This work pointed out new probable synergies between feline viruses that can now be further investigated in laboratory conditions. However, the associations could also result from the existence of an unknown confounding factor common to FHV, FCV and FPV. The feline parvovirus is immunosuppressive, as a result of the strong leukopenia occurring within the two weeks post-infection [95, 96] . This virus could therefore be a confounding factor to the FHV-FCV pair if FPV-seropositive cats are more susceptible to FHV and FCV at the same time. However, as shown in this paper, the FHV-FCV interaction remained significant after correction by FPV (Table 2) .