Causes of Infertility in Female Dogs
The causes of infertility in female dogs are highly varied. Fertility requires a normal heat cycle and ovulation of normal eggs (ova) into a healthy reproductive tract, followed by fertilization of those eggs by normal sperm that are successfully introduced into the female. There also must be successful implantation of the embryos in the uterine lining, normal development of the fetuses, successful maintenance of the pregnancy and a successful delivery of live puppies.
By far the most common cause of infertility in domestic dogs is improper breeding management. This includes failing to properly detect and assess the progress of the bitch’s heat (estrus) cycle, insemination at the wrong time and miscalculating the appropriate number of breedings to be accomplished per estrus cycle. Silent heats, split heats, abnormally long heats, missed heats and irregular heats all occur in female dogs and can contribute to breeding management timing errors.
After “bad timing,” other common causes of reproductive failure in female dogs can be classified as infectious, anatomical, hormonal or other/miscellaneous.
Infections of the vagina or uterus can prevent conception and prevent the bitch from carrying her litter to term. Vaginal infections (vaginitis) are most frequently caused by bacterial microorganisms, especially by Brucella canis. Brucellosis is a venereal disease that causes infertility by either preventing conception or causing spontaneous abortion. Contrary to popular belief, the infectious microorganism, Brucella canis, can be transmitted sexually or orally with equal frequency and has the potential to infect people. Infection with the canine herpesvirus can also lead to infertility. Uterine infections can contribute to infertility, such as endometritis, pyometra/pyometritis and metritis.
Anatomical causes of female infertility are not as common as some of the other causes, but they do occur. Perhaps the most common are congenital strictures or constrictions inside the vaginal canal, and benign or malignant tumors, both of which can physically prevent successful mating and movement of sperm from the vagina through the cervix and into the uterus. Strictures often make mating painful or impossible and can make both the bitch and the stud dog reluctant to try to consummate the union. Blockage of the oviducts with tumors or masses can prevent ovulated eggs from successfully uniting with sperm. Ovarian abnormalities are common anatomical causes of infertility in dogs. They include ovarian cysts and tumors, which can cause prolonged heat cycles or shortened intervals between cycles. Cystic uterine endometrial hyperplasia is another common anatomical cause of infertility. Bitches that have previously been spayed (have had an ovariohysterectomy) obviously have legitimate anatomical reasons for their infertility. Bitches with intersex disorders (hermaphroditism; pseudohermaphroditism; others) have a poor chance of successfully reproducing. Fortunately, intersex disorders are very uncommon in dogs.
Hormonal influences are among the most important reasons for infertility in domestic dogs. Hormones are chemical substances produced by various glands in the body, such as the adrenal glands, pancreas, pituitary and thyroid. The most common hormonal cause of infertility in the bitch is hypothyroidism, or low circulating levels of thyroid hormones. The thyroid gland is located in the neck and secretes hormones that are essential for a multitude of body functions. Low levels of thyroid hormones can lead to lack of sex drive (libido) and infertility. The adrenal glands are paired structures located just above each kidney. They normally produce a number of important steroid hormones.
Adrenal dysfunction or insufficiency can interfere with the production of sex hormones that are necessary for proper reproductive function. Two common adrenal disorders in domestic dogs, Cushing’s Disease (hyperadrenocorticism) and Addison’s Disease (hypoadrenocorticism), usually prevent female dogs from conceiving and carrying a litter to term. The tiny pituitary gland, located deep in the brain, is critically involved in producing and releasing hormones that start and regulate the estrous cycle. Any disease or disorder of the pituitary can cause canine infertility due to abnormal heat cycling. This can include failure to ovulate, abnormal ovulation pattern or timing, persistent heat cycle, primary congenital anestrus (never having a heat cycle), split heat, silent heat and false pregnancy/pseudopregnancy.
Other causes of infertility in female dogs include genetic factors, which can contribute to embryonic defects and fetal malformation. These, in turn, can lead to mummified, reabsorbed or aborted puppies. Many additional factors can influence a bitch’s ability to conceive and carry a litter to term, including environmental conditions (noise, crowding, poor hygiene/sanitation, psychological factors, temperature extremes and exposure to inclement weather), stress, poor or inadequate nutrition, vaccination and immune status, external and internal parasite management programs, administration of certain medications and the bitch’s overall systemic health. Psychological factors can be important contributors to canine infertility. For example, many bitches raised exclusively as house pets are shy breeders, because they have not had much social contact with other dogs. Some males are extremely aggressive during mating, which can traumatize the bitch and create an unpleasant, fearful association with sexual activity. Close inbreeding can also interfere with normal fertility in subsequent generations.
It is impossible to make meaningful generalizations about effective ways to prevent infertility in female dogs, because the causes can be so variable. However, inbreeding should be avoided by all but the most seasoned of breeders. Breeding animals should be kept away from environmental and household toxins and should not be given drugs for longer than medically necessary. Perhaps most importantly, infertility can be prevented by sound, thoughtful, well-managed breeding programs and practices on the part of both the stud and the bitch owners.
Rigorous physical and medical examinations and reliable note-taking are critical to managing the proper breeding a bitch. It generally is no longer considered good breeding protocol to simply place an intact female with an intact male and cross one’s fingers in hopes that a successful mating, with conception and a full-term pregnancy, will take place. Planning is essential. When a bitch is expected to come into season, she should have a thorough pre-breeding work-up by a veterinarian knowledgeable about canine reproduction, which not all veterinarians are. Vaginal swabs and cytology are usually started shortly after a bitch visibly goes into heat, followed by ultrasound, progesterone and/or relaxin testing and ultimately abdominal radiographs (X-rays). Responsible breeders put a tremendous amount of thought, time, energy and money into getting their bitches pregnant and ensuring the production of a healthy litter. It is worth the expenditure on the front end to get the best chance of a wonderful outcome at the other.
Most female dogs first come into heat between 9 and 24 months of age, and then cycle on average every 6 months. Early heat cycles tend to be erratic. By 2 years, the cycles of most bitches have stabilized, although just as in women the cycles of female dogs can vary tremendously. One of the most common infectious bacterial causes of infertility in dogs, Brucella canis, is potentially infectious to people (zoonotic) through direct physical contact with urine, semen, vaginal discharge or aborted fetuses of infected dogs.