Beaudeau et al. Acta Veterinaria Scandinavica 2010, 52:33 http://www.actavetscand.com/content/52/1/33 Page 4 of 7 Figure 1 Prevalence of BCV in Swedish beef cattle by three-digit postal code area (2007). The estimates presented are (a) crude or (b) adjusted by empirical Bayes smoothing applying a spatial weight matrix. Information was missing for the white areas. © Lantmäteriverket Gävle 2010. Permission number I 2010/0055. lowest prevalence for BRSV has a very low beef herd den- sity (2 per 100 km2). This suggests a positive association between herd-density and risk of infection. Statistical analysis confirmed that areas with herd-density >10 per 100 km2 had significantly higher risk of being part of high-prevalence clusters. A probable explanation is that a short distance between herds increases the risk of spread of the viruses. In such situations, there is a higher likeli- hood of direct and indirect contact (through e.g. animals, vehicles or visitors) between herds and animals. However, some PCAs in the low-prevalence cluster Kronoberg-Ble- kinge also had a beef herd-density >10 per 100 km2, sug- gesting that there are other factors with uneven geographic distribution that have an impact on the BCV and BRSV prevalences. Live animal trade, in particular, is considered very important for the spread of infectious diseases and a recent study show that the number of movements and trade patterns in different parts of Swe- den vary considerably [28]. Large herd size has also been identified as risk factor for BCV and BRSV infections in dairy cattle [13,27] and for respiratory disease outbreaks in beef cattle [29]: on increasing the herd size from 20 to 50 animals, the risk for disease outbreak increased 2.1-fold. The size of Swed- ish beef herds differs between regions and there is a ten- dency for smaller herd sizes in the regions covering the identified low prevalence areas compared to the high prevalence areas (10 beef adults per herd in Kronoberg- Blekinge versus 16 in Skåne, based on information from the database of the Swedish Board of Agriculture). Also regional differences in biosecurity and management rou- tines in relation to farming styles can be assumed. Based on a study on Swedish dairy farms, it has been suggested that organic herds may have a reduced risk of BCV and