Hormonal Control of the Estrous Cycle in Cats

Manipulation of the estrous cycle in cats is often even more challenging than in dogs, because of the physiology of their estrous cycle and possible infeasibility of medication administration. Queens are long-day, seasonally polyestrous, induced ovulators with diestrus stimulated primarily by the physical act of copulation or natural breeding. Ovulation can also occur spontaneously.

Megestrol acetate at 5 mg/cat every 24 hours for 3 days, then 2.5–5 mg every 7 days for a maximum of 10 weeks, can be used to suppress estrus in queens (1, 2). Administration of megestrol acetate to queens is an extralabel use of the drug, and informed consent should be obtained from the cat's owner. The queen should be allowed an estrus before resuming therapy.

The American College of Theriogenologists does not recommend the use of megestrol acetate in feral cats, for multiple reasons, including the difficulty of ensuring the dosage that is consumed and the inability to monitor for efficacy, as well as for the development of adverse effects. Adverse effects include the development of cystic endometrial hyperplasia, pyometra, mammary hyperplasia, insulin resistance and overt diabetes mellitus, inappetence, vomiting, and diarrhea.

Mibolerone is also not recommended to suppress estrus in queens, because of the occurrence of hepatotoxicosis (3, 4). Long-term deslorelin implants have been used to suppress estrus in cats; however, the length of suppression reported varies (5).

Melatonin is not approved for use in queens; however, it has been used extralabel for estrus suppression either with subcutaneous implants labeled for sheep in the UK and New Zealand (18 mg/implant [6, 7]) or with oral formulations (4–30 mg/cat, administered 3 hours before dark, every 24 hours [8]). The duration of efficacy in cats is highly variable between individuals (7).

Induction of ovulation is necessary in queens if assisted reproductive techniques, such as semen collection and insemination, are pursued, because cats are induced ovulators after the act of copulation (9, 10). Ovulation of mature follicles (present on day 2 of estrus) can be induced by treatment with human chorionic gonadotropin (hCG) at 250 IU, IM, once (11, 12, 13), or with GnRH at 25 mcg, IM (14, 15). Reported dosages of hCG range from 25 to 500 IU; however, higher dosages, though more effective at inducing ovulation reliably, can also result in oocyte degeneration. Ovulation is reported to occur 25–27 hours after hCG administration to queens (16, 17).

Recommended doses of hCG range from 25 to 500 IU (17). The higher doses are more effective at inducing ovulation; however, they might result in oocyte degeneration, as well as decreased fertility after breeding.

Estrus can be induced in queens with porcine-derived follicle-stimulating hormone (FSH; 2 mg, IM, the first day; then 0.5–1 mg, IM, every 24 hours for 4 additional days) (9, 18, 19, 20).

Use of GnRH agonists has also been described to induce estrus in queens when used similarly to the way it's described in dogs (21).

Use of estrogens to induce estrus is not recommended in cats and can cause signs of toxicosis, such as bone marrow suppression, alopecia, lethargy, vomiting, or diarrhea.