Pyometra is a disease mainly of middle-aged female dogs that have not been spayed.

In the past, vets thought pyometra was simply a uterine infection, but today we know that it is the result of hormonal and structural changes in the uterus lining. This can happen at any age, whether a bitch has bred or not, although it becomes more common as the dog gets older.

The main risk period is for eight weeks after a bitch's peak standing heat (oestrous cycle) has ended. Normally during this period, the cervix, which was open during the heat, begins to close and the inner lining of the uterus begins to adapt back to normal. However, cystic endometrial hyperplasia (CEH) may occur at this time in some animals, as an inappropriate response to the hormone progesterone, and the resulting cysts release a lot of fluid into the interior of the uterus.

In some cases the uterus is distended from a normal pencil thickness, to each uterine horn becoming more the size and shape of a cucumber.

Pyometra can occur with or without infection, but clearly bacteria find the environment favourable to growth, which makes matters worse by causing additional fluid and pus formation.

Pyometra can also occur in the case of an incomplete ovariohysterectomy when part of the ovaries and part of the uterus remain leaving hormonally active tissue in the body.

Frequently recurring heats and false pregnancies can equally increase the risk of pyometra as these symptoms can be an indication of a hormonal dysfunction.

The condition of the cervix is important. 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, as in appendicitis, the uterus may rupture and the pus escapes into the abdomen, causing peritonitis and possible rapid death. This is known as closed pyometra.

Once the cervix is closed, toxins are produced by the uterus as it tries to cope with the problems, and the body will attempt to eliminate the problem by carrying these wastes and excess fluid through the bloodstream to the kidneys. However, the kidneys cannot resolve this, and so the dog effectively self-poisons and can also die from kidney failure.

Sometimes resolving the condition by surgery can come too late to save the kidneys, so it is important to recognise the symptoms of the condition.

The affected bitch will drink excessive quantities of water and urinate large amounts frequently. She will lick at her vaginal area while the cervix is still open and the uterus is discharging a white fluid. She may run a low-grade fever and, if blood tests are done, she will show an elevated white blood cell count.

As the uterus increases in size and weight, the dog shows weakness in the rear legs, often to the point where she cannot rise without help. As the dog enters kidney failure, she stops eating and becomes extremely lethargic.

It is thought that recurring false pregnancies in a bitch can be an early warning that she is more likely than most to get this condition after a subsequent season.

An acute pyometra is an absolute emergency. Since toxicity may develop very quickly in dogs with pyometra, it needs to be treated promptly. In most cases, the preferred treatment is a complete ovariohysterectomy (spay), with supportive treatment.

In some bitches valued for breeding, prostaglandin and antibiotic therapy may be tried instead of surgery. This therapy should only be used in dogs of six years of age or younger, who are in a stable condition, and have an open cervix, and with the understanding of the owner that any decline in the bitch's condition must proceed to surgery in her best interests.