Understanding the diagnosis and risk factors for respiratory disease in dairy calves
The purpose of the work presented in this thesis was to evaluate the use of portable ultrasonography (US) for the diagnosis of respiratory disease in dairy calves. In addition to testing diagnostic accuracy of US, the efficacy of an intranasal vaccine against viral pathogens and the short-term effects of respiratory disease on calf behavior were evaluated. In calves affected with subclinical respiratory disease, the sensitivity and specificity of US in diagnosing lung lesions was 94% (95% CI: 69 100%) and 100% (95% CI: 64 100%), respectively; and the presence of US lung lesions predicted a neutrophil proportion ≥ 4% in bronchoalveolar lavage fluid (OR = 23; 95% CI: 2.6 – 198; P < 0.01). Ultrasonographic lesions were highly correlated to post-mortem lesions (r = 0.92; P < 0.01). After experimental bacterial infection, lesions associated with bronchopneumonia developed rapidly, and progressed to maximum size over 48 hours, after which the lesions remained stable for several days. The odds of observing lung lesions were lower in calves that were vaccinated within the 1st week of life and 6 weeks of age with an intranasal viral respiratory vaccine as compared to calves subjected to a positive or negative control vaccination protocol. Intranasal vaccine was associated with average daily gain within the 1st 8 weeks of life, although this relationship was farm dependent. The presence of lesions as detected by ultrasonography had no effect on lying behavior of young dairy calves. The portable US machine carried by bovine practitioners provided a practical means to accurately assess lung lesions in dairy calves. This tool will help practitioners assist producers in making well-informed treatment and management decisions, hopefully serving to improve both calf health and welfare. Additionally, from a research perspective, this equipment will reduce the number of calves used to study respiratory disease and allow for new outcomes to be measured including the onset, duration, and resolution of lung lesions as we study the impact they have on future performance.