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Canine Ovulation TimingBy Melissa Goodman, DVM The estrous cycle of the bitch consists of four stages: 1. Proestrus: Proestrus begins with the observable signs of heat, such as vulvar swelling, the onset of a bloody vaginal discharge, and attraction of males. Hormonally, proestrus is characterized by rising estrogen levels. 2. Estrus: Behavioral estrus is the period of female receptivity, including such behavior as "flagging" and "winking". The vulva will soften and decrease in size, and often the discharge will change from bloody to clear or straw - colored. Hormonally, estrus begins with the luteinizing hormone (LH) surge and is characterized by declining estrogen and rising progesterone levels. Ovulation and the fertile period occur during this stage. 3. Diestrus: Diestrus begins approximately 8 days (range 6- 10 days) after the LH surge, and signifies the end of the fertile period. Progesterone levels continue to rise and remain elevated during diestrus in all normal bitches whether pregnant or not; this elevation in progesterone is necessary to maintain a pregnancy. 4,.Anestrus: Anestrus is the transition period between one cycle and the next. Progesterone levels return to baseline, either abruptly just prior to whelping, or gradually in the non-pregnant bitch, The reproductive tract "rests" for several months, while some hormonal changes occur to prepare the bitch for her next cycle. IMPORTANT HORMONES Estrogen: During proestrus, serum estrogen levels rise slowly over a period of 1 0 to 14 days, peak 2 to 3 days before estrus, and then decline rapidly. Estrogen's, major role is to prepare the reproductive system for breeding; therefore peak estrogen levels are reached prior to ovulation. These changes in estrogen levels result in: 1 . Behavioral changes 2. An increased turnover rate of vaginal epithelial cells (the cells lining the wall of the vagina). This process results in the changes seen in vaginal cytology. 3. Progressive swelling of the Female reproductive tract and bloody vaginal discharge. Blood estrogen levels are not reliable for timing breedings, however, because levels vary significantly from bitch to bitch, and there is no meaningful way to standardize the results. In addition, changes in estrogen are not directly correlated to the fertile period. Luteinizing Hormone: Normally LH is present in very small quantities. In early estrus, a significant increase in serum LH, followed by a return to baseline values, occurs rapidly, usually over a 24-hour period. It is this surge in LH that triggers ovulation, and thus determines the fertile period of the bitch. All events subsequent to the LH surge are consistent between bitches. Therefore, LH is the most accurate diagnostic tool for timing breedings. Serial daily blood samples can be performed to measure luteinizing hormone and identify the LH surge and the fertile period. Progesterone: Blood progesterone levels stay at a low baseline level during late anestrus and proestrus, and begin to rise at the time of the LH surge. Progesterone then remains elevated for two to three months. These increasing levels of progesterone, together with decreasing estrogen levels, cause the reduction in swelling of the reproductive tract that occurs during estrus. Increased progesterone is also necessary to maintain pregnancy. Measurement of blood levels of this hormone is consistent between bitches and simple to perform. Therefore, serial blood samples can be performed to identify the initial rise in progesterone that coincides with the LH surge, and breedings may be based on this parameter. OVULATION AND THE FERTILE PERIOD Ovulation is triggered by the LH surge, which causes the ovaries to release the developing ova, or eggs. Ovulation occurs 48 hours after the LH surge. The ova cannot be fertilized upon their immediate release from the ovaries, however. A subsequent maturation step, requiring 2 to 3 days, must occur before sperm penetration and fertilization can take place. Once -the eggs are mature, they remain viable for approximately 2 to 3 more days before they begin to degenerate. Thus, the actual fertile period of the bitch is only 2 to 3 days long, begins 4 or 5 days after the LH surge (2 or 3 days after ovulation), and occurs just prior to the onset of diestrus. PROGESTERONE ASSAY Blood progesterone levels are extremely reliable for ovulation timing when doing natural breedings or fresh artificial inseminations Progesterone levels can be determined by many private laboratories, although the turn-around time for test results is often too long to make this useful for ovulation timing. In addition, many laboratories have not standardized their Progesterone assays to be accurate in the range important for canine ovulation timing. Also available are in-house canine specific tests that your veterinarian can run in his office, with results available in less than 30 minutes. Remember that progesterone levels are baseline early in the bitch's heat cycle, but begin to rise at the time of the LH surge. Therefore, if the date of the initial rise in progesterone is identified, the LH surge can be estimated. LUTEINIZING HORMONE (LH) ASSAY The most accurate parameter used in ovulation timing is the actual identification of the LH surge by direct measurement. Since the elevation in LH typically has a duration of only 24 hours, blood sampling must be performed daily to identify this surge. An in-clinic canine LH assay is available which allows practical identification of the LH surge by your veterinarian, and is recommended for breedings using frozen semen and chilled extended semen breedings or as part of a thorough infertility investigation. BREEDING DATES In routine natural breeding or fresh Al with a normal, healthy stud, sperm may be expected to live within the bitch's reproductive tract 5 or more days. Thus, breedings performed a day or two before the fertile period should still be viable at the time of peak fertility. Subsequent breedings during the fertile period will maximize sperm numbers on the bitch's most fertile days. It is recommended to breed on days 2,4 and 6 post-LH surge if three breedings are possible. If not, perform two breedings between days 3 and 7 post-LH surge., If using chilled or frozen semen, or performing a natural breeding or fresh AI using a stud with compromised semen, it may be assumed that the longevity of the sperm cells are decreased and early breedings will likely be wasted. Therefore, these breedings should take place during the true fertile period, days 4-7 post - LH surge, OVULATION TIMING Many diagnostic and ancillary aids are available to assist in the timing of ovulation and breedings. No-single test or assay is fully reliable or completely correlates with the exact stage the bitch is in. A single examination, vaginal smear, LH or progesterone level provides very limited information. Ovulation timing is more accurate and breeding management is more successful when multiple parameters are repeatedly evaluated. About the Author Dr. Melissa Goodman, a 1983 graduate of the University of Florida College of Veterinary Medicine, practiced small animal medicine for five years before joining International Canine Genetics, Inc. where she received specialized training in small animal reproduction and became Vice President of Veterinary Services in 1991. Dr. Goodman is Board Qualified by the American College of Theriogenology (the specialty of veterinary reproduction) and maintains a practice limited to small animal reproduction in suburban Philadelphia. Dr. Goodman exhibits and breeds Golden Retrievers, is active in conformation, agility, field and obedience events, is a past Director of the Golden Retriever Club of America, and is Chairman of the GRCA Health and Genetics Committee. |
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