Pamela A. Davol, 76 Mildred Avenue, Swansea,
The Normal Reproductive Cycle of the Bitch
Throughout the adult reproductive years of the female, the structural composition and hormonal activity of the ovaries are continually changing. Hormones, called gonadotropic hormones, produced by the anterior pituitary gland within the brain initiate such changes. During early development, prior to sexual maturity of the bitch, very little gonadotropic hormones are secreted and the ovaries, therefore, remain inactive. However, around the age of 6 months, the pituitary begins to secrete higher levels of the gonadotropic hormones called follicular stimulating hormone (FSH) and luteal hormone (LH). The rise in FSH and LH in the bitch will initiate the sexual cycle sometime between the age of 6 months and 14 months. Cyclical increases and decreases in FSH and LH, in turn, control the cyclic ovarian changes and, as such, are responsible for the physiologic events in the normal reproductive cycle of the bitch.
The bitch has two ovaries that will produce the ova (eggs). Within the ovaries, the ova are contained within follicles that grow toward the surface of the ovary. When FSH and LH from the pituitary gland begin to be secreted in high quantities during onset of sexual maturity, the ovaries and the follicles within them will begin to grow. Within these follicles, a follicular fluid hormone, secreted by the ovary, called estrogen, surrounds the ovum. This hormone is a biologic chemical that produces physiologic and social/behavioral effects within the bitch that will signal a readiness to mate.
Two days prior to ovulation, there is a surge in the secretion of LH by the pituitary gland proceeded by rapid swelling of the follicle. This LH surge is of critical importance because in its absence, even with the other hormonal physiologic effects taking place, ovulation will not occur. Additionally, the LH surge causes the ovarian cells to switch over to secreting progesterone hormone rather than estrogen. As a result, there is an increase in progesterone levels, and a decrease in estrogen levels. Within two days of the LH surge, the follicle reaches the surface of the ovary and bursts, thereby releasing the ovum into a capsule that surrounds the ovary. This process is referred to as ovulation. If the bitch is bred, then sperm will subsequently fertilize each of the released ova. The ova will then move down the oviducts, which connect each ovary to each of the two uterine horns, and the fertilized ova will eventually implant in the walls of the uterus where they will develop into fetuses. In the meantime, the ruptured follicles from which each ovum was developed will begin to produce a rapidly dividing mass of cells called luteal bodies, which will make up the corpus luteum. In addition to producing progesterone, which will maintain the pregnancy, the corpus luteum will also produce inhibin, the hormone that will signal the pituitary gland to decrease production of FSH and LH. When enough inhibin has eventually been secreted, this will end the mating period.
Though the above explanation mainly describes the mating stage of the bitch, the normal reproductive cycle of the bitch is comprised of four stages: proestrus, estrus, diestrus, and anestrus.
Proestrus: (average duration = 9 days; range = 3-17 days) Swelling of the vulva, the external tissue of the vaginal opening, and bloody discharge marks the beginning of the proestrus stage, also known as the follicular stage. During proestrus, the ovarian follicles, each containing ova, increase in size. Increasing amounts of estrogen hormone, secreted by the ovarian follicles, cause the cells of the vaginal walls to take-on a distinctive shape, a process known as cornification. Both the level of estrogen and vaginal cornification are useful indicators of proestrus.
Estrus: (average duration = 9 days; range =3-21 days) Receptivity to mating marks the beginning of the estrus stage. Physiologically, estrus coincides with the predominant presence of cornified vaginal epithelial cells and an increase in serum progesterone levels to 2 ng/ml. Ovulation usually occurs 2 days following this increase in progesterone and hence, monitoring the levels of progesterone is an excellent indicator for timing breeding.
Diestrus: (average duration = 2 months) Approximately 6 days after ovulation, the cornified vaginal epithelial cells will revert to a non-cornified state. This condition marks the beginning of diestrus. This stage ends when progesterone levels fall to less than 1 ng/ml just prior to whelping in the pregnant bitch or approximately 2 months after ovulation in the nonpregnant bitch.
Anestrus: (average duration =
4-4.5 months) The beginning of this stage is marked by the drop in serum progesterone
levels to less than 1 ng/ml. The beginning of proestrual bleeding marks the end of this
stage. Duration of anestrus is quite variable among bitches and may be governed by both
genetic and environmental variables.
The average bitch will experience the LH surge on Day 10 (where Day 1 is defined as the first day that bloody discharge is observed), will ovulate on Day 12, and will, therefore, optimally conceive on Day 14. Traditionally, bitches were usually bred on the 14th day following onset of proestrus. This was because it was observed that most bitches would display "tail flagging", defined as the lateral deviation of the tail with elevation of the vulva, and "standing heat", defined as the bitches' behavior in allowing the male to mount and breed, at this point in time. Additionally, other physical changes such as a softening of the flesh of the swollen vulva were external signs indicating the onset of ovulation. Later, when it became the norm to perform multiple matings, the bitch was bred on Days 12 and 14 (for double service), or Days 11, 13, and 15 (for triple service). Though these schedules are still adequate for insuring optimal breeding and litter size in the average bitch, not all bitches ovulate on Day 12 following proestrus onset. Some may ovulate as early as Day 5 or as late as Day 25 in which case utilizing this standard mating schedule will result in breeding failure.
