Doctor of Philosophy / Department of Diagnostic Medicine/Pathobiology / Robert L. Larson / Brad J. White / The diagnosis and prevention of bovine respiratory disease (BRD) is challenging due to the multifactorial nature of the disease. Mannheimia haemolytica, Pasteurella multocida, and Histophilus somni are gram negative bacteria that are commonly considered three of the most important bacterial agents involved in the etiology of BRD; all three are also commensals in the upper respiratory tracts of healthy cattle. However, despite decades of study, questions still remain regarding the transmission dynamics and characterization of these bacteria, the ability of diagnostic sampling methods to accurately portray the causative bacteria, and ways to mitigate the effects of risk factors for BRD such as long-distance transportation. Through our research, we have demonstrated how variable the culture results of a single nasopharyngeal swab can be and the challenges of using an individual culture to truly represent animal M. haemolytica status. Additionally, comparison of the diagnostic performance of two antemortem sampling methods, nasopharyngeal swabs (NPS) and bronchoalveolar lavages (BAL), revealed high agreement, high negative predictive values of NPS for the presence of M. haemolytica, P. multocida, and H. somni in the lungs, and the potential for different susceptibility profiles from paired NPS and BAL samples. As agreement and predictive values can vary with disease prevalence, interpretation of diagnostic test results should be done carefully and with due consideration of the sample population in which the test is being applied. An investigation into cattle behavior following a relatively innocuous handling procedure indicated that some behaviors are altered after handling and restraint in a squeeze chute. Additionally, we demonstrated that cattle with different temperament scores may have different activity levels and spend different amounts of time within 1 m of the hay bunk, grain bunk, waterer, and shed after handling. Consequently, there is also the potential to improve upon disease detection algorithms by incorporating behavioral changes that may occur after handling events and the need for careful trial design when behavioral parameters are a trial outcome. Additionally, this study indicated that some behaviors may vary for calves that react differently when handled, which also has potential implications when behavior is considered a variable of interest. Examination of the potential for 1 mg/kg oral meloxicam administered pre-transport to mitigate the effects of long-distance transportation revealed that meloxicam did not have a statistically significant effect on the maintenance of leukocyte function or the reduction of inflammation during or after long-distance transportation in healthy steers. Finally, a report on two separate experiments regarding the effects of 1 mg/kg oral meloxicam administered pre-transport on the movement, feeding, and drinking behaviors and performance of transported and non-transported calves demonstrated that there was a significant day effect on behaviors in transported calves but meloxicam did not affect behavior or performance. However, meloxicam did modify the effect of day on daily distance traveled in non-transported calves but there were no significant within-day comparisons between non-transported meloxicam and non-transported control calves. These results do not provide evidence for the benefit of administering a single dose of 1 mg/kg oral meloxicam prior to long-distance transportation.

