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

Le parasitisme est de plus en plus considéré comme un facteur écologique pouvant créer des variations dans le comportement des individus. Toutefois, la direction de causalité entre le comportement et le parasitisme reste incertaine. Les infections expérimentales sont le plus souvent réalisées en laboratoire, limitant les inférences écologiques. À l’aide d’une infection expérimentale semi-naturelle, nous avons infecté avec succès des crapets-soleils (Lepomis gibbosus) dans un lac où ils ont été exposés à une variété de vers parasites (trématodes, cestodes), permettant d’examiner les effets de la co-infection naturelle chez les hôtes. Nous avons mesuré la témérité, l’exploration et l’activité avant et après l’infection expérimentale. En utilisant une approche bayésienne, nous avons trouvé que les traits de comportement initiaux exercent une forte influence sur la susceptibilité à l’infection : les poissons les plus téméraires et/ou les moins actifs au départ ont acquis une plus grande densité de points noirs (c.-à-d. points noirs visibles sous la peau, les nageoires et dans les muscles du poisson) et de cestodes lors de l’infection. Par ailleurs, nous avons montré que la condition corporelle est réduite par la densité de cestodes, suggérant la débilitation de l’hôte. La condition corporelle était corrélée positivement avec la distance parcourue, quel que soit le statut d’infection individuel. Nous avons également trouvé une relation négative entre la distance parcourue après l’infection et la densité de trématodes, suggérant que l’infection causant la maladie des points noirs diminue le niveau d’activité des hôtes. La témérité et l'exploration n'étaient pas affectées par la densité parasitaire ou la condition corporelle. Nous suggérons que la diminution de l’activité est causée par une réponse adaptative de l’hôte, visant à rediriger son énergie pour combattre l’infection. Sachant que les points noirs ont un cycle de vie complexe et que le crapet-soleil est un hôte intermédiaire, ce changement dans le comportement de l’hôte pourrait aider le parasite à compléter sa transmission aux oiseaux-hôtes piscivores en augmentant la prédation sur les poissons infectés. Bien que nous ne puissions confirmer la direction de causalité, nos résultats montrent qu’il existe un lien étroit entre le comportement et le parasitisme. Nous suggérons que deux mécanismes peuvent simultanément agir : le comportement initial des individus influence le risque d’infection, et l’infection peut créer de la variation au niveau de la plasticité comportementale des individus. / Parasitism is increasingly seen as an ecological factor that can create variations in individual behaviour. However, the direction of causality between behaviour and parasitism remains uncertain. Experimental infections are most often conducted in laboratories, limiting ecological inferences. Using a semi-natural experimental infection, we successfully infected pumpkinseed sunfish (Lepomis gibbosus) in a lake where they were exposed to various parasitic worms (trematodes, cestodes), allowing us to examine the effects of natural co-infection in hosts. We measured boldness, exploration, and activity before and after the experimental infection. Using a Bayesian approach, we found that initial behavioural traits strongly influence infection susceptibility: initially bolder and/or less active fish acquired a higher density of black spots (i.e., visible black spots under the skin, fins, and in the fish's muscles) and cestodes during the infection. Additionally, we found that body condition was reduced by cestode density, suggesting host debilitation. Body condition was positively correlated with distance swam, regardless of individual infection status. We also found a negative relationship between distance swam after infection and trematode density, suggesting that infection causing black spot disease decreases host activity levels. Boldness and exploration were not affected by parasite density or body condition. We suggest that a decrease in activity is caused by an adaptive host response to redirect its energy to combat the infection. However, since trematode parasites have a complex life cycle and pumpkinseed sunfish are intermediate hosts, decreases in activity levels following infection may make infected fish more susceptible to predation by piscivorous birds, which is needed for trematodes to complete their life cycles. While we cannot confirm the direction of causality, our results show a close link between behaviour and parasitism. We propose that two mechanisms may simultaneously operate: the initial behaviour of individuals influences their risk of infection, and infection can create variation in behavioural plasticity of individuals.