Using receptive behavior of the female as an indicator for ovulation and therefore, a method for determining mating schedule has many limitations because these approaches are not always clear-cut. Some bitches may exhibit "phantom proestrus" (displaying little or no outward signs of bloody discharge, etc.) making it difficult to estimate the average date of ovulation, may "flag" and appear receptive to males throughout proestrus, or may remain unwilling to mate even following ovulation. The differences observed from bitch to bitch in regard to mating signs and behavior as well as the fact that unsuccessful mating will result in a 6-month or longer wait to "try again" understandably leads to a sense of anxiety in many bitch owners. Additionally, even cytological analysis, which can be used to determine onset of estrus, is often a poor predictor for ovulation since LH surge, a key precursor to ovulation, may occur 3 to 5 days before to 5 days after the onset of estrus. Therefore, a clinical screening test that accurately predicts ovulation in the bitch and therefore serves to optimize breeding schedule, is used frequently by many breeders.
The serum progesterone enzyme-linked immunosorbent assay (ELISA) is an accurate predictor for ovulation. For this assay, vaginal smears are examined periodically at the onset of proestrus to monitor cornification of the vaginal epithelial cells, which occurs as a result of increasing estrogen hormone. When the cells of the vaginal wall are approximately 60% cornified, as observed by microscopic analysis, testing with the serum-progesterone ELISA should commence. Blood samples are drawn, ideally, every day (though every 2 days may also be utilized), and whole blood or serum (depending on the test kit utilized) is added to a test indicator that has been treated with monoclonal antibodies specific for progesterone. Late in proestrus, the level of estrogen will decrease and levels of LH will surge. This LH surge is concurrent with an increase in progesterone levels, which will rise above 1 ng/ml on the same day. Therefore, detection of increased serum progesterone corresponds to the LH surge. This is an important indicator, since ovulation occurs 2 days following the LH surge. The serum progesterone ELISA manufactured by International Canine Genetics, a division of Synbiotics (Malvern, PA) provides a qualitative color change to indicate when the progesterone level in the bitches' serum rises. Early in proestrus, the test indicator will produce a strong blue color indicating low levels of progesterone (between 0.0 to 1.0 ng/ml). For determination of LH surge, the first appearance of a fading of the test color (as compared to an internal "low-progesterone" control indicator) to a light blue (approximately 2.0 ng/ml) indicates the LH surge. Two days later upon ovulation, progesterone levels will further increase to 5.0 ng/ml or above, at which point the test color will appear white, confirming ovulation. Though different kits utilize different testing methods and procedures, the concept of each kit is virtually identical and provides the means for determining ovulation. However, limitations to the sensitivity of ELISA testing may sometimes result in false-positive and false-negative results. This is because the greatest inaccuracy in measuring serum progesterone levels occurs in the range of 1.5 to 3.0 ng/ml of progesterone, the concentration range of importance for determining the LH surge. Greater accuracy occurs in the high range of greater than 5.0 ng/ml. Occasionally, a test will indicate a "medium" level of progesterone one day (suggesting LH surge), but may indicate a "low" level of progesterone when taken on the next day. This suggests that the former test demonstrated a false-positive because once progesterone levels rise, they should remain elevated and increase throughout ovulation. Therefore, to reduce incidence of inoptimal mating due to false-positives, two consecutive days of testing, whereby increased progesterone levels are indicated on both days, should be obtained prior to establishing the mating schedule. Additionally, a post-ovulatory testing should be performed on a day that mating is performed to confirm high levels of progesterone (5.0 ng/ml or greater), which indicates that ovulation has occurred.
Alternatively, ICG also offers an LH ELISA that works on the same principle as the progesterone ELISA but which specifically detects levels of serum LH. Though the LH ELISA may be used alone for determining ovulation, the major limitation with the LH ELISA occurs as a result of the brief time in which the LH concentration is elevated in the serum. Unlike progesterone concentration, which continues to increase, LH peaks within 24 hours and then quickly dissipates. As such, it is possible to miss the LH surge if one does not test on a consistent, daily basis. For this reason, Synbiotics recommends combined testing with the progesterone ELISA and the LH ELISA, using the latter to reduce the possibility of false-positives and false-negatives occasionally encountered with the progesterone ELISA. For example, on the first day that the progesterone ELISA indicates a rise in serum progesterone levels, one may confirm the concurrent LH surge by re-testing serum using the LH ELISA. If both tests are positive, then there is less likelihood of false-positive results.
Once the day of the LH surge is determined (to be considered Day 0), ovulation will occur on Day 2. Maximal litter size is achieved when the bitch is bred 2 days after ovulation (Day 4 following the LH surge). A single insemination 2 to 3 days following ovulation will result in pregnancy in the healthy reproductive bitch. The reason that optimal conception occurs 2 days following ovulation is because when ovulation occurs, the ova are immature (primary oocytes) and must undergo two meiotic divisions before they can be fertilized. These divisions can take up to 48 to 72 hours to occur. Once matured, the ova remain viable for another 2 to 3 days. Because normal sperm (spermatozoa) of the male delivered by natural insemination can live in the reproductive tract for at least 5 to 6 days, successful conception may occur if a bitch is bred from 2 days prior to ovulation to 4 days after ovulation.
