The estrous cycle in bitches: Mechanisms, influencing factors, and clinical relevance

The estrous cycle of female dogs is a complex, hormonally regulated process comprising proestrus, estrus, diestrus, and anestrus. Its variability is influenced by breed-specific genetics, environmental factors, and microbial dynamics within the reproductive tract. Methods of estrus determination, including vaginal cytology, hormonal assays, ultrasonography, and thermography, remain integral to reproductive management, while emerging artificial intelligence-based approaches offer novel advancements in cycle prediction and monitoring. Despite extensive research on hormonal mechanisms, significant gaps remain in understanding the interplay between endocrine regulation, microbial colonization, and genetic predisposition in cycle variability. The use of hormonal contraceptives and gonadectomy introduces long-term physiological alterations, including chronic luteinizing hormone elevation, which has been linked to metabolic disorders, orthopedic complications,and neoplasia. Additionally, microbial influences on reproductive health, particularly the role of vaginal and uterine microbiota in fertility and disease susceptibility, require further investigation. The prolonged anestrus phase in canines poses a significant challenge for assisted reproductive technologies, limiting the efficacy of artificial insemination and embryo transfer. This review critically examines current methodologies for estrus cycle determination, explores factors contributing to interindividual variability, and evaluates the long-term implications of reproductive interventions on fertility and overall canine health.