Bovine Coronavirus and Bovine Respiratory Syncytial Virus Infections in Dairy Herds Prospects for Control

Bovine coronavirus (BCV) and bovine respiratory syncytial virus (BRSV) are significant causes of enteric and respiratory disease in cattle throughout the world. The aim of this thesis was to gain further knowledge of the epidemiology of these infections in Swedish dairy herds. Two studies were conducted of the association between BCV/BRSV antibody status and health and performance, on herd (79 herds) and individual animal level (65 herds). The analyses were carried out using survival, linear regression, and logistic regression models. Herds that were antibody-positive to BCV and/or BRSV had higher bulk tank milk (BTM) somatic cell count than herds negative to both BCV and BRSV. Cows in herds with recent BRSV infection had a lower milk yield than cows in BRSV-free herds, and cows in herds with clinical signs of BRSV infection had a higher somatic cell count than cows in herds without clinical signs. In the third study, the effect of herd-level risk factors on antibody status was quantified in 257 herds using logistic regression models and spatial analysis. Large herd size, being located in southern Sweden, and not providing boots for visitors were found to be associated with antibody positivity to BCV and BRSV, while short distance to nearest cattle herd was additionally associated with positivity to BCV. Providing boots for visitors likely reflects herds with a high level of biosecurity measures. Neither BCV-positive nor BRSV-positive herds were spatially clustered, indicating that local spread and airborne transmission are not of great importance. The fourth study investigated the long-term dynamics of BCV and BRSV infections in 20 herds from each of two southern and two northern areas. There was a high prevalence of BCV and BRSV amongst the herds in the southern areas, and a lower prevalence in the northern areas; one of the latter was free from BRSV at the end of the study. There was also self-clearance of BCV and BRSV, manifesting as conversion from antibody positive to negative in pooled milk samples and BTM. The results of this thesis suggest that it is possible to develop inexpensive strategies for controlling BCV and BSRV infections, based on biosecurity and antibody monitoring. Such a control strategy would improve cattle and calf health.