Manipulation of the Estrous Cycle in Small Animals
The estrous cycles of dogs and cats are not as easily manipulated as in other species. Most protocols are not based on controlled studies, so manipulation of the estrous cycle in valuable breeding individuals is not advised. Although onset of a particular cycle may be delayed, return to normal cycling is highly variable. Induction of estrus is possible in late anestrus bitches by using prolactin inhibitors (eg, bromocriptine, cabergoline).
Ovariohysterectomy or ovariectomy is the best method to prevent estrus in the bitch and queen. Longterm suppression of estrus by using androgens is not advised, because it is not documented to be safe in breeding bitches. Common adverse effects are breakthrough proestrus, clitoral hypertrophy, vaginitis (especially in prepubertal bitches), increased activity of skin sebaceous glands, mild epiphora, and alterations in hepatic function studies. After treatment is discontinued, return to estrus is ~70–90 days but variable. Conception rates are reportedly normal by the second cycle after treatment. If given to pregnant bitches, synthetic androgens induce severe developmental anomalies in the urogenital system of female puppies. The safety and efficacy of injectable testosterone, as is practiced commonly in racing Greyhounds, has not been supported by controlled studies and is not advised. Androgens should not be given to cats.
The use of megestrol acetate, a synthetic progestagen, is not advised in breeding females because of the increased risk of cystic endometrial hyperplasia and pyometra, as well as other adverse effects (eg, mammary hyperplasia and neoplasia, hyperglycemia secondary to insulin resistance, and rebound hyperprolactinemia and lactation).
Suppression (and induction) of estrous cycles by the use of synthetic GnRH implants has been described in the bitch; it can be successful, but the products are not universally available. Estrus induction in the bitch can be accomplished with oral prolactin inhibitors (cabergoline 2.5–5 mcg/kg/day); anestrus of at least 2 mo duration must precede induction.
Ovulation can be induced in estrual queens physically or, more reliably, hormonally to produce a luteal phase (diestrus or metestrus) of ~45 days. Physical methods include mating with a vasectomized tom (very effective) or inserting a sterile swab or glass rod into the vagina. The latter should be performed repeatedly for best results. Hormonal methods include administration of human chorionic gonadotropin at 500 IU/cat or GnRH at 25 mcg/cat. Both are given IM, once daily for 2 days.