DAO 61:23-32 (2004)  -  doi:10.3354/dao061023

Molecular characterization of Sp serotype strains of infectious pancreatic necrosis virus exhibiting differences in virulence

R. B. Shivappa1, H. Song1, K. Yao1,4, A. Aas-Eng2, Ø. Evensen3, V. N. Vakharia1,*

1Center for Biosystems Research, University of Maryland Biotechnology Institute and VA-MD Regional College of Veterinary Medicine, University of Maryland, College Park, Maryland 20742, USA
2Alpharma, Animal Health Division, PO Box 158, Skøyen, 0212 Oslo, Norway
3Department of Basic Sciences and Aquatic Medicine, Norwegian School of Veterinary Science, PO Box 8146 Department, 0033 Oslo, Norway
4Present address: Wyeth-Lederle Vaccine and Pediatrics, PO Box 304, Marietta, Pennsylvania 17547, USA
*Corresponding author. Email:

ABSTRACT: Infectious pancreatic necrosis virus (IPNV), a prototype virus of the family Birnaviridae, exhibits a high degree of antigenic variability, pathogenicity and virulence in salmonid species. The Genomic Segment A encodes all the structural (VP2 and VP3) and nonstructural (NS) proteins, whereas Segment B encodes the viral RNA-dependent RNA polymerase (VP1). We tested 3 different IPNV isolates (Sp103, Sp116 and Sp122) isolated during field outbreaks in Norway for their ability to cause mortality in fry and post-smolt of Atlantic salmon Salmo salar L. The cumulative mortality following experimental challenge in fry was 29% for Sp122 followed by 19% for Sp116 and 15% for Sp103. In post-smolt, the corresponding mortality rates were 79, 46 and 16%, respectively. Comparisons of the deduced amino acid sequences of Segments A and B of all 3 Sp strains revealed substitutions of residues in 13 positions, of which 6 are in VP2, 2 in VP3, and 5 in VP1. Our results suggest that these residues, especially those in the outer capsid VP2, may be involved in the virulence of IPNV. Genome Segment A of the Sp serotype is 3097 nucleotides long and contains a major open reading frame (ORF) encoding a polyprotein of 972 amino acids, which initiates at the second in-frame start codon at Position 119. This was ascertained by making mutants of Segment A clone using site-directed mutagenesis, followed by in vitro transcription-coupled translation reaction and immunoprecipitation analyses. In addition, Segment A also encodes a 15 kDa arginine-rich nonstructural protein from a small ORF, preceding and partially overlapping the polyprotein ORF, which is truncated to 12 kDa in the virulent Sp122 strain. Moreover, Segment A could encode a novel, putative 25 kDa protein from another ORF between VP2 and VP4 coding regions, which is only detected in the Sp serotype. Segment B is 2777 nucleotides long and encodes in a single large ORF (a polypeptide of 844 amino acid residues), VP1.

KEY WORDS: Infectious pancreatic necrosis virus · IPNV · Sp serotype · Virulence · Molecular characterization

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