Pairwise interactions between FHV, FCV and FPV could have come from the fact that one virus was a common risk factor for the two others. This possibility was tested (see the three last lines of Table 2 ) by adding the serological status to one virus as a common risk factor for the two others. Results led to reject this hypothesis, meaning that the observed associations cannot be solely explained by the fact that one virus interacts with the two others.