The Female Dog’s Reproductive System

Anatomy and physiology of the female dog’s reproductive system.


By Randy Kidd, DVM, PhD The female dog’s reproductive system is the “nest” that nurtures a simple union of two single cells into a weave of billions of intercommunicating cells, which then form into organ systems and ultimately unify into the whole organism we call “dog.” We’re talking about the act of creation, the production of entire litters of living, breathing, best friends of mankind. It’s an amazing, powerful, complex system. And as with the male dog’s reproductive system, very small alterations in the balance of any one of the female functions involved with reproduction can produce profound results throughout her body. Anatomy and physiology of the female dog The vaginal vestibule, a short entryway into the vagina, is oriented at a 60-degree angle to the horizontal (upward, toward the spine, and forward, toward the head). Thus, to pass a speculum or catheter into the vagina requires that it be initially oriented at this upward angle, and if a female needs assistance expelling a puppy from the pelvic canal, best results are obtained when gently pulling in this mostly-downward direction. Just cranial to the vestibule lies the vagina, which is oriented horizontally in the standing female. The vagina terminates at the cervix, an organ that separates the vagina from the uterus, which is a Y-shaped organ in the bitch. In a normal pregnancy one or more fetuses will develop in each of the uterine horns. Each horn of the uterus terminates in a tortuous uterine tube (oviduct), which then expands into a bursa that completely enfolds the ovary. The ovary is the site for production of the ovum and a variety of hormones including the estrogenic compounds (primarily estradiol-17 Beta) and progesterone; other hormones, including testosterone, are also produced in the ovary. In last month’s article on the male dog reproductive system (“All Male Review”), we discussed the vomeronasal organ and its ability to sense the sex-related pheromones that are emitted by an animal in heat. The vomeronasal organ in most species (including the human species) is composed of two short tubes with tiny, slit-like openings into the nares, tucked away just below the floor of the nose. They are processing centers for pheromones. Females also have the vomeronasal organ, and while pheromones don’t seem to have the profound driving effect that they have on the male of the species, they are still an important component of the breeding cycle. In many species the females in the herd or colony will come into heat only in the presence of a male (or in the presence of something that is redolent with his male odor). Some bitches will not display any outward signs of heat until a male dog, along with his male pheromones, is actually present. The ovarian or estrus cycle The dog reaches sexual maturity at from 5 to 24 months of age – earlier in smaller breeds; later in larger breeds. Dogs are what is termed unseasonally monestrous, meaning that their heat cycle is an isolated event that occurs at any time of the year. While traditional lore has it that bitches tend to cycle in the spring and fall, actual observations have indicated that for most breeds heat cycles occur randomly throughout the year. The time between cycles varies with the individual from 3 ½ to 13 months, and the estrus cycle lasts from 2-21 days (6 to 12 days on average). Proestrus and estrus are stages of obvious sexual activity. Estrus (from the Greek oistros, meaning mad, frenzied, any vehement desire), is defined as the period of sexual receptivity in female mammals, and proestrus is the period of heightened follicular activity prior to estrus. The dog’s ovarian cycle is divided into four phases – anestrus, proestrus, estrus, and diestrus. For breeding purposes there are various ways to interpret the changes that occur during these different phases, including monitoring blood hormone levels, microscopically observing vaginal cells, visualizing the vaginal cell walls with an endoscope, and observation of behavioral characteristics. A dog breeder would refer to the combined two stages of proestrus and estrus as “heat” or “season,” with the first day of heat being the first day of proestrus, the last day of heat being the last day of estrus. A horse or cow breeder would use the term “heat” to mean only the period of sexual receptivity or estrus. • Anestrus (65 to 281 days, mean 150.3 days). Anestrus is the quiescent period of the reproductive cycle, behaviorally characterized by sexual inactivity. Microscopic, endoscopic, and hormonal evaluations all reflect a general lack of activity. While there may be hormonal changes during anestrus, these changes are not consistent among individuals. The hormonal concentrations – especially of leutenizing hormone (LH) – often surge in episodic fashion, creating peaks and valleys of blood level concentrations throughout anestrus. • Proestrus (6 to 11 days, mean 9.1 days). This phase is the period when the bitch is sexually attractive yet rejects the male’s advances. Behavioral clues for this stage are often indistinct, however, and most folks mark the first day of proestrus at the time when a vaginal discharge appears that is yellowish or straw colored, or tinged with pink or red (serosanguineous). During this phase of the cycle, the vulva gradually enlarges and becomes quite edematous and firm by the last third of proestrus. Vaginal discharge and vulvar swelling are both variable, and observers may miss them completely, especially if the female frequently licks and cleans her vulva throughout all stages of her cycle. Signs of estrus and proestrus are often indistinct and especially difficult to detect in young females. Microscopic examination of vaginal cells at this stage will reveal red blood cells along with a gradual increase in