Canine parvovirus type 2 (CPV-2) and feline panleukopenia virus (FPV) are single-stranded DNA viruses belonging to the genus Parvovirus of the Parvoviridae family, that are responsible for hemorrhagic gastroenteritis and leukopenia, mainly in pups and kittens, with high mortality rates. CPV-2, emerged in the late 1970s, appeared to be a host-range variant of FPV, since is evolved from FPV or other FPV-like viruses infecting wild carnivores, such as mink, raccoons or foxes. After its emergence, the original CPV-2 continued to evolve and was completely replaced by the new variants CPV-2a, 2b and 2c that have gained the feline host range and they are able to infect both dogs and cats whereas the original CPV-2 does not replicate in cats. The recent emergence of CPV-2 represents one of the few examples where the process of cross-species viral transmission has been observed in real-time. Furthermore, the successive species jump of CPV-new variants from dogs to cats, constitutes a valuable model of a successful cross-species transmission event. Since the process of virus transfer to a new host (viral host switching) is rarely observed directly, we have investigated the evolution of CPV in domestic cats. In the study described here, we report the detection of co-infection by multiple CPV variants, the high genetic complexity of CPV-2 strains and a mixed infection FPV/CPV, detected in domestic cats. These findings display the higher within-host genetic diversity of CPV in cats than in dogs, confirming the importance of the viral host switching as mechanism for the emergence of new viruses.
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