Available ELISA Kits for Determining Ovulation
|K9-Proges-Check (ERK C2008)
K9-LH-Check (ERK C2007)
Cambridge Veterinary Sciences
|Endocrine Technologies Inc.||
Henry Crabb Row
P.O. Box 2251
|35325 Fircrest Street||
P.O. Box 9191
Princeton, NJ 08543
|Newark, CA 94560-1003||
College Station, TX 77842
The use of artificial insemination in canine breeding offers a solution to a number of situations that may prohibit or complicate natural breeding. Such situations include anatomical deterrents (such as a narrow vulva and vagina in a virgin bitch), unwillingness to breed (dominant/aggressive behavior of the bitch or submissiveness in the male), weakness or pain in the spine or hind limbs (in geriatric studs still providing service), reducing risk of sexually transmitted disease to the stud (brucellosis), or geographical distance between stud and bitch.
The collection of semen for artificial insemination will be discussed in a later section in Reproduction in the Male. Here, the procedures for inseminating a bitch with the collected semen will be discussed.
When using the method of artificial insemination, timing of insemination is a critical factor for ensuring successful conception. Freshly collected or chilled semen should be introduced by means of intravaginal insemination at least 2 days after ovulation has occurred in the bitch. For this procedure, the semen is drawn into a sterile syringe and an insemination pipette designed for bitches* (Synbiotics; *some large breeds may require a larger pipette custom-made from pipettes used for bovine uterine infusions) is attached. A gloved finger, lightly lubricated with water, is inserted into the vagina and the pipette is guided over the finger, and extended into the vaginal passage as far as it will go. The semen is then expelled from the pipette followed by some air to clear any remaining semen from the tube. The pipette is withdrawn, but the finger remains to "feather" (gently scrape in a backward motion) the wall of the vagina. This procedure is very important as it simulates the gentle pulling action of the male penis within the vagina following the "tie" in natural breeding. This procedure will induce muscle contractions of the vaginal walls that will assist the locomotion of the sperm toward the awaiting ova. After several minutes of this stimulation, the gloved finger is removed and the bitch should be positioned with her hind-end in an elevated position. Care should be taken to ensure that no pressure is placed around the abdomen, therefore, lifting should be performed by gripping the bitch at her knees. After being in this elevated position for about 5 to 10 minutes, the bitch should be immediately crated for 30 to 60 minutes. She should not be allowed to urinate, and if it is required for her to be placed into a crate in a vehicle, she should be lifted by two people, with the person lifting the hind-end, gripping the knees and not around the abdomen to lift.
Insemination using frozen-thawed semen requires a modified schedule for introduction of the sperm and more sophisticated techniques to ensure successful conception. First, the viability of frozen-thawed sperm is significantly reduced and as a result, compared to fresh or chilled sperm that may live up to 5 or 6 days in the reproductive tract of the bitch, frozen-thawed sperm live only a few hours. As such, the ova must be mature when frozen-thawed semen is introduced. Therefore, insemination with frozen-thawed semen is best performed 3-4 days following the LH surge (2-3 days following ovulation). Secondly, frozen-thawed sperm are insufficiently mobile to reach the ova if introduced by intravaginal insemination. For this reason, higher rates of conception with frozen semen have been achieved using intrauterine insemination. Several surgical methods have been developed for the purpose of delivering semen into the uterus. Laparotomy and laparoscopy utilize a small-gauge needle for delivery. Alternatively, transcervical catherization is performed by passing a catheter by way of the vagina through the cervix and into the uterus for delivery of the semen. In regard to advantages of one intrauterine insemination method to another, there appears to be no significant difference in pregnancy rates or litter size.
Overall variability in pregnancy success rate for artificial insemination methods:
|Fresh AI, intravaginal insemination:||62.3-100%|
|Chilled AI, intravaginal insemination:||59-80%|
|Frozen AI, intravaginal insemination:||52.6-60%|
|Frozen AI, intrauterine insemination:||0-80%|
Following fertilization of the ova by the spermatozoa, the once single-celled, now fertilized egg will immediately begin to divide, becoming a blastocyst. Though not all the ova may have been fertilized at the same time (some may have been fertilized during subsequent matings) the tubo-uterine junction through which the blastocysts will pass to enter the uterus will not open until approximately 10 days following the LH surge. Therefore, although some of the blastocysts may be older than others, all will migrate to the uterus usually between days 12 to 16 and implant in the uterus at the same time usually around day 20. Therefore, determination of pregnancy can only be determined after this time point.
Though some breeders may rely on several external signs such as mammary-gland development, increased appetite, etc. for confirmation of breeding success, these methods are unreliable because many pseudopregnant bitches (those experiencing "false-pregnancy") will display these signs as well. Therefore, breeders routinely use more accurate methods.
