The estrous cycles of dogs and cats are not as easily manipulated as in other species. Controlled studies are lacking with most protocols, and their use 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 using prolactin inhibitors (eg, bromocriptine, cabergoline).
Ovariohysterectomy is the best method to prevent estrus in the bitch and queen. Longterm suppression of estrus in the bitch may be accomplished with a synthetic compounded androgen. The dose is 3 μg/kg/day except for German Shepherds and their crosses, which require 6 μg/kg/day. Therapy must begin ≥1 mo before proestrus. Common adverse effects are clitoral hypertrophy, vaginitis (especially in prepubertal bitches), increased activity of skin sebaceous glands, mild epiphora, and alterations in hepatic function studies. Return to estrus after treatment is discontinued is variable but is ~70–90 days. Conception rates are reportedly normal by the second cycle after treatment. No published studies exist documenting the safety of androgens in large numbers of valuable breeding bitches; their use should not be undertaken without client consent. If given to pregnant bitches, synthetic androgens induce severe developmental anomalies in the urogenital system of female puppies. This synthetic androgen 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).
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 μg/cat. Both are given IM, sid for 2 days.
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.
Last full review/revision July 2011 by Autumn P. Davidson, DVM, MS, DACVIM