This thesis seeks to assess the exposure of children in outdoor play areas of early childhood centres in Auckland City to traffic-related PM10 pollution. An estimated 400 premature deaths occur each year in New Zealand due to motor vehicle emissions. In addition to premature deaths, acute and chronic health effects including asthma, chronic obstructive pulmonary disease (COPD), heart disease and bronchitis, as well as increased hospitalisations and restricted activity days (sick days) are also associated with vehicle emissions. Epidemiological studies have shown that respiratory diseases such as asthma can be exacerbated by increases in the concentration of particulates of less than 10 microns in diameter (PM10) from motor vehicle emissions. Significant positive associations have been found between proximity to heavily travelled roads and increased childhood respiratory disease symptoms including hospitalisations for childhood asthma. In spite of this evidence, many early childhood centres in Auckland are located adjacent to busy roads. Children at these early childhood centres spend much of their time playing in the outdoor areas of these centres with the potential for particulates from motor vehicle emissions to exacerbate symptoms in those children already suffering from respiratory disease and asthma. Very little research has been carried out either in New Zealand or internationally on the air quality of outdoor play areas of early childhood centres in relation to motor vehicle emissions and childhood respiratory disease and asthma. The extent of monitoring is also limited and the amount of exposure data available in New Zealand relatively sparse, particularly in comparison with Europe. Levels of traffic-related PM10 in the outdoor play areas of early childhood centres were measured in centres located adjacent to busy roads and in centres away from a quiet road or adjacent to a very quiet road for comparison. Two of five early childhood centres located alongside busy roads had PM10 levels that exceeded the World Health Organization (WHO) guideline value. While PM10 levels monitored at the other three centres located alongside busy roads did not exceed the WHO guideline value, results were often only marginally within this guideline value. In contrast, PM10 levels monitored at two centres located away from a quiet road and one centre located adjacent to a very quiet road were well within the WHO guideline value. This pilot study provides preliminary evidence that children attending early childhood centres located alongside busy roads have greater exposure to traffic-related PM10 pollution than those attending early childhood centres located away from a quiet road or adjacent to a very quiet road. If the link between PM10 exposure and health effects is causal, as suggested by epidemiological studies, then children attending early childhood centres proximal to busy roads are at a greater risk of respiratory illness than children attending early childhood centres adjacent to quiet roads. Further work is required to confirm the findings in this small sample of air quality around early childhood centres in a larger sample, and possibly to undertake an epidemiological study to confirm the link to health effects. Drawing on the precautionary principle, prudent territorial local authorities should be encouraged to introduce regulations ensuring that any new early childhood centres are located at a specified distance from major roads, and that information for parents, ongoing PM10 monitoring and processes for issuing PM10 advisories when limits are exceeded are available in existing early childhood centres that are adjacent to busy roads.

The objectives of this thesis were to investigate 1) the use of behavior and activity monitoring for the identification of heifers at risk of disease, 2) the use of group level management practices to reduce the risk of disease, and 3) the identification of long-term impacts of bovine respiratory disease complex (BRD). For objective 1, lying posture, a decreased willingness to approach an observer and high lethargy scores were associated with diarrhea in calves under 2 weeks of age and a high lethargy score in 4-6 week old calves was associated with decreased average daily gain (ADG) in the first 8 weeks of life (n = 744). In weaned calves (n = 74) increased activity (increase in steps and decrease in lying), standing at the bunk not eating, and lying far from other calves in the first 3 days post-weaning were associated with decreased post-weaning weight gain. For objective 2, separating social mixing from movement to a novel environment, and administering prophylactic antibiotics to calves at high risk of disease, were investigated. Both mixing and movement to a novel environment increased activity levels in newly weaned dairy calves (n = 64). When calves were mixed prior to movement to a novel environment they had a smaller increase in activity compared to calves that were simultaneously mixed and moved. No differences in weight gain or calf starter intake were observed. Administration of a prophylactic antibiotic, tulathromycin, to 3 day old calves upon arrival at a heifer raising facility (n = 788) and 8 week old calves at first movement to group housing (n = 1,392 ) was found to reduce diarrhea and otitis media, and BRD, respectively. Objective 3 was addressed by monitoring calves that received tulathromycin at 8 weeks of age to determine the long-term impacts of BRD. Bovine Respiratory Disease complex was associated with decreased growth to 9 months of age, decreased survival to first calving, increased risk of dystocia and a greater age at first calving. / Natural Sciences and Engineering Research Council of Canada & Pfizer Animal Health

