Pyometra or pyometrea is a uterine infection, most commonly found in unaltered female dogs. It is a secondary infection that occurs as a result of hormonal changes in the uterus, also known as the womb, where a foetus develops.
Pyometra is often the result of hormonal changes in the reproductive tract. Following estrus (heat), the hormone progesterone remains elevated for up to two months and causes the lining of the uterus to thicken in preparation for pregnancy. If pregnancy does not occur for several consecutive estrus cycles, the uterine lining continues to increase in thickness until cysts form within the uterine tissues (a condition called cystic endometrial hyperplasia). The thickened, cystic lining secretes fluids that create an ideal environment for bacterial growth. Additionally, high progesterone levels inhibit the ability of the muscles in the wall of the uterus to contract.
Pyometra may occur in any sexually intact adult female dog, but is most common in older dogs. During estrus, white blood cells, which would normally protect against infection by eliminating bacteria, are inhibited from entering the uterus. This is a normal occurrence to allow sperm to safely enter the female’s reproductive tract, without being damaged or destroyed by these immune system cells.
The cervix is the gateway to the uterus, and remains tightly closed, except during estrus. When it’s open, bacteria that are normally found in the vagina can enter the uterus rather easily. If the uterus is normal, the environment is adverse to bacterial survival; however, when the uterine wall is thickened and cystic, perfect conditions exist for bacterial growth. In addition, when these abnormal conditions exist, the muscles of the uterus cannot contract properly. This means that bacteria that enter the uterus cannot be expelled.
The typical time for pyometra to occur is about two to eight weeks after oestrus (“heat cycle“). The more heat cycles a dog experiences, the more likely they are to get pyometra.
The clinical signs depend on whether or not the cervix remains open. If it is open, pus will drain from the uterus through the vagina to the outside. Pus or an abnormal discharge is often seen on the skin or hair under the tail or on bedding and furniture where the dog has recently laid. Fever, lethargy, anorexia, 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 ultimately causing the abdomen to distend. The bacteria release toxins that are absorbed into the bloodstream. Dogs with closed pyometra become severely ill very rapidly. They are anorectic, very listless and very depressed. Vomiting or diarrhea may also be present.
Toxins released by the bacteria affect the kidney’s ability to retain fluid. Increased urine production occurs, and many dogs drink an excess of water to compensate. Increased water consumption may occur in both open- and closed-cervix pyometra.