Higher biosecurity level was associated with reduced risk of Danish dairy cattle farms becoming test-positive for Salmonella Dublin in a nested case-control study

Lars Pedersen^1,2*Hans Houe²Erik Rattenborg¹Liza Rosenbaum Nielsen²

• ¹Animal Health and Welfare, Cattle Livestock, SEGES Innovation P/S, Aarhus, Denmark
• ²Department of Veterinary and Animal Sciences, Section for Animal Health and Welfare, University of Copenhagen, Frederiksberg, Denmark

Salmonella Dublin is a cattle-adapted bacterium with enzootic occurrence in cattle populations of many countries. Preventing spread of S. Dublin between cattle farms requires understanding of the local pathways for direct and indirect transmission of the bacteria. It is complicated to identify key risk factors because of the numerous pathways through which the bacteria can be introduced and established on dairy cattle farms. The objective of this study was to provide new knowledge about the effect of biosecurity in dairy farms located in S. Dublin-enzootic areas of Denmark. The association between the researcher-assessed biosecurity level and the risk of introduction and establishment of S. Dublin in farms was investigated by following a monthly recalculated cohort of dairy farms with no test-positive S. Dublin surveillance results over the previous two years. Cases were 37 new test-positive farms that were matched by herd size with 74 control farms that remained test negative in the mandatory S. Dublin surveillance programme. A published Biosecurity Assessment Framework, BAF-SD, was used to systematically and semi-quantitatively assess the on-farm biosecurity practices in 12 farm sections. Each section was scored on a scale from 0 (total lack of biosecurity measures) to 100 (excellent biosecurity) based on observations and interview. Lower biosecurity scores in the sections 'Entrance area', 'Pick-updelivery of calves', 'Calves<130 days', 'Cattle>130 days' and 'Storage of feed and feeding' were associated with becoming S. Dublin test-positive at a 90% confidence interval level in univariable logistic analyses. In the multivariable analysis, a higher weighted biosecurity score across all sections was found associated (p<0.05) with lower odds of becoming S. Dublin test positive (odds ratio=0.64 per 10-unit increase in biosecurity level). None of the study farms had very good or excellent biosecurity (score of 80 or above) highlighting the opportunities for biosecurity improvements onfarm. In conclusion, the current biosecurity levels in Danish farms appear to be insufficient to resist the presenting infection pressure of S. Dublin from the farm surroundings. Hence, either the biosecurity practices need to be improved, and/or the infection pressure needs to be reduced, to lower the number of new test-positive dairy cattle farms in Denmark.