A Pilot Study to Evaluate the Potential of Melatonin Implants to Control Cyclicity in the Bitch

SIMPLE SUMMARY: Current methods to control the reproductive cycle in the bitch are associated with problematic side effects. Melatonin plays a role in the regulation of reproduction in several species. It has been used to control the reproductive cycle in species with seasonal reproduction. The ancestor of the dog, the grey wolf, has strict seasonal reproduction with breeding in early spring. In contrast to the wolf, the domestic dog is a non-seasonal breeder. It is still not known whether melatonin is involved in control of cyclicity in either the wolf or the domestic dog. The aim of this study was, therefore, to evaluate whether a long-acting melatonin implant would be a safe alternative for short-term postponement of heat in the bitch. Five beagle bitches were treated with melatonin implants while another four bitches were untreated controls. There was no effect of treatment with melatonin implants on the cyclicity in this study. Implants with 18 mg are, therefore, not likely to be a useful method to control cyclicity in the bitch, in contrast to some other species. It is still not known whether melatonin is involved in regulation of the oestrus cycle in the domestic dog. ABSTRACT: In short-day breeders such as the sheep, melatonin stimulates oestrus activity; in contrast, a high serum concentration of melatonin inhibits oestrus in long-day breeders such as the cat. Therefore, implants with melatonin have been used to suppress or induce oestrus depending on the species. The aim of this pilot study was to evaluate if melatonin could be an alternative to control the reproductive cycle in the bitch. Nine beagle bitches were observed for three oestrus cycles. Five beagle bitches were treated with 18 mg melatonin implants on average 27 days before the next expected oestrus based on the previous interoestrus interval. Four bitches served as untreated controls. Blood samples for evaluation of serum melatonin were collected at the time of assignment to treatment or control group and 1–4 weeks thereafter. Clinical signs and vaginal smears were used to follow the cycle. Melatonin varied significantly with bitch (p < 0.05) but not with treatment. Treatment did not affect the interoestrus interval (p > 0.05). In conclusion, treatment with 18 mg melatonin implants approximately one month before expected oestrus is not likely to be a useful method to control cyclicity in the bitch. It is still not known if melatonin is involved in regulation of the oestrus cycle in the domestic dog.