Changes in the canine uterus affected by pyometra
Pyometra is a serious infection of uterine tissue which is reported to affect about 23% of intact female dogs at some time in their life. Pyometra kills about 1% of intact female dogs.44,45 Many veterinarians actually recommend spay of a female dog solely to prevent pyometra from ever developing. We consider that recommendation to be excessive upon consideration of the many ill effects that spay has been shown to create in female dogs. Notably, some of these ill effects (e.g., the increased rates of terminal cancers, diabetes) pose a much greater threat to the life of a dog than pyometra. Our approach is to educate our readers. Understanding how pyometra develops helps us to evaluate preventive strategies, opportunities for early diagnosis, as well as potential treatments for this condition.
Pyometra is a secondary infection that occurs as a result of hormonal changes in the female’s reproductive tract. When a female dog reaches puberty, her estrus (heat) cycles begin (a detailed explanation of the hormonal changes in the canine heat cycle can be found here). The hormone that requires our attention with respect to pyometra is progesterone. Progesterone levels rise steadily from the time of ovulation, and ovulation occurs at a variable time within the proestrus phase and estrus phase of the cycle. Progesterone hormone levels remain elevated during the diestrus phase which follows. The diestrus phase usually lasts 2-3 mo in the absence of pregnancy.
During estrus, white blood cells, which normally protect against infection, are inhibited from entering the uterus. This allows sperm to safely enter the female’s reproductive tract without being damaged or destroyed by these immune system cells. The elevated progesterone levels of diestrus cause thickening of the lining of the uterus in preparation for pregnancy and fetal development. If pregnancy does not occur for several consecutive estrus cycles, the uterine lining continues to increase in thickness until cysts often form within the tissues, causing the thickened, cystic lining to secrete fluids that create an ideal environment for bacteria to grow in. Additionally, high progesterone levels inhibit the ability of the muscles in the wall of the uterus to contract and expel accumulated fluids or bacteria. The combination of these factors often leads to infection–which is termed by medical literature as “pyometra”.
In the approximately two months after an intact dog goes into heat, she is at most risk of developing pyometra. This is, in part, because progesterone at a sufficiently high level is necessary for pyometra to develop (more on this later). A guardian’s vigilance can facilitate an early diagnosis of pyometra, which significantly enhances the chances of successful treatment, whether treatment is conservative or the more radical procedure spay is performed.
There are two types of pyometra – differentiated by whether the cervix is open or closed (the literature terms this as open/closed pyometra). While the general symptoms of the two types are identical, certain secondary symptoms are different. In both open and closed pyometra, the dog may exhibit the following:
- Lack of appetite
- Weight loss
- Excessive water consumption
- Excessive urination
If it is open pyometra, there can be drainage of pus and blood from the urinary tract. If it is closed pyometra, there will be no drainage, and because the infection cannot drain, the abdomen can be swollen and painful.
Observing any of the above symptoms during the diestrus phase (i.e., the two month time frame after estrus) warrants an immediate trip to the vet. Definitive diagnosis can then be made with blood tests, urinalysis and microscopic evaluation of the infection if it is draining. Certain blood value abnormalities are characteristic of pyometra. Ultrasound can definitively diagnose the fluid filled uterus of closed pyometra as well as reveal increased thickness in the uterine wall that is seen in cases of open pyometra.
Traditionally, the veterinary community recommends spay (i.e., removal of both ovaries and uterus) for intact female dogs who exhibit symptoms of pyometra. For the optimal, long-term health and happiness of the dog, we strongly recommend a hormone-sparing, minimally invasive method of dealing with pyometra. Once again, we look to the human medical care model to see how pyometra is treated. Pyometra in women is an uncommon condition. Dilatation (i.e., the opening) of the cervix and removal of the infection is the treatment of choice. Antibiotics are given if there is an invasive infection characterized by generalized malaise, fever or abnormal laboratory values. Regular monitoring after initial treatment is warranted to detect persistent and recurrent disease.158
Research is ongoing to establish more conservative methods of treatment for canine pyometra. The concept behind these conservative canine treatments is the same as it is for the treatment of women with pyometra; we want to open the cervix and facilitate drainage of the excess tissue and infection. This canine research started in 1997 and has continued over the years, with results that often approach or actually reach 100% success.