cornified epithelial cells, until they are the predominate cell at about one or two days before the actual serum estradiol peak. • Estrus (7 to 9 days, mean 10.4 days). Behavioral estrus begins when the female allows the male to mount and stands with her tail cocked to the side (flags) when he attempts intromission (insertion). Estrus ends with her refusal of the male’s advances. Ovulation usually occurs about the 11th day of heat (day 2 of estrus). At ovulation, vaginal epithelial cells are almost totally cornified (hardened), so successive microscopic evaluations of slides taken from vaginal swabs are a fairly accurate way to predict the time of ovulation. During estrus, endoscopic examination of the vagina by an experienced evaluator may also be helpful for pinpointing the time of ovulation. It is important to realize, however, that an examiner can’t tell early estrus from late estrus from a single cellular cytology or vaginal endoscopic exam; sequential evaluations are needed to truly evaluate timing of the cycle. Some females may have a “silent heat,” a heat cycle that is not associated with bleeding. Some of these bitches will have vulvar swelling, but this is often difficult to detect. Most bitches undergoing silent heat will accept a male at the time of ovulation; however, determining this time may be difficult, especially if the male is not on the premises. • Diestrus (56 to 58 days pregnant, 60 to 75 days nonpregnant). This phase can be precisely defined by observing changes in vaginal epithelial cells; a less precise way to identify it is by noting the first time the female refuses the male. This usually occurs at the same time she is no longer attractive to males. Diestrus is completely dominated by progesterone; other hormones are essentially at baseline levels. Breeding tips • Be certain the female has reached puberty. • Expect that the female’s ability to accept the male and her breeding efficiency will increase with age and experience. (The male dog’s libido and efficiency will also increase with time and experience, until old age changes begin to take effect.) • Realize that each female is an individual, and each will have her own way of expressing her heat cycle. Some will bleed profusely and show prominent vulvar swelling; others will have a silent or near-silent heat cycle; some will readily accept any male; others may accept for only a few days (or hours) and then only if the male is deemed “acceptable.” • Be certain that the bitch is truly in standing heat (the most common cause of breeding failure is that the female is not truly in estrous). To be sure, use a combination of hormonal, cytological, endoscopic, and behavioral evaluations, especially for the difficult-to-breed female. • When possible, stay out of the way. The second-most common cause of breeding failure is interference from well-meaning folks, disrupting the “ambiance” necessary for good reproductive contact. • Realize that sometimes the mating was simply not meant to be – either the bitch or the male, for whatever the reason, may not be attracted to the other, and they may never be able to “hit it off.” • Poor thyroid function is known to adversely affect libido and breeding soundness in animals, and other organ systems will likely be shown in the future to have intimate connections to the reproductive system. A complete breeding soundness exam will certainly include an evaluation of thyroid function, and an evaluation of other organ systems may also be indicated. Disorders of the canine female reproductive tract There are several common disorders of the female canine reproductive tract that deserve mention. If a female continues to show signs of proestrus or estrus (heat) for a prolonged period (more than 21 days of standing heat or more than 40 days of attracting males along with vaginal bleeding and vulvar swelling), suspect the possibility of follicular cysts. Cysts are fluid-filled sacks that result in a prolonged secretion of estrogen, leading to the signs of heat. The treatment of choice for this condition, if it recurs, is ovariohysterectomy. False pregnancy (pseudopregnancy, pseudocyesis) is fairly common. As the name suggests, it is a condition where the female appears to be pregnant, but she is not. Dogs in false pregnancy may demonstrate swelling of the mammary glands, lactation, nesting, or other “mothering” signs, without the presence of fetuses. Other than the possible need for tranquilizers (herbal or otherwise) for the overly distraught “mother,” no treatment is necessary, as the problem usually resolves itself in one to three weeks. Conventional medicine sometimes suggests hormonal therapy, but the approved medications often lead to pyometra. The only long-term therapy proven to prevent recurrent false pregnancies is ovariohysterectomy. Difficulty during breeding attempts or whelping may lead to metritis, an infection of the uterus. A variety of organisms may be involved in producing clinical signs of purulent vulval discharge, often accompanied with fever, lethargy, and refusal to eat. Also, the mother may neglect her puppies. Some bitches may need stabilizing supportive therapy such as fluids; most cases of metritis respond to antibiotics coupled with treatments (oxytocin or prostaglandins) aimed at evacuating the uterine contents. Pyometra, a hormonally mediated disorder that occurs after estrus, is a serious and potentially life-threatening condition that typically occurs in older females. It can be caused by infections during or after breeding. Or it may be associated with the administration of hormones, such as progesterone compounds given to delay or suppress heat, or estrogens administered to females after an unintended and unwanted mating. There may be an evident mucopurulent vulvar discharge; if the cervix is closed, however, the purulent material may remain in the uterus and enlarge