Canine breeding management--vaginoscopy and progesterone assays (Proceedings)

While vaginal cytology is an inexpensive, accurate way to diagnose the stage of the estrous cycle of a bitch, use of cytology alone will not allow the clinician to determine the truly important events in the cycle: the LH surge, ovulation, and the fertile period.  Use of other diagnostic tests, such as vaginoscopy or serum endocrine assays will aid in accurate determination of these events.

Vaginoscopy involves the use of a device such as a human pediatric proctoscope to visualize the caudal vagina of a bitch.  The vaginal mucosa changes dramatically in color and texture as the bitch progresses from anestrus through proestrus and estrus. Changes noted initially are due to the rise in estrogens, but later changes are due to the rise in progesterone.

Serum progesterone in the bitch first begins to rise in the preovulatory period as some of the granulosa and thecal cells luteinize prior to ovulation.  After ovulation, serum progesterone concentrations rise even more precipitously as the remainder of the follicular cells luteinize.  The concentration of serum progesterone at certain stages and the rate of the rise is relatively consistent between animals so that monitoring the pattern of serum progesterone concentrations in a particular individual can be predictive of specific physiological events.

Anestrus

Vaginoscopic examination of a bitch in anestrus shows the vaginal mucosa to be pink and smooth.  The only fold evident is the dorsomedial fold, which is an anatomic relic of embryological development and always present. The deep pink color comes from the close proximity of the capillaries since the vaginal mucosa is very thin during this stage.

Serum progesterone concentrations are at baseline values during anestrus (< 1.0 ng/mL). The ovaries are relatively quiescent with no luteal activity.

Proestrus

Estrogen concentrations begin to rise at the end of anestrus and throughout proestrus. Rising estrogen concentrations have a vasoconstrictive effect on the blood vessels in the reproductive tract, leading to edema of the vagina and vulva. Vaginoscopic examination during early proestrus shows longitudinal folds, termed "primary folds," throughout the vagina. The mucosa is still a light pink color, but becoming more pale. In late proestrus the edema continues to build and transverse folds become evident on top of the longitudinal folds.  These folds are trend "secondary folds".  The mucosa at this point has become even more pale.

Serum progesterone concentrations begin to rise as proestrus progresses and some of the granulosa and thecal cells undergo preovulatory luteinization. The concentrations will rise around or slightly above 1.0 ng/mL and remain in that range for usually a few days. Eventually, the serum progesterone concentrations will rise above 2.0 ng/mL.  This usually coincides with the peak of the LH surge in the bitch and is regarded by theriogenologists as Day 0. Indeed, the rise in progesterone and a concurrent drop in serum estrogen concentrations signals the LH surge. This is regarded to be the endocrinological boundary between proestrus and estrus.

Estrus

As progesterone concentrations rise and estrogen concentrations fall, the vasoconstrictive effects of estrogen are released. The edema resolves and leaves behind a very wrinkled, pale mucosa. These wrinkles are called "crenulations." The mucosa is very sensitive during this stage and responds to the touch of the vaginoscope with hyperemia, producing an effect known as the "rosette sign" because of the similarity to a rose.

Progesterone concentrations continue to rise and reach a range of 4.0 to 6.0 ng/mL around the time of ovulation (Day 2).  The bitch ovulates a primary oocyte and requires another 2 days before that primary oocyte matures to a secondary oocyte and is fertile and ready to fuse with a sperm cell. Progesterone concentrations during these two days continue rise and are usually around 10.0 ng/mL at the beginning of the fertile period (Day 4). The fertile period continues for another 2 days, at which time it ends because increasing progesterone concentrations result in closure of the cervix.  The serum progesterone concentrations at the end of the fertile period (Day 6) are usually around 22 ng/mL.

It should be kept in mind that these notes reflect what values that are often seen when managing the normal, average cycles of bitches.  Individual variation does occur and is not always abnormal.  In addition to biological variation, abnormalities do occur and include silent heats, split heats, and anovulatory cycles.

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