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Journal of Virology, November 2008, p. 10359-10365, Vol. 82, No. 21
0022-538X/08/$08.00+0     doi:10.1128/JVI.01009-08
Copyright © 2008, American Society for Microbiology. All Rights Reserved.

Differences in Virulence of Marine and Freshwater Isolates of Viral Hemorrhagic Septicemia Virus In Vivo Correlate with In Vitro Ability To Infect Gill Epithelial Cells and Macrophages of Rainbow Trout (Oncorhynchus mykiss){triangledown}

Bjørn E. Brudeseth,1 Helle F. Skall,2 and Øystein Evensen3*

PHARMAQ AS, P.O. Box 267, N-0213 Oslo, Norway,1 Technical University of Denmark, National Veterinary Institute, Hangøvej 2, DK-8200 Aarhus N, Denmark,2 Norwegian School of Veterinary Science, P.O. Box 8146 Dep, N-0033 Oslo, Norway3

Received 14 May 2008/ Accepted 19 August 2008

Two strains of viral hemorrhagic septicemia virus (VHSV) with known different virulence characteristics in vivo were studied (by a time course approach) for their abilities to infect and translocate across a primary culture of gill epithelial cells (GEC) of rainbow trout (RBT; Oncorhynchus mykiss). The strains included one low-virulence marine VHSV (ma-VHSV) strain, ma-1p8, and a highly pathogenic freshwater VHSV (fw-VHSV) strain, fw-DK-3592B. Infectivities toward trout head kidney macrophages were also studied (by a time course method), and differences in in vivo virulence were reconfirmed, the aim being to determine any correlation between in vivo virulence and in vitro infectivity. The in vitro studies showed that the fw-VHSV isolate infected and caused a cytotoxic effect in monolayers of GEC (demonstrating virulence) at an early time point (2 h postinoculation) and that the same virus strain had translocated over a confluent, polarized GEC layer by 2 h postinoculation. The marine isolate did not infect monolayers of GEC, and delayed translocation across polarized GEC was seen by 48 h postinoculation. Primary cultures of head kidney macrophages were also infected with fw-VHSV, with a maximum of 9.5% virus-positive cells by 3 days postinfection, while for the ma-VHSV strain, only 0.5% of the macrophages were positive after 3 days of culture. In vivo studies showed that the fw-VHSV strain was highly virulent for RBT fry and caused high mortality, with classical features of viral hemorrhagic septicemia. The ma-VHSV showed a very low level of virulence (only one pool of samples from the dead fish was VHSV positive). This study has shown that the differences in virulence between marine and freshwater strains of VHSV following the in vivo infection of RBT correlate with in vitro abilities to infect primary cultures of GEC and head kidney macrophages of the same species.

* Corresponding author. Mailing address: Norwegian School of Veterinary Science, P.O. Box 8146 Dep, N-0033 Oslo, Norway. Phone: 4722964500. Fax: 4722597310. E-mail: oystein.evensen{at}veths.no

{triangledown} Published ahead of print on 27 August 2008.

Journal of Virology, November 2008, p. 10359-10365, Vol. 82, No. 21
0022-538X/08/$08.00+0     doi:10.1128/JVI.01009-08
Copyright © 2008, American Society for Microbiology. All Rights Reserved.



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