioration of luteal function and the decline of prolactin secretion seem to be prerequisites. The termination of anestrus is marked by an increase in the pulsatile secretion of pituitary gonadotropins, follicle stimulating hormone (FSH), and luteinizing hormone (LH), induced by gonadotropin-releasing hormone (GnRH). Hypothalamic GnRH secretion is itself pulsatile, its intermittent secretion is a physiologic requirement of gonadotropin release. Mean levels of FSH are moderately elevated, and those of LH slightly elevated, during anestrus. At late anestrus, the pulsatile release of LH increases, causing the proestrous folliculogenesis. Estrogen levels are basal (2-10 pg/mL) and progesterone levels at nadir (<1 ng/mL) at late anestrus. Anestrus normally lasts 1-6 mo.
During proestrus, the bitch becomes attractive to male dogs but is still not receptive to breeding, although she may become more playful. A serosanguineous to hemorrhagic vulvar discharge of uterine origin is present, and the vulva is mildly enlarged. Vaginal cytology shows a progressive shift from small parabasal cells to small and large intermediate cells, superficial-intermediate cells, and finally superficial (cornified) epithelial cells, reflecting the degree of estrogen influence. RBC are usually, but not invariably, present. The vaginal mucosal folds appear edematous, pink, and round. FSH and LH levels are low during most of proestrus, rising during the preovulatory surge. Estrogen rises from basal anestrous levels (2-10 pg/mL) to peak levels (50-100 pg/mL) at late proestrus, while progesterone remains at basal levels (<1 ng/mL) until rising at the LH surge (2-4 ng/mL). Proestrus lasts from 3 days to 3 wk, with 9 days average. The follicular phase of the ovarian cycle coincides with proestrus and very early estrus.
During estrus, the normal bitch displays receptive or passive behavior, enabling breeding. This behavior correlates with decreasing estrogen levels and increasing progesterone levels. Serosanguineous to hemorrhagic vulvar discharge may diminish to variable degrees. Vulvar edema tends to be maximal. Vaginal cytology remains predominated by superficial cells; RBC tend to decrease but may persist throughout. Vaginal mucosal folds become progressively wrinkled (crenulated) in conjunction with ovulation and oocyte maturation. Estrogen levels decrease markedly after the LH peak to variable levels, while progesterone levels steadily increase (usually 4-10 ng/mL at ovulation), marking the luteal phase of the ovarian cycle. Estrus lasts 3 days to 3 wk, with an average of 9 days. Estrous behavior may precede or follow the LH peak—its duration is variable and may not coincide precisely with the fertile period. Primary oocytes ovulate 2 days after the LH peak, and oocyte maturation is seen 2-3 days later; the lifespan of secondary oocytes is 2-3 days.
During diestrus, the normal bitch becomes refractory to breeding, with diminishing attraction of male dogs. Vulvar discharge diminishes and edema slowly resolves. Vaginal cytology is abruptly altered by the reappearance of parabasal epithelial cells and frequently neutrophils. The appearance of vaginal mucosal folds becomes flattened and flaccid. Estrogen levels are variably low, and progesterone levels steadily rise to a peak of 15-80 ng/mL before progressively declining in late diestrus. Progesterone secretion depends on both pituitary LH and prolactin secretion. Proliferation of the endometrium and quiescence of the myometrium develop under the influence of elevated progesterone levels. Diestrus usually lasts 2-3 mo in the absence of pregnancy. Parturition terminates pregnancy 64-66 days after the LH peak. Prolactin levels increase in a reciprocal fashion to falling progesterone levels at the termination of diestrus or gestation, reaching much higher levels in the pregnant state. Mammary ductal and glandular tissues increase in response to prolactin levels.
merck veterinary manual online source