This thesis seeks to assess the exposure of children in outdoor play areas of early childhood centres in Auckland City to traffic-related PM10 pollution. An estimated 400 premature deaths occur each year in New Zealand due to motor vehicle emissions. In addition to premature deaths, acute and chronic health effects including asthma, chronic obstructive pulmonary disease (COPD), heart disease and bronchitis, as well as increased hospitalisations and restricted activity days (sick days) are also associated with vehicle emissions. Epidemiological studies have shown that respiratory diseases such as asthma can be exacerbated by increases in the concentration of particulates of less than 10 microns in diameter (PM10) from motor vehicle emissions. Significant positive associations have been found between proximity to heavily travelled roads and increased childhood respiratory disease symptoms including hospitalisations for childhood asthma. In spite of this evidence, many early childhood centres in Auckland are located adjacent to busy roads. Children at these early childhood centres spend much of their time playing in the outdoor areas of these centres with the potential for particulates from motor vehicle emissions to exacerbate symptoms in those children already suffering from respiratory disease and asthma. Very little research has been carried out either in New Zealand or internationally on the air quality of outdoor play areas of early childhood centres in relation to motor vehicle emissions and childhood respiratory disease and asthma. The extent of monitoring is also limited and the amount of exposure data available in New Zealand relatively sparse, particularly in comparison with Europe. Levels of traffic-related PM10 in the outdoor play areas of early childhood centres were measured in centres located adjacent to busy roads and in centres away from a quiet road or adjacent to a very quiet road for comparison. Two of five early childhood centres located alongside busy roads had PM10 levels that exceeded the World Health Organization (WHO) guideline value. While PM10 levels monitored at the other three centres located alongside busy roads did not exceed the WHO guideline value, results were often only marginally within this guideline value. In contrast, PM10 levels monitored at two centres located away from a quiet road and one centre located adjacent to a very quiet road were well within the WHO guideline value. This pilot study provides preliminary evidence that children attending early childhood centres located alongside busy roads have greater exposure to traffic-related PM10 pollution than those attending early childhood centres located away from a quiet road or adjacent to a very quiet road. If the link between PM10 exposure and health effects is causal, as suggested by epidemiological studies, then children attending early childhood centres proximal to busy roads are at a greater risk of respiratory illness than children attending early childhood centres adjacent to quiet roads. Further work is required to confirm the findings in this small sample of air quality around early childhood centres in a larger sample, and possibly to undertake an epidemiological study to confirm the link to health effects. Drawing on the precautionary principle, prudent territorial local authorities should be encouraged to introduce regulations ensuring that any new early childhood centres are located at a specified distance from major roads, and that information for parents, ongoing PM10 monitoring and processes for issuing PM10 advisories when limits are exceeded are available in existing early childhood centres that are adjacent to busy roads.

Master of Science in Biomedical Sciences / Department of Clinical Sciences / Robert L. Larson / Bradley J. White / Feedlot cattle frequently endure high environmental temperature-humidity index conditions in the summer months within cattle feeding regions of North America. Heat stress develops when the total heat gain (combined effects of environmental and metabolic factors) exceeds an animal’s heat loss capabilities. The objective of my research was evaluating heat mitigation strategies and developing a practical method to identify animals that are of greatest risk of heat stress; thus improving animal welfare and performance. A number of heat abatement strategies have been utilized in US feedlots including shade, sprinklers, nutritional modifications, and misters. A literature review was performed using published journal articles demonstrated significant benefits of providing shade to feedlot cattle. Sprinkling the pen surface may be just as beneficial as sprinkling or misting cattle. Sprinkling the ground not only cooled the ground which increased the thermal gradient between lying cattle and the ground, but also provided increased thermal conductivity and better heat flow down that gradient. A study was performed to develop a noninvasive, remotely applied, practical method to identify animals at risk for heat stress. Infrared thermography images were obtained during the morning hours and pant scores obtained in the afternoon hours. Data mining techniques were employed to evaluate accuracy of potential classification methods to identify heat stress events in the afternoon based on the known morning data. Using infrared technology as a diagnostic test was not accurate for predicting heat stress events in the study presented. Finally a retrospective study of Kansas feedlot performance, medical and weather data was performed. Findings indicate that diagnostic counts of bovine respiratory disease are associated with elevated ambient temperature two days prior. In conclusion, heat stress in beef feedlot animals is an important area of research. Heat mitigation methods such as shade have been proven to be effective at reducing heat stress in beef feeder cattle. Further research is needed to evaluate the use of infrared technology to predict heat stress events in the feedlot setting.

