Canine estrous cycle and ovulation (Proceedings)


Canine estrous cycle and ovulation (Proceedings)

Aug 01, 2008

Bitches generally attain puberty two to three months after reaching adult body size. The average is six to 12 months with a range of four to twenty-four months. Smaller breeds reach adult size earlier and therefore reach puberty sooner than larger breeds. The first estrus can be abnormal in that it can be long or split. It generally will occur sooner if young bitches are housed with mature cycling bitches. The Basenji, Dingo and wolf cycle is usually once a year. Cycles begin at all times of the year but there is a small, yet significant increase in the occurrence of estrus in the late winter and early spring months.

The estrous cycle interval averages approximately six months with a range of 4 to 24. Clinical evaluations to determine the stage of the estrous cycle are based on: behavior, vulvar appearance, vaginal appearance, vaginal cytology, uterine tone, progesterone concentrations and luteinizing hormone concentrations.


Proestrus lasts approximately 9 days with a range of 3 to 17 days. It has been reported that several weeks prior to the onset of proestrus there may be an improvement in appetite and appearance and somewhat more tolerant of the presence of males. Immediately prior to proestrus the bitch may become listless, become inappetent, and demonstrate nervous signs. The beginning of proestrus is designated by the commencement of a serosanguinous vulvar discharge. During early proestrus the bitch will be aggressive toward males. As the bitch approaches late proestrus she no longer attacks the male but still may be unreceptive as evidenced by sitting down. During proestrus estrogen from the developing follicles, which began to increase 3 to 4 weeks prior to the onset of proestrus, continues to increase due to the effects of sustained follicle stimulating hormone (FSH) and luteinizing hormone (LH) release. The estrogen creates a positive feedback on the release of LH and possibly FSH.

The increasing estrogen causes edema of the vulvar lips. The swelling increases until the vulva is very firm and very enlarged. The source of the bloody vulvar discharge is the uterus. The blood vessels within the endometrium increase in size and length and vasodilatation occurs thereby causing a loss of blood into the uterine lumen by diapedesis. This discharge, although bloody, should not contain clots. The appearance of clots could be an indication of cystic ovaries or other abnormalities of the reproductive tract. The increasing estrogen also causes a growth of the endometrial crypts and differentiation of the glandular epithelial cells into well-developed secreting cells.

Vaginal cytology

One of the most commonly employed clinical aids used to determine the reproductive stage of the bitch and whether it is normal is vaginal cytology. A vaginal smear is obtained by moistening a sterile cotton swab with sterile saline and inserting it into the anterior vagina. The swab is gently rolled against the vaginal wall and removed. The swab is then rolled onto a microscopic slide and permitted to air dry. It is fixed with alcohol and stained. The slide is permitted to remain in the alcohol for 15 to 30 seconds, rinsed with tap water, and placed into eosin for 10 seconds. The slide is again rinsed with water, placed into new methylene blue for 15 seconds, and rinsed. The stains most commonly but not solely utilized are eosin and new methylene blue. The slide is rinsed a final time with tap water and air dried, patted dry with a towel or dried with a warm air drier. The slide can be examined at 100X, 200X or 400X magnification.

The order of slide examination that I have found most beneficial is to first note cellularity. It does not matter the type of cells at this time just whether or not there is an increase in number. The importance of this will be explained later. The amount of mucus and debris is next noted. This is followed by determining if there is an increase in the number of WBC's in the sample. Next the presence of RBC's is noted. The last part of the examination is to determine number and types of epithelial cells in the sample.

During proestrus the sample should appear as follows: high number of cells throughout proestrus; increased debris and mucus early decreasing late; few WBC's (1 to 3 per field) early and becoming absent late; numerous RBC's early and disappearing late in proestrus; parabasal epithelial cells will disappear and superficial cells will predominate late in proestrus. The noncornified cells are called parabasal cells. These cells have a rounded uniform cytoplasm with large healthy nucleus. These cells are close to the basement membrane and have a rich blood supply. The cornified cells are divided into intermediate cells and superficial cells. The intermediate cells have a cytoplasmic border that is roughened or irregular. The cytoplasm demonstrates signs of degeneration as evidenced by density changes. The nucleus is still apparent and normal or near normal in appearance. The superficial cell has a cytoplasmic border that is irregular and degenerating with a nucleus that is degenerating. This is apparent by the decreased density and the increase in size. It may also become pyknotic, very dark and small.

An anuclear cell is the most cornified superficial epithelial cell with a cytoplasm that is degenerated and the nucleus is absent.

Since each bitch is unique, it is necessary to do several smears in order to know the rate of progression and what is normal for her.


Estrus lasts approximately 9 days with a range of 3 to 18 days. It is suggested that the total time of proestrus and estrus should not exceed 21 days to be considered normal. The signs of estrus include a willingness to accept the male characterized by standing with the rear legs firmly placed and the tail lifted over her back or held to the side; known as flagging. It is important to note that not all bitches will stand for all males making it sometimes difficult to detect estrus by signs of receptiveness. The normal bitch may stand for the male as early as 11 days before or as late as 8 days following ovulation. The vulvar edema decreases rapidly in estrus such that wrinkles appear in the vulva and thus the vulva appears smaller than during proestrus. The vulvar discharge generally becomes serous or slightly brownish in color. In some bitches the discharge may remain red but examination of vaginal cytology will not reveal intact red blood cells indicating that the cells have previously lysed. The continued release of FSH and the increase in release of LH causes ovulation to occur to within 24 to 72 hours of the luteinizing hormone peak. The LH surge or peak occurs within 24 hours of the beginning of estrus. Estrogen concentrations are dropping and progesterone concentrations begin to rise more rapidly then during proestrus. Younger bitches tend to ovulate within 24 hours of the increase and older bitches ovulate slightly later. The bitch may remain in estrus for 7 or 8 days following ovulation even though a functional corpus luteum (CL) is present. Estrogen concentrations have decreased several days prior to the refusal of the bitch to stand. The increasing concentrations of progesterone and decreasing estrogen reduce the attractiveness of the bitch to the males.