The most common method for determining pregnancy in the bitch is done by palpation, a process of feeling for detectable swellings within the uterus. Abdominal palpation of the uterus is usually scheduled between days 26 to 28 after the first breeding. Uterine swellings are only discernable by palpation for a period of about 10 to 15 days (between days 20 to 35 following breeding), and if no uterine swelling can be detected upon the first examination, a second exam should be scheduled one week later. By day 28, uterine swellings in the Labrador (and other medium- to large-sized breeds) measure about 3 to 5 cm and can be felt as distinct entities. After day 30, rapid enlargement of the uterus may make it more difficult to discern the individual swellings. Later, by day 45 to 50, the fetuses may be individually palpated.
Limitations to palpation do exist and may present as false-negative or false-positive results. False-negatives often occur in some small bitches or short-backed bitches, or in those carrying only a small litter, in which the uterine swellings may be hidden behind the ribs and as such, be difficult or impossible to palpate. Alternatively, false-positives may result particularly when screening bitches with a saccular-form of pyometra (a uterine infection) that often feels similar to swellings palpated in pregnant bitches.[For information on "pyometra" please refer to: Robin Camken's Pyometra Links]
Currently ultrasound is the best method for determining and staging pregnancy in the bitch and is becoming routinely used by many breeders. Ultrasound technology utilizes the safe and non-invasive method of soundwave imaging for detection of embryos/fetuses. Though the expert can detect embryonic vesicles as early as 19 days following breeding, typically screening is performed at or after Day 25 when the actual embryos can be visualized. Additionally, by this time, heartbeats are also usually discernable.
Ultrasound is particularly helpful for differentiating between pyometra and pregnancy in bitches. Furthermore, in addition to diagnosing pregnancy, ultrasound is currently being assessed for its ability to detect fetal distress during pregnancy, which would be indicated by decrease in the heartbeat rate of fetuses (normal rates reported at 200 to 255 beats per minute).
Limitations to use of ultrasound do exist. Often, for best visualization, an area of fur must be shaved on the bitch. Additionally, several factors including the skill of the technician, the temperament of the bitch, and the size of the litter when attempting to estimate litter number will have bearing on the quality of imaging. In regard to the latter, it may be more difficult to determine the number of fetuses accurately in bitches pregnant with large litters.
Survey radiography utilizes radiation in the form of x-rays to visualize fetuses. As such, the use of this method for diagnosing pregnancy is limited to late-stage pregnancy (pre-partum) and post-partum evaluation. This is primarily because fetal skeletons are not sufficiently mineralized to become radiopaque (detectable by x-ray) until sometime between Day 42 and 52 after breeding. Despite this limitation, survey radiography does have an advantage over ultrasound in its ability to more accurately determine the number of fetuses. Survey radiography is also capable of detecting skeletal deformity and excessive flexation, which are indicators of fetal death. It may also be helpful for assessing potential whelping complications by providing comparison between fetal skull size or over-sized fetuses to the width of the bitches' pelvic canal. In regard to post-partum diagnostics, survey radiography is routinely used to assess the bitch for retained fetuses.
Endocrinologic and Biochemical Testing
Unlike humans and horses that produce a pregnancy-specific gonadotropic hormone that can be quantitated by assay and thus utilized as an indicator of pregnancy, no similar such hormone has yet been found in the dog. Recently, however, a hormone, known as relaxin, which is secreted by the placenta of the dog and functions to relax the pelvis before whelping, has been evaluated as specific marker for canine pregnancy. Serum relaxin is not present in non-pregnant dogs; however, concentrations of relaxin increase to detectable levels in the pregnant bitch approximately 25 days following breeding and peak at Day 40 to 50.
Synbiotics (Malvern, PA) has developed an ELISA test, called ReproCHEK, for detection of serum relaxin for the purpose of pregnancy testing in the bitch. For routine testing, a blood sample is obtained from the bitch 25 days after breeding or between days 22 to 26 after the LH surge if ovulation testing has been performed. Antibodies specific for relaxin will detect presence of the hormone in the blood sample. In preclinical studies, the relaxin ELISA was specific and sensitive for detecting pregnancy in bitches without the occurrence of false-positive results. Additionally, the relaxin assay is quite effective at distinguishing pseudopregnancy from actual pregnancy since relaxin hormone is absent in pseudopregnant bitches. Other applications for this assay include pregnancy monitoring, since sudden decrease in relaxin is an indicator of spontaneous abortion in the bitch.
Testing of plasma proteins as indicators of pregnancy in the bitch have included assessment of fibrinogen and C-reactive proteins, which are commonly elevated in pregnancy and some other conditions. Serono Diagnostic (United Kingdom) offers one test for measurement of fibrinogen, which is elevated 25 to 30 days after breeding and as such may be used as an indicator between days 30 and 50 of pregnancy, after which levels decline. Testing for C-reactive protein also has indications for ability to diagnose pregnancy in that levels of this protein also increase between days 30 and 50 of pregnancy. However, because inflammatory responses associated with infection as well as placental development within the uterus may also lead to elevation of these proteins, there is a high incidence of false-positives when utilizing these indicators for determination of pregnancy.