Asthma is a chronic respiratory disease characterized by reversible airway obstruction, persistent airway hyperresponsiveness, chronic airway inflammation, and chronic airway remodeling. Most adult asthmatics have neutrophilic airway inflammation that correlates to increasing disease severity, and fail to respond to corticosteroid therapies that mitigate other asthma endotypes. Accordingly, there is a need to investigate the molecular mechanisms responsible for neutrophilic asthma. Pasture heaves, a respiratory disease affecting horses housed on pasture in conditions of high heat and humidity, shares the aforementioned characteristics of human asthma, including neutrophilic inflammation. The cause is undetermined, but genetic propensities for reactivity to seasonally inhaled, pasture-associated, aeroallergens are presumed. Complexities of diseases like asthma and pasture heaves, that include temporal interactions between environmental and genetic factors, lend themselves to exploration using -omics technologies. An emergent paradigm in disease pathogenesis views disease as the result of imbalances in a biological system of thousands of proteins that maintain eukaryotic homeostasis. Consistent with this paradigm, this dissertation describes systematic efforts to identify groups of proteins in the bronchoalveolar lavage fluids of horses with pasture heaves that are altered in a manner that influences neutrophilic airway inflammation, and are similarly changed in human asthma. This is the first use of -omics technologies to investigate pasture heaves. This was accomplished first by improving functional annotation of the equine genome by providing functional annotation for an equine oligoarray, thereby facilitating future functional modeling of equine gene products. Next, through comparative modeling of protein functions in normal bronchoalveolar lavage fluid (BALF) proteomes from horse, human, and mouse, we demonstrated conservation of protein functions in lung fluids across these species. Finally, comparative modeling of pasture heaves-affected and non-diseased BALF proteomes demonstrated that proteins in diseased BALF favor airway neutrophilic inflammation by increasing neutrophil migration, chemotaxis, adhesion, detachment, transmigration, and degranulation, while reducing activation, cell spreading, infiltration, phagocytosis, respiratory burst, apoptosis, and clearance. Collectively, these molecular events contribute to airway neutrophilic inflammation in pasture heaves, and are conserved in human asthma. This method further validates pasture heaves as a robust model for human neutrophilic asthma, and highlights proteins of potential clinical and therapeutic relevance.

Pneumonic pasteurellosis is a significant disease in beef production medicine. The most recent information suggests that this disease is a $700 million dollar per year economic burden in bovine food animal production The medical and pathological characteristics of this disease are well documented. Many pathological findings associated with pneumonic pasteurellosis may be explained by disruption of the pulmonary vascular adrenergic system. However, only a limited amount of research has addressed the adrenergic system and its relationship to the etiology and pathophysiology of this disease. In an attempt to further investigate the contributions of the vascular adrenergic receptor mechanism to the development of pneumonic pasteurellosis a series of six experiments have been completed. It is to be noted, that in 1999 the organism Pasteurella haemolytica was renamed Mannheimia haemolytica. The name change was based on the taxonomic features of the organism from other closely related organisms, in particular Pasteurella multocida.. The differences noted were identified and described by Dr. Mannheim in 1974. The familiarity of the past nomenclature and the lack of familiarity for the new nomenclature suggests that the more commonly recognized name of Pasteurella haemolytica should be used throughout this document. Scientific evidence suggests that the disruption of the normal homeostatic mechanisms of the pulmonary vasculature to beta adrenergic agents may be part of the etiology of pneumonic pasteurellosis. The dynamics and kinetics of the involvement of the beta receptors, following prophylactic vaccination and in the disease state, has yet to be fully investigated with respect to the