It is has been suggested that 25% of all entire bitches will suffer from pyometra by the age of ten
Pyometra is a serious condition of the womb in bitches but is not unique to the dog as it also affects other animals too. Despite being relatively common in entire bitches there is surprisingly little detail known about why it occurs and how to treat it. Perhaps one reason for this is that, even today, the commonest recommendation is surgical removal of the ovaries and womb (ovariohysterectomy).

Why does pyometra occur? It is almost certainly based upon a bacterial infection but this requires the presence of a pathogenic bacteria in the womb at a time when conditions are right for bacterial growth. The most likely period for this is the two months following a season. At this time the womb lining undergoes hormonally induced changes in preparation for pregnancy and then develops to maintain it. This process takes place in all entire bitches whether or not they become pregnant and provides a perfect medium for bacterial growth.
If bacterial infection occurs this appears to influence a cystic development in the womb lining and induces increased secretions providing further growth medium for bacteria to develop. The result is the production of pus and in relatively large amounts. The womb may expand to contain this increasing volume of pus.
At this point two scenarios may develop. One where the neck of the womb (the cervix) remains tightly closed and thus pus fills the womb which enlarges to accommodate the volume. In some cases the enlarged womb is sufficient to cause abdominal distension. The alternative is where the cervix is dilated. In these cases the pus will overflow into the vagina and result in a discharge from the vulva. In these cases the womb tends to distend less.

The effect on the bitch is significant. Pus is the breakdown of cells, bacteria and secretions. It releases toxins into the bloodstream adversely affecting liver and kidney function and often causing depression, a reduced appetite and increased thirst. Clearly a ‘closed pyometra’ may be seen as a more acute situation than the open pyometra where the pus accumulation is slower due to constant discharge. Nevertheless reality often diverges from the textbook description.

Symptoms of a pyometra are reasonably classic. A depressed dog, increased thirst, not keen to eat but will on good days, some vomiting, possible excessive urination or on occasions more frequent attempts to pass small amounts of urine. A vaginal discharge might be expected with an open pyometra. However, some bitches may have no immediately obvious symptoms except a discharge. Pyometra is normally diagnosed around one to two months after a season but this can vary.

You can find more information on pyometra here