Health Management of the Bitch, Before Breeding and During Pregnancy
Of the many factors that will come to play a role in the sound development of a dog from puppyhood to adult, intrauterine influences on the early embryological development of that dog and their impact on subsequent development are often not emphasized enough. Such factors may be external or internal and include nutrition, hormones, chemicals, presence of infection, etc. to which the bitch may be exposed. As such, ensuring health of the bitch before and during pregnancy increases the likelihood of producing genetically sound offspring.
All health problems that may potentially compromise the bitch and offspring should be identified and treated prior to breeding. In addition to obtaining routine genetic screening and certifications to confirm that the bitch is free of common, breed-specific genetic disorders that could potentially impact upon the quality-of-life of future offspring, the bitch should receive a thorough physical examination. External and/or internal parasite infections should be diagnosed and treated prior to breeding. Additionally, the bitch should be brought up-to-date on all vaccinations before breeding since administration of vaccines to a pregnant bitch is contraindicated.
Besides routine physical health examination, a vaginal examination should be performed to confirm normal anatomy and identify any abnormalities or infections (vulvar and vestibulovaginal stenosis, vaginal septa, vaginal neoplasia, vaginitis, etc) that could present complications to the actual mating, conception, gestation or delivery. Additionally, vaginal cultures may be obtained for the purpose of screening for increased levels of organisms (mycoplasma, E. coli, etc) that may be indicative of potential pathogenic state (although the benefit of this procedure is considered questionable by some clinicians because of the difficulty in differentiating between normal and pathogenic organisms inhabiting the vaginal tract of the bitch). In addition to the routine blood screening to insure that the bitch is not infected with brucellosis prior to breeding, a test required by most stud owners, other laboratory blood work including complete blood count, serum biochemistry profile and urinalysis should be obtained to both acquire base-line values (in the event that the bitch develops a health problem during pregnancy for which blood values at that point in time may be compared to these primary normal values) and identify any asymptomatic infections or abnormalities that may surface under the physiological stresses of pregnancy.
In addition to ensuring a healthy pregnancy, proper diet and exercise prior to breeding increases the conception rate of bitches. Ideally, any correction of inoptimal conditioning should also occur prior to breeding. For example, bitches that are underweight at breeding may be further compromised nutritionally around the third week of gestation when many bitches experience nausea, vomiting and anorexia. Increasing weight prior to breeding or feeding a growth or performance food early in the pregnancy will correct this condition. In contrast, bitches that are too obese should undergo weight reduction prior to breeding. Obesity decreases rate of conception and increases risk for whelping complications. Once bred, however, an over-weight bitch should not be dieted.
Pregnant bitches in overall good condition will require an increase in their protein and carbohydrate intake during the last trimester of their pregnancy. This is usually accomplished by increasing daily food rations beginning sometime between day 28 and day 35 (fourth to fifth week) of pregnancy, when research indicates the bitches' energy requirements will increase above the nutritional provisions of her usual maintenance ration. Although some breeders switch their bitches to specialized growth or performance formulas, this is not viewed as a necessary practice in the average, well-conditioned pregnant bitch. Additionally, switching foods may lead to gastrointestinal irritation, which only serves to be counter-productive. Large quantities of food at a given feeding, however, may result in discomfort to the bitch particularly in the last several weeks of pregnancy when the expanding uterus is putting pressure on the stomach and other digestive organs. Therefore, multiple feedings of small meals is recommended to circumvent this problem.
Dietary supplementation of the pregnant bitches' usual food with vitamins and other minerals is a practice discouraged by veterinarians in recent years. It is now recognized that many vitamins and minerals may actually be harmful to the developing fetuses or produce serious pregnancy-related complications in the bitch. For example, excess vitamin A is associated with congenital defects such as cleft palate, and vitamin D may compromise the calcium mobilization within the bitch. Excess vitamin C may interfere with normal processes of bone development, and because dogs produce sufficient amounts of this vitamin, supplementation with vitamin C is simply unnecessary. Some breeders insist upon administering calcium supplements to their pregnant bitches, which is an extremely dangerous practice. Although bitches do need to increase their calcium intake, this added source of calcium will be attained by simply increasing the quantity of food they consume. Excessive levels of calcium in the body, which usually occurs when a bitch is supplemented with calcium above the amount found in a high-quality dog food, interferes with the normal processes of calcium storage and metabolism by suppressing parathyroid hormone production. Ironically, therefore, calcium supplementation actually has the opposite effect of its intended purpose because excess calcium interferes with calcium storage and places the bitch at risk for developing hypocalcemia (pre-eclampsia or eclampsia), an often fatal metabolic disorder caused by insufficient calcium availability in pregnant and nursing bitches, respectively.
In addition to contraindications for vitamin and mineral supplementation, administration of any type of medication is not recommended between days 13 and 30 of the pregnancy. When administration of medication is warranted during any point in the pregnancy, the risk to benefits should be carefully weighed. Though some drugs have been deemed as relatively safe for use during pregnancy, others have been found to be dangerous, and effects of some others on pregnancy and the developing fetuses are still unknown.
|TABLE 1. Drugs Considered Safe in Pregnancy||TABLE 2. Some Drugs to be Avoided in Pregnancy|
|Amoxicillin with clavulanic acid||Trimethoprin|
[The above Tables are from: Freshman JL. Current therapeutic recommendations for pregnant dogs. In Kirk's Current Veterinary Therapy XIII, Bonagura, J.D. (ed.), W.B. Saunders Co., Philadelphia, 1999. pp. 931-933.]