events associated with pneumonic pasteurellosis. Evaluation of the time frame of the onset and duration of the events associated with the disruption of pulmonary vascular beta adrenergic receptor mechanisms revealed that an escalating level of dysfunction occurs over the first 24-48 hour period after exposure to parenteral Pasteurella haemolytica and lasts for at least 21 days. A component of P.haemolytica organism or contained in the vaccine using the organism is likely associated with the disruption of vascular beta adrenergic mechanism. This factor is, as yet, not specifically identified, however the likely culprit is the lipid A moiety of the endotoxin. Using the well defined and purified Escherichia coli endotoxin, trials were run to examine the effect of endotoxin on the pharmacological response of vascular associated beta adrenergic receptor mechanisms. The effects of Escherichia coli endotoxin, administered parenterally, on beta adrenergic receptor mechanisms were pharmacologically indistinguishable from those effects following parenterally administered Pasteurella haemolytica. The nature of the disruption in the beta adrenergic receptor remains a mystery. The receptor mechanism involves at least two second messengers to initiate vascular relaxation. Initial activation of the beta adrenergic receptor with a beta selective drug starts a cascade of events involving adenylylate cyclase and cyclic adenylylate monophosphate (cAMP) and nitric oxide. A disruption in the receptor mechanism, as a result of the parenteral administration of Pasteurella haemolytica, which is "upstream" of adenylyl cyclase, would result in a diminished amount of cAMP when compared to the unvaccinated negative controls. An investigation of cAMP accumulation, at the receptor level was inconclusive. The assessment of some previously used vaccines has demonstrated that there is an, as yet unidentified virulence factor, associated with these vaccines that results in the pharmacological disruption of beta adrenergic receptor mechanisms. Two newer vaccines, Once PMH® and One Shot® have been evaluated and there is evidence to suggest that these currently used vaccines also have the ability to disrupt beta adrenergic receptor mechanisms in rats. The effects of parenteral P. haemolytica on the alpha-2 adrenergic receptor mechanism, is described. The alpha-2 receptor mechanism, unlike the beta receptor mechanism appears to increase the amount of vasoconstriction. The possibility that the alpha-2 adrenergic receptor could also mediate vasorelaxation under certain conditions was investigated. The evidence suggests that in the presence of high alpha-1 mediated vascular tone, the alpha -2 receptor can cause vasorelaxation. Evidence, from other scientists active in this area of investigation, suggests that a vasorelaxation response may be mediated by nitric oxide. Elimination of the nitric oxide mediated relaxation may offer an explanation for the increased vasoconstriction noted with alpha-2 selective drugs after exposure to parenteral P. haemolytica. Finally, the importance of the beta adrenergic receptor to the disease process is addressed by elucidation of one of the mechanisms by which Micotil 300® (tilmicosin phosphate) acts to improve cattle with symptomatic pneumonic pasteurellosis. The rapid improvement of animals on Micotil 300®, with-in 24 hours suggests that there is a mechanism beyond the antimicrobial effect of the drug that mediates the clinical improvement. Evaluation of the effect of Micotil 300® demonstrates a pharmacologically measurable amount of beta adrenergic activity with respect to the bovine pulmonary artery and vein. Based on the conclusions drawn as a result of these experiments, the adrenergic system in general, and the beta adrenergic system in particular are important to the development of pneumonic pasteurellosis in cattle. The beta adrenergic system is affected by endotoxin. Further, these receptors maybe responsible for the mediation of the pathological and clinical signs associated with pneumonic pasteurellosis. In conclusion, these investigations have suggested, that it is likely that a disruption in the homeostatic mechanisms mediated by the beta and alpha-2 adrenergic receptors are intimately involved in the development of post vaccination receptor failure as well as the pathophysiology associated with pneumonic pasteurellosis in cattle. / Ph. D.

