Statistical analyses:
To evaluate the strength and direction of the associations of the horse and stud farm management risk factors with the seropositivity to BoDV by IFAT in the group of 235 SP breeding horses sampled between 2011 and 2013, a multivariable logistic regression model was carried out, after checking the independence of the stud farm factor. Odds ratios (ORs) with their corresponding 95% confidence intervals (CIs) were used to measure the associations. All analyses were performed with the software Small STATA version 12.0.
Possible risk factors were included as individual variables. The variables tested (n= 24) with their frequencies are listed in the Supplementary Data. All variables showing a P value < 0.25 in the univariable screening analysis of variables controlling for important confounders (age) were selected for inclusion in a multivariable model, which included those confounders. The multivariable model was built using a stepwise backward variable selection procedure in which variables with a P < 0.05 were retained in the model, or where their removal resulted in a significant change in the effect of other variables indicating confounding. Collinearity between variables was checked and excluded. To improve the goodness of fit, biologically plausible first-order interactions were determined between significant and confounders variables. The possible multivariable significant models were compared to each other by the Akaike Information Criterion (AIC) and the Bayesian Information Criterion (BIC) (Dohoo et al., 2003). The Hosmer-Lemeshow goodness-of-fit test (Hosmer, 2000) was used to evaluate the fitting of the proposed model and the area under the receiver operating characteristic (ROC) curve was also calculated.