CDV is an enveloped virus with a single-stranded, negativesense RNA genome, 15,690 nucleotides long. The genomic RNA is enclosed by a helical nucleocapsid composed of the nucleocapsid (N) protein and is replicated by the viral polymerase (L) protein and its cofactor phosphoprotein (P). The P gene also encodes two nonstructural proteins (C and V). The N, P, and L proteins together with the viral RNA constitute the ribonucleoprotein (RNP) complex, which directs the synthesis of capped and polyadenylated mRNAs.4 The viral envelope contains two glycoproteins, the fusion (F) and hemagglutinin (H) proteins, and a membrane-a ssociated protein (matrix [M]), which links the envelope with the RNP. The H protein facilitates viral attachment to receptors on the host cell and is therefore the major determinant of viral tropism.4 Neutralizing antibodies produced against

H gene epitopes are important for protective immunity by preventing viral attachment to host cells. The F protein executes the fusion of the host cell plasma membrane and the viral envelope, resulting in entry of the viral RNP into the cytoplasm. The F protein also promotes fusion between host cell membranes4,5; however, experiments with recombinant viruses suggest that it is the H protein, not the F protein, that is actually the main determinant of fusion efciency and, therefore, the cytopathogenicity determinant at least in vitro.4