Pasteurella multocida is an important multi-host animal and zoonotic pathogen that is capable of causing respiratory and multi-systemic diseases, bacteremia, and infections resulting from bite wounds. The glycosaminoglycan capsule (CPS) of P. multocida is an essential virulence factor, protecting the bacterium from host defenses. However, chronic infections such as bovine respiratory disease (BRD) and avian cholera may be associated with biofilm formation. Biofilm formation was inversely related to capsule production (determined by uronic acid and N-acetylglucosamine assays), and was confirmed with capsule-deficient mutants of mucoid strains. Capsule-deficient mutants formed biofilms with a larger biomass that was much thicker and smoother than encapsulated strains. Gas chromatography-mass spectrometry, nuclear magnetic resonance, and enzymatic digestion demonstrated that the matrix material of the biofilm was composed predominately of a glycogen exopolysaccharide (EPS). Therefore, CPS may interfere with biofilm formation by blocking adherence to a surface or by preventing the EPS matrix to encase large numbers of bacterial cells. Chemical mutagenesis was performed on P. multocida strain P1059, resulting in isolation of an acapsular mutant designated as P1059-R8. A uridyltransferase encoded by gene P1059_01979 was mutated in such a way that a polar amino acid was changed to a non-polar amino acid near the active site. The protein product of P1059_01979 is important for the biosynthesis of the CPS subunit N-acetylglucosamine. CPS quantification revealed that the subunit glucuronic acid was produced in equal concentrations to the parent, but the CPS subunit N-acetylglucosamine was not detected in the chemical mutant. Biofilm formation in the chemical mutant was significantly higher than in WT P1059 and the capsule-deficient mutant. We hypothesize that P1059_01979 is essential for CPS production in P. multocida serogroup A. Histophilus somni and Pasteurella multocida cause bovine respiratory disease (BRD) and systemic infections in cattle. Following respiratory infection of calves with H. somni, P. multocida is also often isolated from the lower respiratory tract. Because H. somni normally forms a biofilm during BRD, we suspected that P. multocida may co-exist with H. somni in a polymicrobial biofilm. Interactions between the two species in the biofilm were characterized and quantified by fluorescence in situ hybridization (FISH), and the biofilm matrix of each species examined by fluorescently-tagged lectins (FTL), confocal scanning laser microscopy of in vitro biofilms and bovine pulmonary tissue following dual H. somni and P. multocida infection. FISH and FTL were used to show that P. multocida and H. somni were evenly distributed in the in vitro biofilm, and both species contributed to the polymicrobial biofilm matrix. COMSTAT z-stack image analysis revealed that the average biomass and biofilm thickness of the individual and polymicrobial biofilms were greatest when both species were present. Encapsulated P. multocida isolates not capable of forming a biofilm still formed a polymicrobial biofilm with H. somni, but only the EPS of H. somni could be detected by FTL staining of bovine tissues from which both species were isolated. Bacteria within a biofilm are more quiescent than during planktonic growth and induce less of an inflammatory response, indicating encapsulated P. multocida may take advantage of the H. somni biofilm to persist in the host during less severe, but more chronic, BRD. These results may have important implications for the management of BRD. Acute avian cholera is associated with encapsulated P. multocida, while chronic and asymptomatic cases of avian cholera are associated with acapsular P. multocida isolates. We hypothesize that biofilm formation is present and an important factor for chronic and asymptomatic avian cholera. Experimental infections of chickens with biofilm deficient P. multocida strain WT X73, proficient biofilm forming P. multocida strain X73ΔhyaD, and proficient biofilm forming clinical isolates 775 and 756 showed that virulence inversely correlated with biofilm formation. Histopathological analysis showed that biofilm forming isolates induced little inflammation in the lungs, heart, and liver, while biofilm deficient isolates induced greater inflammation. Biofilm material was located in pulmonary tissues of chickens diagnosed with chronic avian cholera using FTL staining.. Quantitative real-time PCR for expression of cytokine genes in the spleens of infected chickens indicated that P. multocida induced Th1 and Th17 immune responses during acute and chronic avian cholera. Chickens that succumbed to acute avian cholera after experimental challenge with WT X73 had high levels of INF-ƴ, IL-1β, IL-6, IL-12A, IL-22, IL-17A, and IL-17RA expression in the spleen compared to all other experimental groups. Antibody titers were low, indicating that antibodies may be less important in managing and clearing P. multocida infections. / Ph. D.

