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Diseases of Aquatic Organisms

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DAO 40:163-176 (2000)  -  doi:10.3354/dao040163

Genetic diversity and epidemiology of infectious hematopoietic necrosis virus in Alaska

E. J. Emmenegger1,*, T. R. Meyers2, T. O. Burton3, G. Kurath1

1Western Fisheries Research Center, Biological Resources Division, U.S. Geological Survey, 6505 NE 65th St. Seattle, Washington 98115, USA
2Juneau Fish Pathology Laboratory, Alaska Department of Fish and Game, PO Box 25526, Juneau, Alaska 99802, USA
3Anchorage Fish Pathology Laboratory, Alaska Department of Fish and Game, 333 Raspberry Road, Anchorage, Alaska 99518, USA

ABSTRACT: Forty-two infectious hematopoietic necrosis virus (IHNV) isolates from Alaska were analyzed using the ribonuclease protection assay (RPA) and nucleotide sequencing. RPA analyses, utilizing 4 probes, N5, N3 (N gene), GF (G gene), and NV (NV gene), determined that the haplotypes of all 3 genes demonstrated a consistent spatial pattern. Virus isolates belonging to the most common haplotype groups were distributed throughout Alaska, whereas isolates in small haplotype groups were obtained from only 1 site (hatchery, lake, etc.). The temporal pattern of the GF haplotypes suggested a Œgenetic acclimation¹ of the G gene, possibly due to positive selection on the glycoprotein. A pairwise comparison of the sequence data determined that the maximum nucleotide diversity of the isolates was 2.75% (10 mismatches) for the NV gene, and 1.99% (6 mismatches) for a 301 base pair region of the G gene, indicating that the genetic diversity of IHNV within Alaska is notably lower than in the more southern portions of the IHNV North American range. Phylogenetic analysis of representative Alaskan sequences and sequences of 12 previously characterized IHNV strains from Washington, Oregon, Idaho, California (USA) and British Columbia (Canada) distinguished the isolates into clusters that correlated with geographic origin and indicated that the Alaskan and British Columbia isolates may have a common viral ancestral lineage. Comparisons of multiple isolates from the same site provided epidemiological insights into viral transmission patterns and indicated that viral evolution, viral introduction, and genetic stasis were the mechanisms involved with IHN virus population dynamics in Alaska. The examples of genetic stasis and the overall low sequence heterogeneity of the Alaskan isolates suggested that they are evolutionarily constrained. This study establishes a baseline of genetic fingerprint patterns and sequence groups representing the genetic diversity of Alaskan IHNV isolates. This information could be used to determine the source of an IHN outbreak and to facilitate decisions in fisheries management of Alaskan salmonid stocks.

KEY WORDS: IHNV · Salmon · Alaska · Diversity · Epidemiology · Phylogeny

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