Assessment of concurrent infection with bovine viral diarrhoea virus (BVDV) and Mycobacterium bovis: A herd-level risk factor analysis from Northern Ireland.

Bovine viral diarrhoea virus (BVDV) is a significant pathogen of cattle, leading to severe economic and animal-welfare impacts. Furthermore, the pathogen has been associated with impacting the progression or spread of other pathogens (e.g. Mycobacterium bovis, the causative agent of bovine tuberculosis (bTB)). During this study we investigated (i) risk factors for BVDV at a herd-level and (ii) whether there was any association between BVDV and herd-level bTB risk. The data for this study were gathered from a voluntary BVDV control programme in Northern Ireland (2013-2015) based on the identification of virus positive animals through tissue tag testing of calves. We assigned a herd-level BVDV status to 2827 participating herds, where a herd was assumed "infected" if one or more animals tested positive for BVDV. Two model suites were developed. Firstly, we assessed risk factors for BVDV herd status using multivariable logit random-effects modelling, aggregating to the calendar year level (2013-2015; n=4828; model 1). Secondly, we aggregated data across the three years of the study to give an overall status for the whole study period (n=2827; logistic model 2). Risk factors included year, herd-type, herd size, number of births, inward trade moves, calf mortality, and region. Furthermore, the herd-level bovine tuberculosis status (based on the single intradermal comparative cervical tuberculin (SICCT) test outcomes, or confirmation at post-mortem), or the size of bTB breakdowns (number of SICCT test positive animals), of herds was also investigated to assess whether there was an association (co-infection) with herd BVDV status. The final models suggested that BVDV herd status was positively associated with increased levels of calf mortality, herd size, number of births, the number of BVDV tests undertaken and the number of animals introduced to the herd. There was a significant univariable positive association between BVDV status, and SICCT breakdown risk, breakdown size and confirmed bTB status in model 2. However, there was no evidence of significant associations between bTB status (using SICTT status, confirmed status or herd breakdown size) and BVDV status in final multivariable models when controlling for other significant confounders. These results provide information for action for the future control and eradication of BVDV in Northern Ireland, though these data provide little support for the hypothesised association between BVDV and bTB status at herd-level. Further animal-level analyses are necessary to investigate whether there is support for a BVD-bTB co-infection association, including the impact of co-infection on the severity of infection.