Invited review: Herd-level udder health indices derived from repeated individual somatic cell count measurements

Subclinical mastitis persists as a challenge in dairy production, contributing to significant economic losses, compromising animal health and welfare, and driving antimicrobial use. It adversely affects milk quality, farm productivity, and overall profitability. This narrative review synthesizes historical literature, knowledge, and tools for herd-level udder health monitoring, with a focus on indices derived from repeated individual cow SCC measurements. Grounded in epidemiological principles, this review is structured around the life cycle of dairy cattle, using SCC data collected through routine milk recording tests to support meaningful interpretation of udder health dynamics across different physiological stages, not solely the lactation period. While SCC measurements are exclusively obtained during lactation, they may also be used to infer udder health status and dynamics around nonlactating periods; however, this approach may not fully represent the entire nonlactating stage or capture infectious events occurring between widely spaced tests or long before a cow's first SCC measurement. This framework also acknowledges that udder health is influenced by age, parity, and physiological and production stages, and that effective monitoring requires thoughtful interpretation of available data across the entire lactational trajectory. It supports more comprehensive and sustainable udder health herd-level management strategies. The review presents key herd-level udder health indices, discusses considerations for their use, and highlights the importance of standardized definitions and thresholds to ensure consistency and comparability. It also provides a curated selection of indices, including definitions, calculation methods, strengths and limitations, and reference values reported in the literature. This review aims to support veterinarians and dairy producers who routinely use summarized SCC reports generated through milk recording systems in implementing robust mastitis monitoring strategies throughout the cow's life cycle. The objective is to provide conceptual guidance and practical considerations that can be applied across a wide range of dairy production systems and herd sizes.