Given the length of time spermatozoa remain fertile (about forty-eight hours in the female genital tract), it is possible to optimize the chances of fertilization by making sure that sperm meets egg when both are at the height of their fecundity, to ensure the best fertility and optimal litter size. Ideally, mating or insemination should occur within the forty-eight hours following release of the oocytes, so that most of the eggs and sperm can reach the rendezvous point (in the oviducts). The ova remain capable of being fertilized for two days after maturation (in some breeds they even seem to remain fertile for up to four days), which explains why superfecundation by two different males can occur in dogs.
The most difficult part of the process is observing the biological signs of ovulation as accurately as possible. Several complementary tools, of varying precision, are available to breeders for this purpose:- Clearing of the vaginal discharge generally signals the end of proestrus, although it is not a reliable indicator of ovulation. Some bitches, such as Chow Chows, may show discharge until the end of estrus.- Systematically mating the bitch about twelve days after the first bloody discharge, then again two days later, is a practical technique if the first discharge is accurately noted. This remains an imprecise method, however, as some bitches (about twenty percent) do not ovulate during this period, and so do not conceive or conceive only a few puppies.- Acceptance of the male or teaser dog and appearance of the tail reflex are not indicative of ovulation. For example, bitches have been observed that allowed mounting from the beginning of proestrus, although they did not ovulate until later (thirty days later in the most extreme cases).
Many bitches also allow mounting during the false heats before whelping, when urinary infections are present, or when estrogen secretion by follicular cysts leads to nymphomania.
- Use of a galvanometer to measure the electrical resistivity of vaginal mucus can lead to a reasonably precise evaluation of the fluidity of vaginal secretions. This parameter usually decreases just after ovulation, signifying the end of the period of estrogen saturation and thus the rapid replacement of vaginal cells. However, this diagnostic measurement comes too late for breeding purposes, since predicting imminent ovulation is more useful than knowing that it has just occurred.
- Reagent strips that reveal biochemical changes in the vaginal mucus are difficult to introduce far enough into the vagina to avoid contamination by the urine. The results are usually imprecise (since the color change is seen within the three days preceding or following ovulation) and therefore not very reliable.
- Depending on the stain used, vaginal smears allow direct observation of changes in the vaginal cells related to hormonal variations (particularly of estrogen). This simple, economical technique is now routinely used by veterinarians and breeders to obtain a preliminary evaluation of which phase of the sexual cycle a bitch is in.
First, examine the swelling of the vulva. Pull the vulvular commissure downwards and introduce the swab vertically along the caudal wall of the vagina. Avoid bumping the clitoral fossa. When the swab has reached the roof of the vagina, pivot the swab until it is horizontal and push it in as far as possible without forcing. Collect exfoliated cells and secretions from the cervix by using circular movements.
The swab will usually look red at the beginning of the heat period, pink or colorless at the end of proestrus, and purulent if vaginal or uterine infection is present.
Roll the tip of the swab delicately on a previously cleaned slide. Avoid touching the same place twice, so that there will be no clumps of cells.
Fix the sample using a fixing agent. It can then be taken to the veterinarian or stained for immediate examination.
Vaginal smears provide a great deal of information in addition to the estimated time of ovulation.
When a bitch has run away, or if mismating has occurred through the kennel fence (!), the veterinarian can determine how long the spermatozoa remain in the vagina (up to six hours after coitus). He can also estimate the chances that fertilization has occurred, depending on the observed stage of the sexual cycle. For example, if the bitch is found to be in anestrus, the beginning of proestrus, or metestrus, the risk of pregnancy is minimal, and certainly less than the risk associated with an early abortion performed for convenience.
Vaginal smears also allow certain treatments to be performed during anestrus that would be contraindicated during periods of sexual activity. This includes most hormonal treatments.
Finally, along with hormone levels, they also help to diagnose certain causes of infertility (silent or anovular heats, persistence of a secreting corpus luteum, vaginal infection, etc.).
Thanks to their ease of execution, speed, low cost and the information they provide, vaginal smears are very useful in the field of canine reproduction. However, in certain cases when the interpretation of the smear is not certain, or does not match clinical symptoms, or if the costs of moving or inseminating the bitch are high, the owner can supplement this analysis with a more accurate tool: a measurement of progesterone levels in the blood.
Near the time when the bitch ovulates, the concentration of progesterone in the blood plasma normally rises over several days (five days, on the average) from its basal level (less than two nanograms per milliliter) to more than forty nanograms per milliliter. This rise takes place more quickly in some bitches than in others, and even varies from cycle to cycle in the same bitch. While eighty percent of bitches ovulate on about the twelfth day of their heat period, it is not rare to observe earlier or later ovulation, especially in breeds with this tendency (such as Dobermans and German Shepherds).
Traditionally, ovulation has been considered to have occurred when the concentration of progesterone exceeds 15 ng/ml (although variations due to different measurement methods in different laboratories must be taken into consideration). Mating or insemination should take place within 48 hours of this result, taking into consideration the time required for the oocytes to mature and to allow time for a second mating two days after the first.
This relatively accurate indication of ovulation not only increases the number of successful matings and inseminations, but also improves fertility. In fact, small litters, which are frequently blamed on the age of the bitch or an insufficient number of ova, are often simply a result of a poor choice of mating date.
By judicious use of both vaginal smears and measurements of progesterone levels in the blood, with due attention to careful methods, the heat periods can be tracked in a way that is both highly satisfactory and economically sound: fertility and litter size increase, unnecessary travel for unproductive matings is reduced, etc.
Before infertility in bitches can be treated, it must be very accurately diagnosed. For example, regular measurements of progesterone and other hormone levels, along with clinical examination, can distinguish between an anovular cycle, embryonic resorption due to involution of the corpus luteum, sexual immaturity and androgen saturation, which all have radically different treatments.
Most brachycephalic breeds (breeds with short faces), such as Bulldogs and Pugs, have whelping problems (dystocia) that cause the veterinarian to plan to deliver the puppies by Cesarean section. If a Cesarean is done too early, the puppies are premature, and usually die within a few hours after birth due to respiratory insufficiency. If the Cesarean is performed too late, the puppies suffer from cerebral anoxia due to the long wait in the birth canal. Viability of the fetus in dogs is linked to the late development of the lung surfactant that determines the respiratory capacity of puppies at birth.
In fact, the lungs mature at the same time that progesterone levels fall during the days prior to the ideal whelping date. So the measurement of the bitch's progesterone level gives the veterinarian an excellent tool for accurately determining whether the puppies are ready to survive if a Cesarean is done. This technique has considerably increased the survival rate of puppies born by Cesarean section, particularly in Bulldogs, where 90% of births are by Cesarean section.
Luteinizing hormone (LH) changes the oocyte's nourishing matrix into a corpus luteum that secretes progesterone. It is secreted by the hypophysis, and triggers ovulation. Thus, determination of the time of this hormone's maximum secretion predicts ovulation ahead of time, instead of merely revealing that it has occurred, as does an increased level of progesterone. This measurement is not yet routinely used by veterinarians when diagnosing infertility, except in a few very specific cases.
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