HORMONAL USE

Indications for estrus induction in the bitch include missed breeding opportunities or conception failure, the treatment of primary or secondary anestrus and synchronization of ovulation for embryo transfer programs. Reported methods for canine estrus induction include the use of synthetic estrogens (diethylstilbesterol), dopamine agonists (bromocryptine and cabergoline), GnRH agonists (lutrelin, buserelin, fertirelin, deslorelin, and leuprolide) and exogenous gonadotropins (luteinizing hormone, follicle stimulating hormone, human chorionic gonadotropin, pregnant mare serum gonadotropin, and human menopausal gonadotropin). These methods vary widely in efficacy of inducing estrus, as well as in the fertility of the induced estrus. The applicability of some of these methods for clinical practice is questionable. 

Dopamine Agonist Administration. Several studies have reported the use of a dopamine agonist (DA) administered orally at doses sufficient to lower plasma prolactin as a means to terminate anestrus either prematurely in normal bitches or therapeutically in cases of prolonged or persistent anestrus. The efficacy has been anecdotally estimated to be about 70%, and possibly higher in bitches with prolonged anestrus; the resulting proestrus, when induced, has occurred after a variable duration of treatment ranging from 8 to 40 days; the average appears to be about 20 days; duration appears to be dependent on the stage of anestrus, with longer treatment required in early anestrus . Whether the simultaneous reduction in prolactin is part of the mechanism of action or if the mechanism involves other or additional dopaminergic effects is not known. However, efficacy appears to depend on a dopamine responsiveness sufficient to also cause suppression of prolactin; bitches that fail to experience suppression of prolactin also fail to show a clinical proestrus response. Two DA treatments reported to be effective have included bromocriptine (Parlodel) at 0.05.or 0.1 mg/kg, p.o., q.d. or bid, and cabergoline (Galostop) at 5 ug/kg, p.o, q.d.; administration is until an induced proestrus is pronounced for 2 days or until the onset of estrus.