Unspayed older female dogs are at risk for developing pyometra, a life-threatening disease of the uterus. It occurs most commonly in females over six years of age. Pyometra is caused by one of the normally occurring female hormones, progesterone. In some cases, excessively high levels of the hormone are produced. In others, the uterus simply becomes overly sensitive to progesterone. In either case, in response to this substance, cysts form in the lining of the uterus. Additionally, the secretory cells lining this organ are also stimulated and large quantities of fluid are released into the uterus.

All of this brings about a dramatic increase in the overall size of the uterine horns and body. Normally, the horns of the uterus are no larger or longer than a lead pencil. In pyometra, they become large, sac-like pouches the size of a cucumber and twelve to eighteen inches long. As the disease continues, bacteria that normally live in the vagina move through the cervix and colonize these intrauterine fluids. This results in a defense response by the body, with millions and millions of white blood cells being shunted into the area to combat these organisms (an accumulation of large numbers of white blood cells and bacteria is what we commonly refer to as pus).

Early in the disorder the cervix remains open, allowing this accumulation of pus, fluid, bacteria and their by-products and toxins to drain out through the vagina. With time, however, the cervix closes, trapping all of this within the uterus. At this point, the uterus continues to grow, and can easily rupture. More probably, however, the body will attempt to eliminate the accumulated material through the bloodstream. But in the case of pyometra, there is more material than the body can eliminate. Therefore, the toxins and waste products spread out and poison the entire system.


Pyometra follows a heat cycle in which breeding did not occur. Typically within two to four months following the cycle, the female dog starts showing clinical signs of the disorder. The body attempts to flush the accumulated wastes and toxins with the kidney system. To do this the animal must drink huge quantities of water and therefore urinate frequently.

These are typically the signs first noticed with pyometra. Later, the owner may notice the animal licking at the vulva or a drainage occurring there. The animal will probably have a decreased appetite and due to the increasing mass in the abdomen, seem weak on the rear legs.


Animals with pyometra that do not receive treatment almost always die. The uterus will continue to increase in size and will finally rupture, spilling pus, bacteria and fluid into the abdomen. This causes peritonitis combined with toxic shock syndrome that results in death within a few hours. Even if this doesn't occur, over time the build-up of toxins within the system will lead to kidney failure.


In female pets the treatment of choice is an ovariohysterectomy or spay. By totally removing the ovaries and uterus, the cause and the major result of the disease is eliminated. It is important that this be done as soon as possible to prevent damage to the kidneys. Surgery is usually followed with long-term antibiotic therapy.


The only sure way to prevent pyometra is to spay the dog.