Nonetheless, questions remain on certain aspects of parvovirus evolution. Although virus-host co-evolution had been proposed as an important mechanism in the emergence of new animal parvoviruses [22] , this hypothesis cannot be easily reconciled with the high evolutionary rates observed in diverse parvoviruses, as the evolutionary timescale of the genetic distance between the animal hosts would be several orders of magnitude greater than that of the genetic distance between the corresponding parvoviruses. While the effects of negative selection on parvoviruses highly adapted to their animal host may slow further evolution of the virus, it is unlike to account for the discrepancy between host and virus evolution to a significant degree. In comparison, observations on the emergence of canine parvovirus from feline panleukopenia parvovirus [45] and the experimental cross-species infection of rodent parvoviruses provide more support to the role of crossspecies transmission in the evolution of parvoviruses. This would also be more consistent with the degree of recombination seen in diverse parvoviruses [46] , which would have required frequent cross-species transmission and co-infection of different parvoviruses in the same host [47] . The present detection of these partetraviruses in organs of otherwise healthy animals suggests the possibility of viral persistence, although comprehensive sampling of other sites such as the respiratory tract would be needed to exclude acute infections. Relatively little is known about persistence of partetraviruses in animal hosts, although the phenomenon is well-described for other parvoviruses. Genomic DNA of human erythrovirus genotype 1 has been found to persist in the synovial membranes of both patients with chronic arthropathy and healthy individuals [48] . A larger study has additionally uncovered information on the distribution and persistence among different genotypes of human erythrovirus, which revealed the epidemiological history of these genotypes in the human population [49] . The exact mechanism for viral persistence has not been fully elucidated for many parvoviruses, though site-specific integration has been described for adeno-associated viruses [50] . Among the partetraviruses, the phenomenon of persistence is best studied in the human partetravirus genotype PARV4, which has been found in the blood, lymphoid tissue and bone marrow of HIV-infected patients [12, 18] . Results from interferon c enzyme-linked immunospot assays suggested that PARV4 persistence may be present at 26% of hepatitis C virus-positive individuals [51] .