In its simplest terms, Pyometra (Pyo means "pus" and Metra refers to "uterus") is an infection in the uterus. The disease mainly affecting middle-aged female dogs that have not been spayed. Pyometra is a hormonal abnormality, and a secondary bacterial infection may or may not be present. Pyometra follows a heat cycle in which fertilization did not occur. Typically, within two to four months after the cycle, the female starts showing signs of the disease.

Female dog ovaries are producing tow main hormones estrogen and progesterone. Overquantity of progesterone or oversensitiveness of the uterus to progesterone causes Pyometra and cysts are forming in the lining of the uterus. These cysts contain numerous secretory cells, and large quantities of fluids are produced and released into the interior of the uterus. This fluid, along with a thickening of the walls of the uterus, brings about a dramatic increase in the overall size of this organ into sac-like pouches the circumference of cucumbers and 12 to 18 inches long and weighs around 1 to 4 pounds compare to normal 2 to 4 ounces.

Bacteria commonly colonize the uterus by entering through the cervix. This produces an even greater response by the body, as it showers additional fluid and white blood cells into the affected organ. This effectively traps all of the fluid within the uterus. As the body attempts to flush out the build-up of waste products through the kidneys, the animal will drink excessive quantities of water and urinate large amounts frequently. 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 very lethargic.

Diagnostic test were; complete blood cell count, a chemistry profile to evaluate function of internal organs, radiograhs of the abdomen, ultrasound of the uterus, culture of pus in the uterus, and biopsy of the uterus if cancer is suspected.
Since toxicity may develop very quickly in dogs with Pyometra, it needs to be treated promptly. Dogs will receive intravenous fluids, usually for several days before and after the surgery, and antibiotics for 1 to 2 weeks. In most cases, the preferred treatment is a complete ovariohysterectomy (spay). This removes the ovaries, oviducts, uterus, and all associated blood vessels. Other treatment includes transfusions of plasma, Hetastarch and while blood. In mild cases, when the cervix is still open and the fluid is draining, the success rate is excellent. This therapy should only be used in dogs 6 years of age or younger, who are in stable condition, and have an open cervix. Prostaglandins help the uterus to expel pus and alters the hormone levels of the ovaries. Prostaglandins can have side effects, especially after the first dose, including restlessness, panting, vomiting, increased heart rate, fever, and defecation.  

The best prevention is to have all female animals spayed at or before six months of age. If the animal is used for breeding, then spaying the animal after she is past her breeding years is highly recommended. Pyometra is a fairly common and serious problem and is just one of many compelling reasons to have female pet spayed at an early age.