On the contrary, the three other pairs (FHV-FCV, FHV-FPV and FCV-FPV) were significantly associated after correction by their known risk factors. It is to our knowledge the first evidence of a possible interaction between those viruses. As more double seropositive cats than expected under the independence hypothesis were observed, possible synergies are suggested. After an acute infection, FHV is known to persist life-long in a latent form, which can be reactivated in stressful conditions [73] . Infection with FPV or FCV could thus be responsible for the reactivation of FHV in latently infected animals, resulting in seroconversion against both FHV and the new infecting virus. This could explain the FHV-FCV and FHV-FPV associations. In addition, since FPV is more immunosuppressive than FCV, the interaction between FPV and FHV is expected to be stronger than that between FCV and FHV, which is consistent with our results. The immunosuppressive effect of FPV could also explain the association with FCV. In that case however, contrary to FHV, it would require that the FCVinfection occurs at the time of the immunosuppression occuring within the two weeks post-FPV infection. Interestingly, a similar association between FPV and FCV antibodies was described in free-ranging lions in East Africa [99] .