Faecal shedding of a novel live recombinant canine parvovirus type 2 vaccine: new challenges in diagnostics?

Aim of the Study: Prevention of canine parvovirus type 2 (CPV-2) infection is based on the use of modified live virus (MLV) vaccines, which may be shed in canine faeces for several days after the administration, highlighting the need for a differential laboratory diagnostic strategy (1). Alongside historical MLV vaccine formulations, a recently licensed live recombinant vaccine expressing a capsid-coding insert derived from a currently circulating CPV-2c variant (2) has become available on the Italian market. To investigate potential diagnostic interference, a preliminary molecular characterization of this novel vaccine strain and its faecal shedding profile was conducted. Methods Used: Sequence analysis of the vaccinal CPV genome sequence was carried out using a conventional PCR followed by direct Sanger sequencing; sequences were assembled and analysed using BioEdit ver 7.2.5 software and with nBLAST, to identify related sequences in public domain databases. Faecal samples collected from puppies prior to vaccination and at days 3, 7, 14, and 21 post-vaccination with the CPV-2c vaccine were tested for CPV-2 DNA detection and sequencing. Genetic relationships between the CPV-2 vaccinal and detected field strains were assessed by analysing distinctive substitutions through sequence alignments. A phylogenetic tree based on partial VP2 gene sequences was generated using the maximumlikelihood method and selecting the general time reversible (GTR) substitution model with discrete Gamma distribution (1000 bootstrap replicates). Results and Conclusions: Sequence analysis identified characteristic residues (219Val, 386Lys) in the predicted VP2 protein sequence, shared with a limited number of field or likely vaccine-derived CPV-2 strains (3). Vaccine-derived parvovirus DNA was detected in faecal samples up to 21 days after initial vaccination. Phylogenetic analysis demonstrated a close genetic relationship between the recombinant vaccinal strain and currently circulating CPV-2c strains. These preliminary findings confirm faecal shedding of vaccine-derived viral DNA in healthy puppies and identify VP2 molecular markers associated with this novel live recombinant vaccine. The results highlight the need to refine the currently used diagnostic tools to reliably distinguish between vaccinal and field CPV-2 strains.