In most species infection may arise at oestrus or mating, at parturition or following embryonic death. In cattle pyometra develops when the corpus luteum of the oestrus following parturition does not regress. This is usually as a result of infection acquired at parturition or after early embryonic death. In dogs and cats inflammation and infection may be superimposed on endometrial hyperplasia. This disease in dogs typically develops in older bitches after repeated cycles at which mating has been prevented.
Oestrogens also play a part as the cell receptors for progesterone are oestrogen-dependent.
The disease has been divided into four categories, depending on the clinicopthological features. Endometrial hyperplasia, without inflammation, may lead to formation of cysts. Clinically this type or stage of the disease complex is inapparent. . . - cystic endometrial hyperplasia . . . These cases also have endometrial hyperplasia but in addition have histological signs of inflammation. Type 1 changes are thought to be the predisposing lesion but the relationship between type 2 and type 3 is unclear.