Selective regimen shift and demographic growth increase associated with the emergence of high-fitness variants of canine parvovirus.

The natural evolution of Canine parvovirus (CPV) is characterized by a variety of mutations, mainly in the VP1/VP2 gene. Although positive selection has been previously reported in CPV, little is known about its overall contribution to viral adaptation in the canine population. Herein, the influences of the evolutive constraints on CPV during a period of viral adaptation into a previously uninfected population are more clearly investigated. To do this, 31 sequences of VP1/VP2 gene obtained from symptomatic domestic dogs in Brazil were used, sampled from 1980 to 2000. Marked evolutionary changes in CPV associated with a process of fine-tuning adaptation were observed. Specifically, sequences from the 1980s revealed two distinct antigenic types (i.e. 2a and 2b) cocirculating in Brazil. Moreover, analysis of the selective regimen showed that 90% of the VP2 sites were conserved (d(N)/d(S)=0). In contrast, sequences from the 1990s were composed solely of CPV-2a with 96% of VP2 sites under purifying selection (d(N)/d(S)<1) and site 297 under strong positive selection (omega=4.9). Important features regarding the demographic history of CPV in Brazil were also observed. The viral population size passed through a short period of explosive growth that declined and then stabilized into a constant rate of spread. Remarkably, the explosive growth coincided with the appearance of CPV variants that presented a unique repertoire of mutations never before seen in other worldwide strains. The analysis also showed that the estimated nucleotide substitution was similar to those commonly observed in fast evolving RNA viruses. The present results demonstrated the adaptive potential of CPV to acquire, in short interval of 10 years, key mutations in the VP1/VP 2 gene that increased viral fitness and enabled the virus to disseminate even in vaccinated dogs.