Beaudeau et al. Acta Veterinaria Scandinavica 2010, 52:33 http://www.actavetscand.com/content/52/1/33 more blood samples (19% of the studied herds) were col- lected are not evenly geographically-distributed. To investigate this potential bias in relation to the sampling strategy, possible clustering of herds with more than one sampled animal was explored in a preliminary step by using Moran's I. This test indicated that these herds were proportionally distributed over Sweden (data not shown). To test whether or not a high beef herd-density was a risk factor for significantly higher seroprevalences of BCV and BRSV than expected, univariate logistic regres- sion analysis was performed at PCA level, where the binary outcome was "PCA with a significantly higher number of positive-tested samples vs. without" and the putative explanatory variable was the PCA herd-density (in herds/100 km2) in 3 classes [<5; 5-10; >10]. Data management, statistics and creation of map shape-files were performed using SAS 9.2 (SAS Institute, Inc., Cary, NC, USA) and ArcGIS 9.1 (ESRI Inc., Red- lands, CA, USA). Results The overall prevalence of animals testing positive to BCV and BRSV was 43.1 (95% CI: 41.3-45.0) and 39.2% (95% CI: 37.3-41.0) respectively. There was a statistically signif- icant (P < 0.01, χ2-test) relationship between BCV and BRSV serological status, i.e. BCV-positive animals were more likely to be BRSV-positive and vice versa. Animals testing positive to BCV were predominately located in the central-western and southern Sweden, as well as in some northern areas (Figure 1a). Animals testing positive to BRSV were predominately found in the same central- western and southern parts of the country (Figure 2a). When the prevalences were adjusted by SEB, these ten- dencies became even clearer (Figures 1b and 2b) and the northern areas were no longer considered having high prevalences for BCV. The findings were confirmed by significant Moran's I tests for both infections (0.15, P = 0.0001 for BCV; 0.16, P = 0.001 for BRSV), suggesting that the test positive ani- mals were not randomly distributed throughout the country. Using the spatial scan statistic with elliptic clusters and a maximum cluster size of 10% of the population at risk identified two areas with higher prevalence of BCV than expected: Skaraborg (central-south part of Sweden) and Skåne (extreme south), as well as one area with lower prevalence than expected: Kronoberg-Blekinge (south- east of Sweden; Figure 3a). Almost the same high preva- lence areas (Skaraborg and Skåne) were identified as being clusters for BRSV. Two areas with low prevalence of BRSV were also detected: Kronoberg-Blekinge and Page 3 of 7 Värmland-Dalarna (middle-west) (Figure 3b). The char- acteristics of the identified clusters are displayed in Table 1. The beef-herd density per PCA is presented in Figure 4. Among the 34 PCAs included in the clusters of high BCV prevalence, 20 had a beef herd-density >10 herds/100 km2. This proportion was 14 out of 22 for BRSV. For BCV, the risk for a PCA to be part of a cluster of high prevalence was 5.3 times (95% CI: 2.5-11.1) higher if its beef herd-density was >10 herds/100 km2 than if it was <10 herds/100 km2; this risk was 6.1 times (95% CI: 2.4- 15.1) higher for BRSV. Discussion This study showed, from visual inspection and descrip- tive analyses, that both BCV and BRSV infections are very frequent in Swedish beef cattle, especially in some central-western and southern parts of the country. The results are in accordance with what has been reported for dairy cattle where prevalences of BCV- and BRSV- infected dairy herds have been shown to increase with a gradient southward in the country [9,13,25]. A recent study has also shown substantial production effects (e.g. reduced milk yield) associated with BRSV infection in Swedish dairy herds [26]. There was a strong relationship at animal level between being tested positive to BCV and BRSV, and exploratory spatial analyses also indicated two areas with particularly high prevalences for both BCV and BRSV, i.e. Skaraborg and Skåne. The association between BCV and BRSV is biologically plausible because both viruses are relatively contagious and have transmission routes that are to some extent similar. A concomitant BRSV and BCV infection burden and spread in calves has also been demonstrated [10,27]. For both infections, the prevalence of animals testing positive was approximately 40%. However these apparent prevalences at individual level are probably underesti- mated. Because a few individuals were sampled from each herd (one or two in 95% of herds in the study sam- ple) the herd sensitivity was here low. As a consequence, it can be assumed that a proportion of infected herds and of infected individuals within these herds could have been missed (as both viruses are highly contagious within herd), thus decreasing the apparent individual-level prev- alences. This study was not predominately designed to estimate the seroprevalences of BCV and BRSV infec- tions in Sweden, but to explore their spatial distribution. Both high prevalence areas have a relatively high den- sity of beef herds (13 and 22 per 100 km2 in Skaraborg and Skåne, respectively). In addition, the area with the