In addition to proper diet, exercise is essential for maintaining fitness. Pregnant bitches that are maintained in good physical condition have fewer complications during whelping. Walking is considered the ideal form of exercise for the pregnant bitch. Late in pregnancy, however, length of walks should be shortened because the bitch will usually tire easily, especially when carrying large litters.
Though low-impact exercise is recommended, more demanding physical exertion, such as is required of working bitches used in hunting, herding, agility, or advanced obedience, is discouraged. These forms of high-impact exercise increase risk for abdominal trauma and may induce stress to the fetuses. Indeed, studies exploring the environmental stimuli experienced by the bitch in regard to effects on developing fetuses indicate that even mild forms of stress such as handling and restraint, show exhibition, classes and performance events induce effects on the hypothalamic-pituitary-adrenal axis of the fetuses. As such, puppies born of bitches exposed to stress during pregnancy demonstrate a permanent condition of hypersensitivity to stress stimuli demonstrated by increased and prolonged cortisol production when encountering stimuli in their own environments. Therefore, it is advantageous to the physical well being of the offspring to avoid factors that may produce stress in a pregnant bitch.
[This section describes the physiologic events that will occur in the bitch during a normal, uncomplicated delivery; this section does not discuss complications associated with delivery or neonatal care and management. A discussion of complications that may produce abnormal deliveries and warning signs related to such deliveries will be presented in Part 3 of this article: Medical Management of Complications Affecting Delivery (Whelping). For information and a list of related links on neonatal delivery/care, please refer to: "A Typical Whelping at Wing-N-Wave."]
There is really not a great variability in the length of gestation from bitch to bitch, unless of course one compares bitches for which ovulation time has not been accurately determined by serum progesterone or LH testing. In such an instance, using the first day of a mating to estimate the date of whelping can be very inaccurate with length of gestation ranging from 57 to 72 days. However, when actual time of the LH surge is determined, bitches in general have been found to have a very precise length of gestation, with little variability, that spans 65±1 days from the day of the LH surge (63±1 days from ovulation).
Late in the pregnancy, prostaglandin concentrations begin to increase. This hormone is believed to restrict the blood flow to the corpus luteum resulting in its degeneration. About 24 to 48 hours prior to whelping, the temperature of most bitches will drop to below 99oF (normal body temperature is about 101.8oF) and remain below 99oF. This drop in temperature is caused by a sudden decrease in serum progesterone concentrations caused from loss of the corpus luteum, which secretes this hormone. This drop in serum progesterone induces an increase in the pituitary hormone prolactin, which in addition to initiating the secretion of milk also has specific effects on maternal behavior. As a result of increased prolactin, panting, restlessness, and intense nesting (digging, scratching, and tearing at blankets, etc. to create a place to whelp) are behaviors that will coincide with the bitches' temperature drop. Additionally, the bitch may vomit and/or refuse all food at this point. Uterine contractions may be noted, but they are usually asynchronous and mild.
Eventually, first-stage labor will begin and uterine contractions will become intense and synchronous as the cervix of the uterus dilates. At this time, clear or white discharge from the vagina may be observed confirming that the cervix has dilated. Usually, within 24 hours of the temperature decrease, second stage labor will begin with strong abdominal thrusts, which will lead to expulsion of the puppies. Puppies are born enclosed within a fluid-filled sac of embryonic membrane and usually each puppy is attached to an individual placenta. The puppy will be whelped first, typically a headfirst delivery (though feet-first, breach-births may also occur without complications) followed by the placenta. In normal deliveries, puppies will be born in succession anywhere from several minutes to 1-1/2 hours apart. When long intervals between births occur (20 min or more), it is normal for the bitches' contractions to decrease in frequency. Contractions will increase in frequency as the next puppy's birth approaches. Since a full bladder or rectum may slow the progress of labor, even in a normal delivery, if a long interlude between deliveries occurs, leash-walking the bitch and allowing her to urinate or defecate may help to speed-up labor. The bitch, however, should be carefully supervised while voiding to ensure that a puppy is not whelped and left behind outside. Additionally, allowing puppies, which have already been whelped, to nurse between deliveries will stimulate uterine contractions and may assist to speed-up the delivery process.
Interludes between births are also a good time to provide some general supportive measures to the bitch. Though it is not recommended for the bitch to ingest substantial quantities of food and water prior to the successful completion of delivery of the entire litter, small amounts of water or ice cubes will help to prevent dehydration, especially during long deliveries. Additionally, labor is very taxing on the bitches' energy metabolism and sugar storage, therefore, providing a source of sugar in the form of small amounts of Karo syrup or Nutri-cal will help to replace depleted energy sources.
A normal whelping, without complications, ranges in duration from several minutes to several or many hours and will be dependent on the litter size as well as the size of the puppies. Though palpation of the abdomen will often confirm that delivery has been completed, it is sometimes difficult to determine this with certainty. Typically, only when the uterus is completely empty of puppies and all the placenta will the uterine contractions cease.
