A normal uterus in an average-sized dog weighs just a few ounces. But when pyometra is present, the organ can weigh up to four pounds due to the accumulation of fluid and diseased tissue.
The fluid accumulation in the uterus starts leaking out through the vagina. The dog's natural response is to lick the area clean. Excessive licking can introduce still more bacteria through the cervix and into the uterus.
The body's response to the secondary infection is to increase fluid production and white blood cells to the uterus, which continues to flow out of the vagina. This is called an open pyometra, because the cervix is open, allowing fluid and accumulated debris to be flushed from the body through the vagina.
At some point, the cervix closes and the fluid can no longer flow out of the uterus. Meanwhile, the body continues to produce more and more fluid and white blood cells. The result is an enlarged uterus. This condition is called closed pyometra, because the cervix does not allow the accumulated material to exit through the vagina.
In worst-case scenarios, the uterus can rupture and empty all of its contents into the abdominal cavity. When this happens, the animal usually dies of septic peritonitis and/or acute kidney failure from uremic poisoning within about 48 hours, even with very aggressive medical intervention.
Obviously, the goal is to catch this condition long before it becomes this serious. Catching symptoms early on is very important in treating pyometra successfully.