In 1997, on the basis of a previous study showing the effectiveness of an antiprogestin (drug which blocks the action of progesterone) treatment, seven female dogs with pyometra and documented levels of progesterone over a certain threshold level, were treated with an antiprogestin, i.e., aglepristone. Antibiotics were administered. The authors concluded,
“In all dogs general condition and feed consumption improved rapidly and were normal within 8 days. No side effects were noted. Two of the dogs were mated subsequently and produced two healthy litters. These observations confirm those of a preceding study and show that the treatment of pyometra with an antiprogestin may lead to a clinical recovery. These results justify further studies to enable the optimum treatment regimen to be established.” 159
In 2002, at the World Small Animal Veterinary Association Congress (WSAVA), the presentation of current treatments for pyometra included the following information:
“When the cervix is open the uterus can be easily emptied stimulating contractility of the myometrium. Of all the drugs causing contraction of the uterine musculature…PGF2a [a prostaglandin] is the one which is most indicated for the clinical treatment of pyometra.”
In the treatment of closed-cervix pyometra, the presentation further states:
“The availability of antiprogestin-based drugs has completely changed the clinical approach to a problem whose only solution for the last decades has been ovariohysterectomy.”
Additionally, with respect to the prevention of pyometra:
“Antiprogestins can be used to avoid recurrence of pyometra at subsequent cycles should the owner decide not to breed the bitch immediately.”
Several different protocols combining aglepristone and cloprostenol were studied in 2003, with the authors concluding that both protocols proved to be efficient and safe in reversing clinical signs of open cervix pyometra.160
Over time, we see specific drugs emerging as the favored antiprogestin (aglepristone) and prostaglandin (PGF2a or a synthetic analogue of prostaglandin, cloprostenol) utilized in studies to evaluate the success of conservative approaches to the treatment of pyometra. Aglepristone treatment is intended to block the effects of progesterone and, in cases of closed-cervix pyometra, to promote cervical relaxation and opening. The addition of prostaglandins is intended to induce uterine contractions and promote uterine emptying.161 Cloprostenol also halts the production of progesterone within the ovaries. Broad spectrum antibiotics may be injected, while culturing of the infection may later dictate a change to an antibiotic more effective against the specific bacteria found.
In 2016, a review of the studies which included aglepristone alone or in combination with a prostaglandin was published in the veterinary journal Theriogenology. The authors conclude conservative treatment (i.e., removal of the infection by a protocol of injections only), as described within this web site is a viable alternative to surgical removal of vital sex organs as a means to treat pyometra. The limitation of conservative treatment is that it is only safe when the pyometra is diagnosed at an early stage.161
The vigilant dog guardian who recognizes the signs of pyometra at their early stage has much greater latitude as to what treatment is available and likely to be successful in treating their dog’s pyometra. With respect to reoccurence, it is important to remember that the presenter at the WSAVA Congress offered the advice that the administration of progesterone-blocking drugs during subsequent cycles when not intending to breed, would prevent pyometra from developing in the future. We can envision administering these drugs during the diestrus phase of future cycles for maximum protection against recurrence.
In conclusion, we cannot justify spay (i.e., ovariohysterectomy – the removal of both the ovaries and the uterus) for the purpose of preventing or treating pyometra. We have established there are other viable treatments for pyometra that are less invasive (i.e., a series of injections) and protect a dog’s hormones over its lifetime. However, it is critical the guardian be vigilant, as early diagnosis ensures success when utilizing the less invasive medical treatments. The case where initiation of conservative treatment would be contraindicated is the dog who presents with advanced (i.e., the uterus is badly enlarged and in danger of bursting) closed cervix pyometra. Closed cervix pyometra can become life-threatening in the time it takes for the prescribed injections to open the cervix and allow the infection to drain. Therefore, in the case of advanced closed cervix pyometra, removal of the infected uterus is the appropriate care option. We would contemplate, in that circumstance, a treatment plan that would entail removal of the uterus only, leaving the ovaries intact.