Pyometra is a disease of the uterus most commonly seen in female dogs, but also seen in queens (female cats), rabbits, ferrets, rats and guinea pigs. Pyometra is an important disease to be aware of for any dog owner because of the sudden nature of the disease and the deadly consequences if left untreated. It has been compared to acute appendicitis in humans, because both are essentially empyemas within an abdominal organ.
Cause of pyometra
Pyometra is a result of hormonal and structural changes in the uterus lining. This can happen at any age, whether she has bred or not, and whether it is her 1st or 10th heat (although it becomes more common as the dog gets older). The main risk period for a female is for eight weeks after her peak standing heat (or estrus cycle) has ended.  Normally during this period, the cervix, which was open during her heat, begins to close, and the inner lining begins to adapt back to normal. However, cystic hyperplasia of the endometrium (inner lining of the uterus) - known as cystic endometrial hyperplasia (CEH) - may occur at this time for some animals, as an inappropriate response to progesterone.
Under these circumstances, bacteria (especially E. coli) that have migrated from the vagina into the uterus find the environment favorable to growth, especially since progesterone also causes mucus secretion, closes the cervix (preventing uterine drainage), and decreases uterine contractility. The condition of the cervix is a major factor in the severity of the condition.
- If the cervix is open, the infected material can leave the body, and this is far easier and safer to treat. This is known as open pyometra.
- If the cervix is fully closed, there is no discharge from the vulva, and like in appendicitis, the uterus may rupture and pus escapes into the abdomen, causing peritonitis and possible rapid death. This is known as closed pyometra.
Hormonal influences and mis-mating shots
Bitches that have received estradiol as a mismating shot (abortifacient) in diestrus are at risk for more severe disease because estrogen increases the number of progesterone receptors in the endometrium. 25 percent of bitches receiving estradiol in diestrus develop pyometra.  Pyometra is less common in female cats because progesterone is only released by the ovaries after mating.
The most obvious symptom of open pyometra is a discharge of pus from the vulva in a bitch that has recently been in heat. However, symptoms of closed pyometra are less obvious. Symptoms of both types include vomiting, loss of appetite, depression, and increased drinking and urinating. Fever is seen in less than a third of bitches with pyometra.  Closed pyometra is a more serious condition than open pyometra not only because there is no outlet for the infection but also because a diagnosis of closed pyometra can easily be missed due to its insidious nature. Bloodwork may show dehydration, increased white blood cell count, and increased alkaline phosphatase. X-rays will show an enlarged uterus, and ultrasound will confirm the presence of a fluid filled uterus.
The most important aspect of treatment of pyometra is quick action. Bitches are often septic and in shock (see septic shock). Intravenous fluids and antibiotics should be given immediately. The treatment of choice is an emergency spay, to remove the infected organ, but as this prevents a bitch from breeding it is worth noting that some advances have been made in treating pyometra with longterm antibiotics and an agent to cause contraction of the uterus and expulsion of the pus.
Spaying (ovariohysterectomy) completely and promptly removes the infection, prevents uterine rupture and peritonitis, and of course prevents recurrence, in most cases. Spayed animals do very rarely develop pyometra in the uterine stump. Still, OHE is the most effective and safest treatment.
There is another treatment option for bitches that the owner wishes to breed. Prostaglandin F2-alpha (PGF2-α) and longterm antibiotics can be used to expel the pus from the uterus and treat the infection. PGF2-α stimulates the uterus to contract, and requires at least three to five days to completely remove the infected material.  This treatment should only be used with an open pyometra, because otherwise uterine rupture may occur. Less than 30 percent of bitches with closed pyometras are successfully treated in this way.  Use of PGF2-α should only be considered in bitches that are medically stable due to the length of time treatment takes. These bitches should be bred at the next estrus cycle and then spayed after parturition, because 70 percent will develop pyometra again in the next two years. 
Stump pyometra is a serious health condition that may occur in bitches that underwent Ovariohysterectomy (spaying). In this condition, the remaining stump of the resected Uterus becomes infected and filled with a purulent fluid. The symptoms are similar to those of true pyometra. The risk of this condition is significantly reduced if no uterine or ovarian tissue remains from the original ovariohysterectomy. Diagnosis of a stump pyometra may be challenging as pyometra is often discounted as a possibility if the bitch has been previously spayed.
- Ettinger, Stephen J.;Feldman, Edward C. (1995). Textbook of Veterinary Internal Medicine, 4th ed., W.B. Saunders Company. ISBN 0-7216-6795-3.
- Romagnoli, Stefano (2002). Canine Pyometra: Pathogenesis, Therapy and Clinical Cases. Proceedings of the 27th World Congress of the World Small Animal Veterinary Association. Retrieved on 2006-12-14.
- Brooks, Wendy C. (2003). Pyometra. The Pet Health Library. VeterinaryPartner.com. Retrieved on 2006-12-14.
- Wingfield, Wayne E. (1997). in Hanley & Belfus, Inc.: Veterinary Emergency Medicine Secrets. ISBN 1-56053-215-7.
- Pyometra. The Merck Veterinary Manual (2006). Retrieved on 2006-12-14.
- Pyometra. American College of Veterinary Surgeons (2004). Retrieved on 2006-12-14.
- Pyometra from The Pet Health Library
- Pyometra Surgery Photos and Description from The Pet Centerde:Pyometra