Infection in the lining of the uterus is established as a result of hormonal changes. Following oestrus ("season", or "heat"), hormone levels remain elevated for 8-10 weeks to prepare the uterus for, and maintain, pregnancy. If pregnancy does not occur the lining continues to increase in thickness until cysts form in the walls. This thickened cystic lining secretes fluids which are ideal for bacteria to thrive in. The wall of the uterus is also inhibited in its ability to contract and remove the cystic fluid. When infected by bacteria this fluid becomes pus. Sometimes the fluid becomes purrulent without bacteria being present.
This situation tends to become more intense as the bitch gets older, particularly if she has not been used for breeding. This explains why this condition usually effects older dogs. It can, however, occur in dogs of any age. Pyometra can be a possible complication of the use of hormonal treatments, for example when used as contraceptives, or to treat misalliance, and this is one of the main causes in younger bitches.
The cervix is the gateway to the uterus. It remains tightly closed except during a season or when giving birth. When it is open bacteria that are normally in the vagina can enter the uterus very easily. If the uterus is normal, the environment will not encourage bacterial survival, however when the uterine wall is thickened or cystic, perfect conditions exist for bacterial growth.
The typical timing for the disease is 1-2 months after your bitch has been in season.

The clinical signs will depend on whether or not the cervix is open.
If open, pus will drain from the uterus through the vagina to the outside. It is often noted as a purrulent (pusy), bloody discharge on the skin and hair under the tail or on bedding and furniture where the dog has laid. It may or may not have an odour. Fever, loss of appetite and depression may or may not be present.
If the cervix is closed, pus that forms is not able to drain to the outside. It collects in the uterus, which will cause an apparent swelling of the abdomen. The bacteria and pus in the womb release toxins, which are absorbed into the blood. These bitches will become very ill rapidly. They are off food, very listless, depressed, may vomit and have diarrhoea and will be drinking very excessively. The toxins will affect the kidney function and may cause permanent kidney failure in the bitch if the condition is not treated quickly. A blood test may be taken to estimate the damage to the kidneys before and after surgery. Efforts are made in treatment to minimize the risk of kidney damage.
A female dog which is not spayed, drinking an increased amount of water, has had a recent season and is unwell is always suspected of having a pyometra. If confirmation is required, such as with a closed pyometra, blood tests will show signs of severe infection, an increased no of white cells in the blood and signs of kidney failure. These are present in all very serious infections however, so an x-ray is often used for confirmation. This also has the advantage of being done immediately at the surgery, and will show if your bitch has a uterus which is full of pus. If the cervix is open there may be minimal uterine enlargement and x-rays are more difficult to interpret, however in this case the discharge from the vulva is usually diagnostic.

An IV drip is used to minimize the kidney damage and to help repair dehydration. An anaesthetic will not be attempted until the risks of anaesthetic complications are made as small as possible. Usually 6-12 hours of fluids will be administered before the anaesthetic and at least 24 hours after the surgery. This will depend on the amount of kidney damage and the recovery of your bitch. She may be on a drip for several days after the surgery.
A general anaesthetic and ovario-hysterectomy, removing the ovaries and uterus of the bitch, is the preferred treatment. Antibiotics will often be given for 1-2 weeks after the surgery.
Stitches will be in for 10 days and the dog must not lick them.
There is a medical treatment for pyometra, although it is very risky. Prostaglandins are a group of hormones that reduce the blood level of progesterone hormone, relax and open the cervix and contract the uterus to expel bacteria and pus. They are not always successful and have some serious limitations.
Prostaglandins cause side effects of restlessness, panting, vomiting, diarrhoea, salivation and acute abdominal pain. These occur within 15 mins of an injection and last for several hours. Successive treatments are always required 2-3 times daily.
There is no clinical improvement for at least 48 hours, so dogs, which are very ill at presentation at the veterinary surgery, have a very poor prognosis with this treatment. While waiting for the treatment to work, a bich can deteriorate to a level which makes surgical treatment very much less likly to succeed, and the chances of permanent damage from the toxaemia to be much more likely.
Because prostaglandins cause contraction the uterus, it is possible for the uterus to rupture and spill infection into the abdominal cavity. This is most likely to occur when the cervix is closed and is often fatal.
There are some statistics you should consider before choosing this treatment.

For all of the above reasons we strongly advise that the best course of action for a pyometra (open or closed) is ovariohysterectomy as soon as surgical conditions are optimal for the bitch's survival.

The chance of successful treatment without surgical or prostaglandin treatment is exceptionally low. If treatment is not given quickly the toxic effects from the bacteria will be fatal. If the cervix is closed, it is also possible for the uterus to rupture and spill the infection into the abdominal cavity causing a fatal peritonitis.
Spayed bitches do not get Pyometra. If you do not intend to breed from a bitch we strongly recommend that she is spayed when young and fit, and when surgical risks to her are minimal.

From an Original Text by Jenny King