Occasionally following the estrus cycle, a bitch that was either bred or not bred during estrus develops overt signs associated with pregnancy and lactation. If the bitch was bred, these signs may misguide the breeder to confirm a successful mating. Complicating the situation further, around day 60 of the "suspected" pregnancy, the bitch will often experience a decrease in body temperature and will display the familiar nesting, restless, panting behavior typical of a bitch about to whelp. However, no puppies will be delivered because the bitch is experiencing a condition known as pseudo-pregnancy.
During estrus, the surge in LH that results in ovulation also stimulates the ruptured follicles, from which the ova are released, to secrete progesterone. At this point the follicular cells become the corpus luteum. If the bitch becomes pregnant, the corpus luteum is preserved and continues to produce progesterone, which is required to maintain the pregnancy. If the bitch is not bred or mating is unsuccessful, a hormone produced by the uterus called prostaglandin F2a will cause the corpus luteum to regress, progesterone levels will decrease, and diestrus will follow estrus.
In bitches that experience pseudopregnancy, the corpus luteum does not regress despite the fact that conception has not taken place. As a result, increased progesterone levels are maintained bringing about the signs consistent with pregnancy. Around day 60, progesterone levels will abruptly drop as is observed at the end of gestation in pregnant bitches. This drop in progesterone results in elevation of the hormone prolactin, which is responsible for the typical nesting-behavior in pregnant bitches. As such, the pseudopregnant bitch will display the same behavior as a bitch that is actually pregnant.
Many bitches are only mildly affected and do not require treatment for pseudopregnancy. In those bitches that may demonstrate more severe physiological and behavioral symptoms, treatment by ovariohysterectomy (after lactation ceases), testosterone and/or estrogen (though hormonal treatment should not be used in breeding bitches because they may increase the likelihood and intensity of future episodes of pseudopregnancy within the same bitch), or metergoline, a serotonin antagonist, that blocks effects of prolactin may be utilized.
Complications associated with pseudopregnancy are few but may include mastitis, an infection of the mammary glands, particularly in those bitches who may lactate and through self-nursing induce an ample milk supply.
Regimens for the Management of Mismating
Occasionally, a breeder will have a reason for terminating a pregnancy in a bitch. An accidental mating (mismating) of a young, first-estrus bitch, an unanticipated health problem that suddenly develops shortly after breeding a bitch, or evidence that a bitch may develop life-threatening complications if allowed to deliver are some of the situations that may necessitate aborting a litter. In situations of chronic illness or where pregnancy in general may present a serious health risk to the bitch, ovariohysterectomy (spaying) up to 3 to 4 weeks after mating is the first choice of many veterinarians to end an unwanted pregnancy and a finite way to ensure against the possibility of future pregnancies. However, in the case of mismated bitch for which a breeder intends to use at a later time in a breeding program, ovariohysterectomy is not a viable option. Therefore, more acute methods are required to terminate the current unwanted litter while preserving the reproductive function of the bitch. Because hormones play an essential role in reproductive development, mating, and maintenance of a pregnancy, it is not surprising that such non-surgical approaches to abort unwanted pregnancies focus on inhibiting or interfering with the intended function of these hormones.
There are three stages (trimesters) of pregnancy in the bitch. The first stage begins with fertilization and ends when the embryos implant in the uterus (approximately 20-22 days after the LH surge); the second stage begins at implantation and ends when the fetuses undergo ossification (40-42 days following the LH surge); the final stage begins at fetal ossification and ends at whelping. Ideally, if a pregnancy is to be terminated, this process will take place in the second stage. During the first stage, pregnancy diagnosis is uncertain and treating a possibly unpregnant bitch to induce abortion exposes her to potentially unnecessary health risks associated with treatment. Abortion during the third stage of pregnancy will result in expulsion of the fetuses and because of the wide margin of variation in duration of pregnancy, the later the abortion the greater the risk for delivery of live puppies. Therefore, second stage abortions are the preferred time period because they will result in fetal resorption, which poses the least risk to the bitch, and offer less unpleasantness to the owner.
No matter what method is employed, follow-up monitoring should be utilized to insure the successful completion of the procedure. For this monitoring, ultrasonography is the best method. Ultrasound imaging of the uterus should be performed 5-7 days following the treatment regimen. At such a time point, changes in fetal anatomy, placental detachment, fetal decomposition and resorption can usually be observed. If imaging indicates evidence of unsuccessful or incomplete termination, a repeat of the drug-treatment regimen is usually employed. Alternative methods for monitoring include utilizing tests that measure the decrease in serum progesterone levels associated with pregnancy termination.
One of the most successful and safe anti-progestin therapies for preventing unwanted pregnancy in dogs is use of the progesterone antagonist, algepristone (Alizine). As with most antiprogestin drugs, algepristone works by competitively binding to the progesterone receptor and thereby blocking the binding and biological effects of progesterone hormone, which is required to maintain pregnancy. Like the antiprogestin mifepristone (RU 486), an effective abortant in humans and several other species, aglepristone is extremely effective demonstrating a 94.8% to 100% efficacy rate at terminating pregnancy when administered to bitches from 0 to day 45 of pregnancy. Treatments with antiprogestines, in general, are more effective and produce fewer side-effects when used early in the course of the pregnancy, prior to embryo implantation. Once embryos implant, treated bitches have a higher risk for developing symptoms of pseudopregnancy including mammary development and lactation. However, no other side-effects were observed in dogs treated with aglepristone. The major limitation to use of aglepristone is that it is currently unavailable in some countries including the US because of the controversial use of antiprogestins in human contraception.
