Association of endometrial degeneration in bitches with insufficient plasma progestagen concentrations.

: The relationship between changes in plasma progesterone concentrations and endometrial degeneration in bitches was determined. Mature bitches (n = 14) were ovariectomized and treated with oestradiol benzoate for 11-12 days, followed by progestagen (2 mg megestrol acetate kg-1 body weight per day) for 35-37 days. Two bitches were necropsied at this stage (progestagen group). The other bitches were treated once a day for a further 3 weeks with 0.5 mg megestrol acetate kg-1 (decreased dose group; n = 3), 2 mg megestrol acetate kg-1 (standard dose group; n = 3), or 3 (1 week), 4 (1 week) and 5 (1 week) mg megestrol acetate kg-1 (increased dose group; n = 3), or received no treatment (withdrawal dose group; n = 3). A further five bitches with intact ovaries were examined during dioestrus (n = 4) and anoestrus (n = 1; 3 weeks after plasma progesterone concentration 80% cells) of the luminal epithelium was observed in the withdrawal dose and decreased dose groups, and in the intact bitches with plasma progesterone concentrations of 21, 36 and 39 nmol l-1. Medium (40-60% cells) degeneration was detected in the standard dose group and in the anoestrous bitch. However, a very small proportion (< 10%) of degenerated cells was found in the increased dose and progestagen groups, and no degeneration was detected in the dioestrous bitch with a plasma progesterone concentration of 90 nmol l-1. The numbers of endometrial leucocytes were low in all groups except for the withdrawal dose group and the anoestrous bitch. These results indicate that endometrial degeneration and exfoliation in bitches reflects a reduction or insufficiency of plasma progesterone concentrations. The mechanisms involved are unclear.