Abstract
Over the last several decades, wildfires have been a concern across the United States. Smoke emitted by wildfires is a major source of air pollution emissions, leading to widespread degradations in air quality and concerns for public health. Although wildfire smoke is a heterogenous mixture of air pollutants, particulate matter less than 2.5 μm in diameter (PM2.5) is considered particularly hazardous to health because of its ability to infiltrate the innermost portions of the respiratory tract upon inhalation. PM2.5 derived from wildfires (wildfire-PM2.5) elicits potent toxic effects and may be more dangerous than other sources of PM2.5 pollution. Indeed, exposure to wildfire-PM2.5 induces and exacerbates many health conditions in humans, leading to elevated risk of premature mortality and respiratory disease, among other conditions. Populations such as the young or the elderly are particularly prone to health concerns associated with wildfire-PM2.5 exposure.A substantial portion of US dairy operations reside in regions commonly afflicted by wildfires. Over the last several decades, many researchers have aimed to document and alleviate the adverse effects that various environmental factors, such as management and climate, have on health and production in dairy cattle. However, few have assessed the effects of poor air quality on dairy cattle, and until recently, none had documented the effects of wildfire-specific sources of air pollution. Wildfires are expected to continue to contribute to a large portion of air quality deficits across the US in the coming decades, highlighting the importance of addressing the effects of wildfire smoke on the dairy industry. Calves may be especially susceptible to wildfire-PM2.5 as they are born with immature immune systems, and respiratory disease is one of the leading causes of morbidity and mortality in young calves. Calves that have had respiratory disease in early life have reduced performance in their first lactation. Thus, the objective of this dissertation is to investigate the impacts of wildfire-PM2.5 exposure on calf health and development.
The first chapter of this dissertation provides a detailed overview of the current knowledge of the effects of wildfire smoke exposure on humans, and its implications on cattle. Empirical studies investigating the effects of wildfire-PM2.5 exposure on calves are covered in chapters 2, 3, 4, and 5. Across all empirical chapters, wildfire-PM2.5 concentrations and temperature-humidity-index (THI) were recorded from monitoring stations located close to farms and used for analyses. In chapter 2, the effects of exposure to wildfire-PM2.5 in the preweaning period of Holstein heifer calves on health parameters and metabolic and immune markers is investigated. When exposed to elevated wildfire-PM2.5 in conjunction with high THI, calves showed signs of respiratory irritation, indicated by greater eye discharge and coughing, in addition to greater fat catabolism, altered inflammatory marker concentrations, and depletions of certain immune cell populations. In chapter 3, the effects of wildfire- PM2.5 on Holstein heifer calf standing and lying behavior was assessed. Elevations in daily average wildfire- PM2.5 resulted in greater time spent lying down, however, shorter-term hourly fluctuations in wildfire-PM2.5 resulted in greater time spent standing. Additionally, the responses were greater upon exposure to the first wildfire smoke event as compared to the second. In chapter 4, the effects of wildfire-PM2.5 on calf respiratory pathophysiology and inflammatory status of circulating immune cells (peripheral blood mononuclear cells, PBMC) were assessed. Elevated wildfire-PM2.5 and THI together was associated with an initial depletion of several immune cell populations, increased eye discharge, and increased thoracic ultrasound (TUS) scores, which is indicative of pulmonary health deficits. During wildfire smoke exposure relative to before, fluid collected from the respiratory tract contained a greater proportion of macrophages and a lower proportion of neutrophils, although neutrophils appeared to be depleted to a greater extent. PBMC showed upregulated transcription of pro-inflammatory mediators. Calves continued to show upregulated transcription of pro-inflammatory mediators in PBMC and elevated TUS scores one month following wildfire smoke exposure. In chapter 5, the impacts of wildfire smoke exposure of pregnant dams on postnatal calf health and development were investigated. Calves born to wildfire exposed dams (WFS) were smaller at birth but showed compensatory growth patterns in the pre-weaning period relative to calves born to unexposed dams (CON). However, WFS calves showed evidence of greater fat catabolism during the preweaning period. Across the study, WFS calves also had lower inflammatory marker concentrations.
The work described in this dissertation comprises the first studies on the impacts of wildfire smoke exposure on young dairy heifer calves and provide evidence of health, metabolic, and growth effects from prenatal or postnatal exposure to wildfire pollutants. These studies address the growing concern for the impacts of wildfire smoke-derived air pollution on dairy cattle health and performance, particularly within critical developmental periods of calves.