McNeil Labradors

The Bitch – A Wolf in Dog’s Clothing 

Dog breeding is a fascinating science.  Reproduction in dogs is vastly different from that of the other domestic species.  Dog breeders expend considerable effort educating themselves in the areas of canine reproductive physiology and endocrinology. Focusing on some of the unique aspects of canine reproduction, the goal of this article is to help breeders understand why, when breeding dogs, you see some of things you do. 

The domestic dog evolved from several, now extinct, species of wolves.  A study of wolf reproduction helps to shed light upon some of the idiosyncrasies of reproduction in the dog.  With rare exception, bitches like wolf females “cycle” only twice per year.  In most other domestic species (the horse, cow sheep, pig, goat, and human), females cycle continuously at regular intervals during their breeding seasons.  In wolves, this strategy results in offspring being born at times of the year when food is plentiful, weather is temperate and survival of young is optimized.

When dogs are housed together, bitches often cycle at the same time.  This phenomenon is also observed in female wolves.  In wolf packs, only the highest-ranking female is bred and delivers young.  Other females in the pack are not bred, but they do ovulate right along with the alpha female, have the same long diestral phase (period of high progesterone levels), produce milk, and serve as “nurse maids” to the wolf cubs once they are born.  This ensures survival of the cubs in the event of the demise of their birth mother.  This is also the reason false pregnancy occurs fairly commonly in the bitch when it is rarely seen in other species.  In other species, including the human (you’ve all heard the college dormatory tales), females cycle together but the diestral period is short unless pregnancy actually occurs.  In the bitch, false pregnancy, occurring as a result of the long period of high progesterone, is a normal hormonal event.

So hormonally, the bitch “thinks” she’s pregnant every time she experiences estrus (heat) even if she has not been within a mile of a male.  In addition to false pregnancy, this “hormonal pregnancy” has other, more serious ramifications to canine reproduction, namely PYOMETRA.  Pyometra is a uterine infection so extensive that pus accumulates within the lumen of the uterus.  A uterus with pyometra can contain gallons of pus.  This severe uterine infection occurs only when the bitch is in her long diestral phase and the uterus is being exposed to high levels of progesterone.

How does this happen?  The bitch’s ability to ward off infection in her uterus is highest during estrus when estrogen levels are high.  This makes sense because this is the time when her reproductive tract is being exposed to a multitude of pathogens (and for any that doubt, just take a look at the end of your dog’s penis).  It’s a miracle that the uterus can eliminate billions of bacteria without harming the delicate sperm cells. 

In contrast, uterine defense mechanisms are minimal while under the influence of progesterone when the uterus “thinks” it is pregnant.  It is helpful (albeit an over-simplification) to think that when a bitch conceives and becomes pregnant, the uterine immune defense systems must be turned “off” so that the antigenically foreign conceptuses are not destroyed.  If this did not happen, then the embryos would be rejected and destroyed in much the same manner as rejection occurs in mismatched organ transplants.  When the uterus “thinks” it is pregnant, it fiercely protects whatever lies therein.  And that whatever could be puppies in the case of a normal pregnancy, or it could also be pus in the case of a pyometra. 

In other species (human, horse, cow sheep, goat, pig, and cat to name a few), progesterone levels drop after about two weeks of diestrus if the female does not become pregnant.  Once progesterone levels are low, the “off” switch is released, uterine defenses gear up and the uterus can clear itself of any pathogens.  In the bitch, however, uterine defense systems remain “off” for a much longer period of time (45 days or longer).  When pathogens invade the uterus and are not removed during estrus, they are protected within the uterus and allowed to multiple for the entire diestral period.  This explains why we see pyometra commonly in the bitch and rarely in the other domestic species.  And this is why you should think of pyometra as not just an infection.  It is also a hormonally induced disease.

Successful treatment of pyometra must include hormonal therapy.  A series of injections of prostaglandin cause the ovary to cease production of the hormone progesterone.  All the antibiotics in the world will not effectively treat the infection if progesterone levels remain high.  Once the ovary stops producing progesterone, the uterus no longer “thinks” its pregnant, the “off:” switch is released, uterine defense systems can begin to work, the uterus dumps its offending contents, and with the aid of antimicrobial therapy, the condition can be cured.

Following along with the idea that the prolonged diestral period in the bitch can be deleterious, some specialists recommend estrus suppression except when the bitch is to be bred.  When you prevent a bitch from coming into heat, then you also avoid the long diestral period that inevitably follows.  Remember that hormonally, the bitch has experienced three or four “pregnancies” before she is ever bred for the first time.  An argument can be made that these “pregnancies” place a lot of wear and tear on the uterus.  The rationale for continual estrus suppression is that by preventing estrus, the bitch’s hormonal “pregnancy” can be prevented each time, and the bitch’s reproductive lifespan can be prolonged.  Unfortunately, the drug (also a hormone) used for this purpose has recently been withdrawn from the market and is no longer available.

Hopefully this discussion on canine reproduction, from an evolutionary perspective, adds to your knowledge and success in dog breeding.  The next time your bitch is in estrus, don’t be surprised if you hear her howl at the moon in tribute to her wolf ancestry. 

Jane Barber, DVM, MS, Diplomate American College of Theriogenologists

About the author

Dr. Jane Barber recently opened a referral practice, Veterinary Specialties at the Lake, in the Piedmont region of North Carolina.  Dr. Jane received her DVM from the University of Georgia.  After 5 years in general practice, she returned to UGA to complete a MS degree and residency training program in Veterinary Behavior.  After completing her behavior training, Dr. Jane entered a combined PhD and residency training program in theriogenology at the University of Minnesota.  She is board certified in theriogenology and is a Diplomate of the American College of Theriogenologists.  Dr. Jane is certified to perform PennHIP radiographs in addition to standard OFA films for hip evaluation.  And she is a strong proponent of natural nutrition and energy  medicine.  She can be reached  by email at:

Please do not copy or use portions of this article without permission of the author.   

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Last Updated:  January 29, 2008

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