it to the point where abdominal swelling is evident. Bitches with pyometra often become dehydrated, and they are typically lethargic and refuse to eat. They may also drink and urinate excessively (polyuria and polydipsia), and they may vomit. Further signs such as fever or a change in the WBC count are variable; x-rays or ultrasonic exams may be indicated for a final diagnosis. Cases of pyometra often do not respond well to antibiotic therapy, and this is only attempted when there is a definite need to salvage the reproductive potential of the female. Ovariohysterectomy is the treatment of choice. Vaginitis, inflammation of the vagina, is usually due to a bacterial infection, but viruses, conformational abnormalities, foreign bodies, or therapeutic use of steroids may also be involved. There is usually a vulvar discharge, which the female may constantly lick, and she may attract male dogs. Bacterial infections usually respond to local treatments (vaginal douches) using antibiotics or herbs with antibiotic activity. Systemic antibiotic therapy may be necessary in some cases. If it is a young female, the condition almost always resolves itself after her first estrus cycle. The reproductive tract typically has a normal flora of bacteria, often comprised of several different species. Care should be exercised when diagnosing vaginitis based solely on the finding of bacteria; a profound overgrowth of one species of bacteria may be a more important indicator. Abortion can be caused by a variety of organisms, hormonal imbalances, and physical factors such as trauma, malnutrition, or severe stress. Brucellosis deserves special mention as an infection that causes resorption of the fetuses early in gestation or sudden abortion during the last trimester of pregnancy without any previous symptoms. It is a highly contagious disease that can spread rapidly through a kennel by contact with infected fetuses, vaginal discharge, or occasionally by venereal means. Brucellosis can be diagnosed by isolation of the organism; however, a serologic test is usually more practical. Whenever breeding problems occur in a kennel, the entire kennel should be tested. Mammary tumors will be discussed more fully in an upcoming article on pregnancy and lactation. For now, I’ll just say that their exact cause is unknown, but if a female is spayed before her first heat cycle, her chances of developing mammary tumors is near zero. Transmissible venereal tumors (TVTs) occur frequently in some geographic areas and rarely in others. They are almost always located on the dog’s genitalia (male or female) and are spread by dog-to-dog direct contact. They typically spread to regional lymph nodes and sometimes to other tissues. Other tumors of the lower urinary tract are relatively common in dogs. Neoplasia may also involve any of the other tissues of the reproductive tract. Tumors vary in their potential for growth and in their propensity to spread (metastasize) to other tissues. They are treated via Western medicine by the usual means: surgical excision, and/or some form of chemo- or radiation-therapy. Alternative therapies for tumors of any type include homeopathy or acupuncture; nutritional supplements and herbal remedies may be included to support the primary therapy of choice. I discussed urinary incontinence in “All Male Review” (WDJ May 2005) and much of what was said there especially applies to females, since the incidence of incontinence is somewhat higher in females than in males. Some feel that estrogen-type compounds are more effective for treating incontinence in bitches. In my mind, this makes the phytoestrogens (estrogens from plants) a good option for treatment. To spay or not to spay Castration is the correct term for removal or destruction of the gonads, whether the subject is male or female. (In the male, the procedure is most accurately called bilateral orchiectomy – removal of both testes; in the female, it’s called a bilateral oophorectomy – removal of both ovaries.) However, common usage in animals generally refers to female castration as “spaying,” and in the male the procedure is called either castration or neutering. Most veterinarians, when they perform a “spay” are actually performing an ovario-hysterectomy – removal of both ovaries along with the removal of both horns of the uterus to the cervix (hysterectomy). I discussed my opinions last month about spaying and neutering in “All Male Review.” For this article, suffice it to say that I feel that castration (of both male and female dogs) is a positive step to take to help alleviate our overpopulation problem, even at the possible expense to our dogs of the benefits of normal hormone levels. I suggest that all castrated animals receive herbal (phytohormones) and nutritional supplements to help the body replace its lost hormones. Some plants that provide estrogenic steroidal precursors include wild yam (Dioscorea villosa), black cohosh (Cimicifuga racemosa), feverfew (Tanacetum parthenium), lion’s ear or lion’s tail (Leonotis leonurus), and pleurisy root or butterfly weed (Asclepias tuberosa). Check with an herbalist experienced with using herbs for treating animals for proper dosages and delivery methods. Providing the female with pain relief immediately after spay surgery is fortunately becoming more common. My suggestion is the homeopathic remedy Arnica (available in health food stores). I recommend giving the female a 30c dose every hour or so, for a few doses after surgery, then maybe twice daily for a few days. Also consider a mind-calming herb or flower essence. -Dr. Randy Kidd earned his DVM degree from Ohio State University and his PhD in Pathology/Clinical Pathology from Kansas State University. A past president of the American Holistic Veterinary Medical Association, he’s author of Dr. Kidd’s Guide to Herbal Dog Care and Dr. Kidd’s Guide to Herbal Cat Care.