Doctor of Philosophy / Department of Diagnostic Medicine/Pathobiology / David G. Renter / Bradley J. White / The objective of my research was to quantify epidemiologic parameters associated with feedlot mortality and bovine respiratory disease complex (BRDC); the most significant cause of morbidity in U.S. feeder cattle. We conducted four retrospective studies utilizing individual health and cumulative cohort-level feedlot data. We developed a database that represented 33 U.S. feedlots from six states over ten years. Each project used a subset of these data. We found that the timing of BRDC was associated with important performance and health indices. In the first study, we evaluated the effect of the timing of individual BRDC treatments on standardized net returns. We found important performance and health measures (e.g. hot carcass weight and number treatments) driving net return differences associated with the timing of BRDC. For the second cohort-level study, we classified temporal patterns of BRDC, and evaluated associations among temporal patterns and performance and health. Temporal patterns were significantly associated with mean daily weight gain, days on feed, carcass weight, yield grade, quality grade, cumulative mortality, and retreatment risk. We also evaluated combined mortality and culling risks and quantified the effects of risk factors using count models. All risk factors (arrival weight, gender, and arrival month) were significant and the effects were modified by one another; effects of these covariate patterns have been impossible to quantify in smaller studies. Finally, we assessed the ability of regression models to predict cumulative BRDC morbidity based on arrival risk factors; then assessed the additional value of incorporating daily BRDC morbidity and mortality information. The percent of correctly classified cohorts did increase across days, but the effect of day was modified by weight, month, and feedlot. Information on daily morbidity was beneficial in predicting cumulative morbidity, but daily mortality provided little benefit. Our database containing animal health and cohort-level data allowed us to generate novel information on the effects of the timing of BRDC in feedlot populations. We also demonstrated effects of covariate patterns on adverse health outcomes that heretofore had been difficult to quantify. Finally, we showed that a predictive model for BRDC may be useful for the feedlot industry; this model should be further developed with future research.

Doctor of Philosophy / Department of Diagnostic Medicine/Pathobiology / David G. Renter / Bradley J. White / The objective of my research was to generate novel information concerning the epidemiology, diagnosis and prevention of bovine respiratory disease complex (BRDC), a common pre-weaning and post-weaning beef calf disease. To reach my objective, I conducted three prospective field trials within post-weaned calf populations, and one retrospective study of pre-weaned calves utilizing survey data. I evaluated differences in behavior, health and performance in calves receiving multiple component health programs. Calves in a minimally invasive program, which included primarily non-injectable products, displayed less aversion to initial product administration but experienced higher BRDC morbidity (P = 0.02) and poorer performance (P = 0.04) compared to calves in a more invasive (all injectable products) program. Secondly, in a study of Mannheimia haemolytica inoculated calves, I found that no parameter included in physical examinations, or common blood component evaluations could discern health from disease. However, disease recognition was aided by the measurement of the number of steps taken by a calf in a 24 hour period. None of the parameters that were evaluated predicted the severity of lung pathology. Thirdly, I conducted a study in post-weaned feeder calves that determined prevalence estimates for Mollicutes in general, and Mycoplasma bovis specifically, and their respective associations with health and performance. Nasal Mollicutes prevalence was high on arrival, and differences in calf performance were associated with (P < 0.01) nasal prevalence. More than half of the calves seroconverted to M. bovis; calves not seroconverting gained more weight (0.49 kg/head/day) during the study than those calves that did seroconvert (0.35 kg/head/day). Finally, I conducted a retrospective analysis of national U. S. cow-calf survey data to identify herd level management practices associated with pre-weaned calf BRDC. I found feeding antibiotics to pre-weaned calves, importing cattle, the number of outside visitors, economic purpose of the cow-calf operation, and breeding management of the herd were associated with herd-level pre-weaning BRDC rates. My research projects generated unique information concerning the epidemiology of important pathogens, differences among preventive health programs, objective BRDC diagnostic parameters, and pre-weaning BRDC risk factors. These research studies reinforce the complexity of BRDC and demonstrate the pathogen, animal and management factors affecting BRDC risk in pre- and post-weaned beef calves.