Pyometra, Canine - contd.

Description
Pyometra refers to uterine infection that occurs when contaminating bacteria overcome the normal uterine protective mechanisms. This usually occurs when blood levels of progesterone are elevated. Progesterone increases may occur naturally, as part of diestrus or ovarian phase of reproduction, or iatrogenically, secondary to the administration of reproductive hormones.

As intact female dogs age the uterus may become progressively thickened and cystic from repeated hormonal stimulation. This condition called cystic endometrial hyperplasia. Uterine secretions are greatest during diestrus, the period following estrus, when blood levels of progesterone hormone are maximal. Some inflammatory cells are always present in the secretions. Despite frequent opportunities for bacterial contamination from the lower reproductive tract, the fluid accumulation in the thickening uterus remains free of bacterial infection in most dogs. Pyometra occurs when the excessive uterine secretions become infected with bacteria.

Escherichia coli is the most common bacteria isolated from the uterus of patients with pyometra. This bacterium has the ability to bind specifically to uterine lining cells changed by progesterone. External sources of estrogen hormone potentiate the effects of progesterone, increasing the risk for pyometra. Young dogs that are unlikely to have significant cystic endometrial hyperplasia may develop uterine infections if hormones are administered. Pyometra may occur one to eight weeks after an injection of estrogen.

Bacterial infection of the uterus causes increasing inflammation within the organ and leads to systemic effects. Bacteria or bacterial toxins may enter the blood stream from the diseased uterus, leading to the clinical signs described previously. The severity of the resulting illness is greatly influenced by the degree of drainage from the uterus. If the cervix is closed, then fluids and toxins accumulate, like an abscess, with potential for toxic effects. Rupture or slow leakage from one of the uterine horns can release inflammatory products into the abdominal cavity, causing peritonitis. If the cervix is patent, or open, then drainage limits the accumulation of inflammatory products and bacterial toxins, and increases the likelihood of early recognition of the problem.

Signs of increased thirst and urination have been linked to the direct influence of bacterial toxins on the kidneys' urine concentrating mechanisms. Bacterial infection and toxins may cause secondary damage to the liver as well. Endotoxic shock alters blood supply to all tissues and can disrupt normal blood clotting mechanisms. Microscopic blood clots or clumps of circulating bacteria can further impact upon the blood supply to vital organs such as the heart and brain, permitting seizures, cardiac rhythm disturbances and other grave consequences.