Prostaglandins are natural inhibitors of progesterone that are synthesized late in normal pregnancy to reduce the amount of circulating progesterone and induce birth. Prostaglandins work by inhibiting the blood supply to the corpus luteum and promoting its degradation. Therefore, as prostaglandin levels rise, progesterone levels fall. Additionally, prostaglandin also induces smooth muscle contractions to promote expulsion of the fetuses from the uterus.
To be effective, prostaglandin treatment must be administered at least 13-15 days following the LH surge. Because many side-effects are obtained by using natural prostaglandins, synthetic analogues have been created to imitate the natural molecule while reducing the side-effects associated with use. Such side-effects can be severe and include excessive salivation, vomiting, diarrhea, loss of coordination, respiratory distress, anxiety and pupil dilation/constriction. However, even the analogues can cause significant complications so many veterinarians will combine prostaglandin treatment with parasympatholic agents such as atropine or butylscopolamine to reduce severity of the symptoms. Despite these approaches, bitches must still be hospitalized for the extent of the treatment regimen. Additionally, abortion with prostaglandins is more easily induced when treatment is administered beginning at least 30 days after mating, when implantation of the embryos has already occurred. Lower doses are required to achieve termination of pregnancy at this point in time, and hence, fewer or less severe side-effects occur.
Dopamine Agonist Agents
Prolactin is the primary LH in dogs and is required for sustenance of the corpus luteum, which produces progesterone. Prolactin synthesis by the pituitary gland is stimulated indirectly by serotonin because serotonin inhibits dopamine, which is a direct inhibitor of prolactin. Therefore, increased levels of serotonin will block dopamine's ability to inhibit prolactin and prolactin levels will increase. Around day 25 to 30 of pregnancy after the LH surge, prolactin levels significantly increase. When drugs that stimulate dopamine (dopamine agonists), such as bromocriptine or cabergoline, are administered to the pregnant bitch during this prolactin increase, abortion occurs. The major limitation to use of these drugs is that they are less effective and require high dose administration when used prior to day 40 of the pregnancy. Because inducing abortion in the pregnant bitch is not recommended after day 40, high-dose use of dopamine agonists has the effect of producing greater side-effects including vomiting and inappetence.
New and Controversial Approaches
Dexamethazone, a corticosteroid, has been used to induce abortion in dogs when administered intramuscularly every 12 hours for 10 days beginning at day 35 or 40 of pregnancy or when given orally 2-3 times a day for 5 days followed by gradual reduction of dose over the following 3-5 days. The exact mechanism of dexamethasone efficacy for terminating pregnancy is not understood, however, it is believed that it may cause degeneration of the corpus luteum and thereby reduce the level of circulating progesterone. Side-effects related to this treatment include loss of appetite, increased thirst, and increased urination. Because the exact mode of action is still unknown and because no studies have explored long-term effects of dexamethasone treatment on future fertility, this treatment is not recommended until further information is obtained.
Other drugs which are being explored for their potential use as abortifactants include the drug, epostane, a steroid which interferes with progesterone secretion by inhibiting the synthesis of the precursor molecule that becomes progesterone, and certain drugs that suppress pituitary secretion of LH and FSH. Though these drugs effectively terminate pregnancy without adverse side-effects when administered during mid-pregnancy, they are not currently available for use in small animal medicine.
Treatments No Longer Advocated
At one time, large doses of estrogen prior to implantation of the fertilized blastocysts were utilized as a method to prevent pregnancy after mating. Fertilization of the ova takes place in the oviduct, the tube leading from the ovary to the uterus. For approximately 5-10 days after fertilization, the blastocyst will continue developing outside the uterus in the oviduct before the tubo-uterine junction opens allowing the blastocyst to finally migrate to the uterus and implant. When large doses of estrogen are administered to the bitch immediately after breeding, the tubo-uterine junction will remain closed and the blastocysts will degenerate in the oviduct. If given after day 10 of the pregnancy when the embryos have already migrated into the uterus, the estrogen will interfere with the development of the uterine glands, which will prevent the embryos from implanting.
Side-effects resulting from use of high-dose estrogen are severe and potentially fatal. Bone marrow toxicity with severe hematological side-effects often lead to death. Additionally, the abnormal ratio of estrogen: progesterone induced by high-dose estrogen treatment often leads to cystic hyperplasia of the uterine glands and pyometra, conditions that will preclude future fertility in the bitch. Though orally administered estrogen results in fewer and less severe adverse effects, clinical studies demonstrate that orally administered estrogen is ineffective at inducing pregnancy termination.
Because effective termination of pregnancy is achieved only when utilizing high-doses of estrogen and because of the serious side-effects associated with this treatment, this method is no longer advocated as a means to end unwanted pregnancy in the bitch.
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