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A canine pyometric uterus immediately after surgery to remove it. It is extremely distended with pus.
Pyometra is a uterine infection. Though it is most commonly known as a disease of the unaltered female dog, it is also a notable human disease. It is also seen in female cats (queens), rabbits, hamsters, 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 estrous 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 
Females 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 females 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 Female 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 female dogs 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 and/or increased white blood cell count. 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 to provide supportive care. Female dogs are often septic and in shock (see septic shock). Intravenous fluids and antibiotics should be given immediately. Once the female dog has been stabilized, then the treatment of choice is an emergency spay,
Surgical treatment 
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. Even so, ovariohysterectomy is currently considered the most effective and safest treatment.
Stump Pyometra 
Stump pyometra is a serious health condition that may occur in female dogs that underwent Ovariohysterectomy (spaying), particularly if there is residual functional ovarian tissue. 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 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 dog has been previously spayed.
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