Virulence Comparison of Three Buhl-Subtype Isolates of Infectious Pancreatic Necrosis Virus in Brook Trout Fry

Abstract

Infectious pancreatic necrosis virus (IPNV) is an important aquatic pathogen that can cause high mortality in populations of young salmonids. To determine the molecular basis of virulence, the fry of brook trout Salvelinus fontinalis were experimentally infected with three different A 1 -serotype, Buhl-subtype isolates of IPNV. The three isolates were selected on the basis of results of previously completed virulence assays, which indicated that the isolates had substantially differing virulence levels. To confirm this, mortalities from each treatment were recorded for the duration of the experiment (62 d), along with observation of any clinical disease signs. Mortalities began on day 5 postexposure, peaked on day 7, and then rapidly decreased for all three isolates tested. Diseased fry exhibited whirling, ascites, abdominal hemorrhaging, and prostration on the bottom of the tank. Daily virus titers from live fish were determined for 10 d postexposure (dpe), as well as at 28 and 62 dpe. Viral titers were correlated with fish weight to determine statistical significance. Fish weight was found to correlate negatively to virus titer for the two least-virulent isolates. Initial sequencing results demonstrated sequence homology for the viral capsid protein VP3, whereas slight differences among isolates were observed for the viral capsid protein VP2. The VP2 region was sequenced for each isolate at three times-before being introduced into the fish, during the epizootic, and 2 months after exposure-to determine whether major changes existed in the VP2 region that might account for the differences in virulence. Data indicate that two amino acid differences in the VP2 region exist, at residues 217 and 286, distinguishing the least-virulent isolates and the most-virulent isolate. These amino acid differences might account for the disparity in expressed virulence.

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

“…This result is also in accordance with that of Sano et al (1992), who generated reassortants between virulent and avirulent strains of 2 different serotypes and demonstrated that the virulence of IPNV is associated with Segment A. Similarly, Bruslind & Reno (2000) found 2 amino acid differences in the VP2 at Positions 217 and 286 between the virulent and avirulent Buhl-subtype isolates of IPNV. VP2 is a major capsid protein and is responsible for the production of type-specific monoclonal antibodies (Nicholson 1993).…”

“…Since virulence of IPNV has been associated with Segment A (Sano et al 1992), there may be differences at the molecular level within Segment A that makes the closely related isolates of IPNV so varied in their virulence. Recently, Bruslind & Reno (2000) identified 2 amino acid differences within the VP2 region of a virulent and avirulent Buhl strain of IPNV that was used to infect brook trout fry. However, there are no reports regarding the virulence of Sp serotype of IPNV.…”

Aquabirnavirus polyploidy: a new strategy to modulate virulence?

“…At present, there are relatively few studies on the virulence factors of IPNV, and they are mainly focused on the connection of virulence to specific regions and residues of the genomic sequence (Sano et al, 1992;Bruslind & Reno, 2000;Santi et al, 2004;Shivappa et al, 2004;Song et al, 2005). Another well-known determinant of virulence is reassortment (Schrauwen et al, 2014), which has been suggested to be responsible for the existence of virulent variants among other viruses (Cárdenas et al, 2014).…”

Molecular characterisation of Australasian isolates of aquatic birnaviruses

“…Several studies (Bruslind & Reno 2000, Santi et al 2004, Shivappa et al 2004) have reported the presence of threonine and alanine at positions 217 and 221 of virulent strains, whereas low virulent strains have a proline and alanine at the same residues. As seen in Fig.…”