Investigating plasma proteome, fecal microbiome, and salivary biomarkers as prospective features of heat stress response in preweaned Holstein heifer calves

Abstract

In recent decades, heat stress events have shifted toward extremes in parameterization with increased frequency, duration, and intensity due to global climate change. Respectively, these shifts have resulted in industry-scale economic outcomes that accumulate from operation-scale losses in herd productivity and welfare in dairy systems. Heat stress induces detrimental effects through increased morbidity and mortality, decreased milk production and reproductive performance, among others. Heat stress has remained more characterized for lactating cattle in dairy systems, perhaps due to the availability of real-time performance records kept by the farm. Preweaned dairy calves have received less attention in this area of research, presenting a critical knowledge gap when considering dairy system sustainability. Chapter 1 of this thesis provides comprehensive consideration of factors that contextualize the value and relevance of previous heat stress physiology research and offers insight and constructive direction for future studies, specifically in the framework of a dairy system. The molecular, microbial, and endocrinological mechanisms both biomarking and programming the heat stress response in preweaned dairy heifer have not received thorough characterization. This has resulted in a lack of understanding of the relationship between calfhood heat stress events and individual animals immediate and long-term outcomes. Chapter 2 describes a proteomic study that provides an analysis of differential expression in matched plasma samples obtained from a cohort of dairy heifer calves at seven days of age under thermoneutral and heat stress conditions. Under heat stress conditions, proteasome 26S subunit, ATPase 1, histone H2A type 2-C, histone H1.3, histone H4, histone H2B type 1, and bisphosphoglycerate mutase were identified as occurring in significantly higher relative abundance compared to calves experiencing thermoneutral conditions. These results suggest cellular damage and dysfunction with indications of systemic outcomes, possibly occurring from oxidative stress. Chapter 3 contains a microbiome study that explores the development of fecal microbiome profile for preweaned dairy heifer calves throughout the first sixty days of life, a timeframe coinciding with the preweaning period to avoid confounding by the stress of the weaning event. Data analysis revealed that Faecousia sp000434635 showed the largest increase in abundance over the course of the sixty-day period while Ruminococcus gnavus showed the largest decrease in abundance. Matched fecal samples under thermoneutral and heat stress conditions were obtained at seven days of age and thirty days of age to evaluate whether heat stress exerts an immediate effect on microbiome profile, and it was concluded that heat stress should be considered as a deterministic factor since no significant differences were detected between these matched samples. However, disease states, animal age, and previous heat stress events must be considered in the experimental context. Chapter 4 provides a study of hypothalamic-pituitary-adrenal axis activity through salivary cortisol and cortisone analysis. Salivary samples were obtained from dairy heifer calves starting at two weeks of age, with two complete sampling days including three timepoints. For Day 1, significant differences were observed between the 5:00 AM and 10:00 AM timepoints for salivary cortisol concentration. On Day 2, significant differences in salivary cortisol concentrations were also observed between the 5:00 AM and 10:00 AM timepoints. Salivary cortisone showed significant differences between the 10:00 AM and 2:00 PM timepoints on Day 1, and no significant differences in salivary cortisone concentrations were observed on Day 2. Based on these findings, salivary cortisol and cortisone should be considered together as to account for the enzymatic conversion of salivary cortisol to cortisone within the salivary glands, and to fully capture the temporal and pulsatile dynamics of these analytes when describing in the context of a heat stress study.