ISSN 2398-2942      

The estrous cycle


Mushtaq Memon

The estrous cycle with various approaches to infertility of unknown origin

  • A knowledge of the canine estrous cycle will solve the majority of cases of 'infertility' as most are merely cases of inappropriate management.
Hormonal cycle
  • Unlike other domestic animals, ovarian follicles in bitches start to secrete progesterone 2-3 days before ovulation occurs and, because the first increase in serum progesterone is closely correlated to the time of the LH peak, rapid ELISA assays for progesterone can be used to detect the approximate time of the LH peak. Although not as accurate as determining the LH peak itself, it is still a valuable 'first approach' to use in cases of infertility.
  • Besides bringing about changes in the vaginal mucosa, the peak of estrogen also triggers a reflex release of LH which matures ovarian follicles and causes them to ovulate. If one uses a rapid 'dog-side' LH test to test the blood every day once there are more than 50% superficial cells in the vaginal swab, the LH surge can be detected very reliably.
  • When the LH surge occurs it means that ovulation will occur between 24 and 72 hours later and that fertile oocytes will be available about 48 hours after that, ie on day 5 after the LH surge. (Oocytes in dogs need to undergo reduction division to the haploid state after they are ovulated, which accounts for the delay between ovulation and the time the oocytes can be fertilized.)
  • Detection of the LH surge is very important when frozen semen is being used because the best results are achieved by surgical insemination and obviously, abdominal surgery cannot be repeated every other day.
  • With fresh, transported semen, only one shipment of semen is required if the LH surge is detected accurately.
  • In the case of infertile animals where sperm longevity or asynchrony between female and male gamete life has been a problem, detection of the LH surge can be very valuable because the optimal time for insemination and fertilization can be determined.
  • Detection of the time of the LH surge is also very important if one wishes to know when whelping will occur. Although gestation may appear to vary between 58 and 72 days based on the time that bitches first allow copulation, the actual duration of gestation from the time of the LH surge is consistently 65+ 1 day (SD).
  • If there is a single puppy Dystocia , the precise time to do an elective Cesarean section Cesarean section will be known if the LH surge is detected and the true duration of gestation is known.
  • In routine whelpings, owners can be sure when to stay home to attend the birth of puppies if the LH surge has been detected. This is easier and more accurate than using basal body temperature to determine when a bitch will whelp.
  • Measuring LH is not easy but a progesterone assay Progesterone assay provides an indirect way of predicting the LH peak.
Histological changes in ovary and vagina
  • While circulating estrogen concentrations are elevated, ie before the onset of acceptance or estrus, there is sodium retention by the vaginal mucosa and it swells due to water uptake Vagina: prolapse. When estrogen concentrations begin to decrease, sodium retention decreases too and the vaginal mucosa shrinks, enlarging the vaginal lumen to accommodate the penis for copulation. Therefore, sudden shrinkage of the vaginal mucosa, called 'crenation', is a good indication that estrogen secretion has recently decreased and the fertile period has begun. Similar changes occur in the vulvar lips and these changes are at least as valuable as vaginal cytology for deciding when to breed a bitch. Breeding should occur every second day as soon as crenation is noted.
Vaginal cytology
  • The vaginal cytological changes Vaginal examination that occur before and after ovulation are well described in many basic texts and the clinician is encouraged to review these changes. In essence, vaginal swabs taken approximately every 2 days as ovulation approaches will show an increase in the population of cells that resemble crisps/potato chips (superficial cells), having small pycnotic nuclei or no nuclei at all, and a large amount of light basophilic cytoplasm. Shorr's trichrome stains show the cornified cells as eosinophilic and the non-cornified parabasal cells as basophilic. This change (from smallish cells that have large vesicular nuclei and relatively little cytoplasm - called parabasal cells) is mediated by estrogens. Hormonal monitoring is more accurate as this progressive increase in cornified cells is not always sequential, and false troughs and peaks may be seen, and the cell changes follow the hormonal changes and may be slightly delayed.
  • Vaginal cytology should be recommended in all infertile bitches as soon as there are signs of pro-estral hemorrhage. The time of the LH peak occurs at the time when the first maximum of superficial cells is seen (+3 days SD). In practice, there is time to assess the first maximum in retrospect because the fertile period only occurs several days after the LH surge. If the fertility of the sire is assured and the bitch is bred either naturally or artificially when the superficial cell population is about 60% of all the cells present (60% superficial index or SCI), and repeated every second day, good fertility will be achieved in most cases.
  • It is important to realize that a paradox appears to exist in bitches because circulating estrogen concentrations actually decrease before the onset of estrus. Even though estrogen concentrations decline, behavioral estrus and an increased superficial cell index continues for a total of 6-9 days.
  • After 6-9 days, the population of superficial cells suddenly decreases, signalling the end of the fertile period. If a case is presented once estrus has already begun and the first maximum of superficial cells has been missed, the change back to parabasal or intermediate type cells is still fairly useful to predict the approximate time of the LH surge and therefore whelping. It is the least accurate of all methods for determining the time of the LH surge.
Acceptance of the male/vulvar discharge
  • Detection of the first day of diestrus (DI) is important (when exfoliative vaginal epithelial cells change from cornified to non-cornified as there will be no chance of conception occuring after this time).
  • In cases of infertility, it should never be recommended that breeding commences when the bitch first accepts the male or when the vulvar discharge becomes clear instead of hemorrhagic; some bitches may never accept a male and bleeding (macroscopic or microscopic) can occur throughout the fertile period. However, the dog often does 'know best' and his reaction should be noted.

Further Reading


Refereed papers

Other sources of information

  • Concannon P W (1986) Pregnancy and parturition. Vet Clin North Am Small Anim Pract 16, 453-475. (This article contains some important data generated from the authors' Beagle colony that is the basis for an educated approach to reproduction in all breeds.)

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