Infectious pancreatic necrosis virus (IPNV) is a double-stranded RNA virus from the family Birnaviridae, in the genus Aquabirnavirus. Causing the highly infectious disease Infectious pancreatic necrosis, the virus primarily affects young salmonids resulting in high mortality, occasionally surpassing 90 percent in the early stages. IPNV or IPNV-like viruses have been isolated worldwide from at least 32 families of saltwater and freshwater salmonids and non-salmonids fish including salmon, flatfish, pike, eels and others. Other aquatic organisms infected include 11 molluscs and 4 species of crustaceans. Due to its wide host range and high mortality, the virus is of great concern to global aquaculture. In addition to persistence in hosts, IPNV is also perpetual in the environment, surviving across a range of conditions and capable of infecting fish with as little as 101 TCID50/ml of the virus. Found in Europe, North America, South America, Africa, Asia, and Australia, the virus has led to significant losses in the mariculture of Atlantic salmon, brook trout, and rainbow trout. First described in 1940 in farmed trout, IPNV outbreaks occur primarily in fry as the most susceptible stage, first in freshwater and then again after a transfer to seawater as post-smolts. Survivors as asymptomatic carriers pose a significant threat to continued outbreaks through both horizontal and vertical transmission of the virus by way of excreted matter. The virus is classified into serogroups A and B, with group A containing nine serotypes and B one serotype. With a virion 65-70 nm in diameter and icosahedral structure, the genome of the virus is split into two segments of double-stranded RNA, encoding 5 proteins. An infection of the host is often identified based on visual inspection, with various key external and internal characteristics. A commonly used molecular biological method of diagnosis is RT-PCR assays. Attempts at controlling IPNV have included the reduction of horizontal and vertical transmission, vaccinations, and selective breeding.
Classification
Infectious pancreatic necrosis disease virus (IPNV) causes infectious pancreatic necrosis disease (IPN) in fish – most disease has been documented in young, farmed salmonid fish. Infectious pancreatic necrosis disease virus (IPNV) is a bi-segmented, double-stranded RNA virus belonging to the Aquabirnavirus subgroup of the Birnaviridae. IPNV's virion has a hexagonal profile without envelopes, and a diameter of approximately 60 nm. To date, only two other Aquabirnavirus are listed by the International Committee on Taxonomy of Viruses (ICTV), the Tellina virus and the Yellowtail ascites virus.
Serotypes As a member of Aquabirnavirus, IPNV is divided into two serogroups, A and B, each containing numbered serotypes. Within serogroup A, Serotype A1 includes variants like VR-299 and the Buhl strain found in the US. Serotypes A2 to A5 have been found in Europe and serotypes A6 to A9 have been found in Canada. Serogroup B is made up of a single variant called TV-1.
Segments Infectious pancreatic necrosis virus has two segmented RNA genomes, segments A and B. Segment A contains the genetic information necessary to produce the polyprotein, consisting of genes pVP2, VP3, VP4, and VP5. Segment A also contains virulence factors. Segment B contains genetic information for creating an RNA-dependent RNA polymerase enzyme made of VP1. The composition of the surface of virus particles involves a specific area of the capsid protein VP2 linked to host specificity, virulence and serotype.
Microbiology
Structure Infectious pancreatic necrosis virus is 65-70 nm in diameter, with icosahedral structure. It has unenveloped, single-shelled icosahedrons. Its buoyant density is 1.33 grams per cm3. Similar to other Birnaviruses, it has a single T = 13 icosahedral cell and no inner capsid. The shell is made up of 120 subunits of a coat protein.
RNA Infectious pancreatic necrosis virus has 2 segments of double-stranded RNA, segment A and segment B. Segment A is 3097 base pairs long, and has two partially overlapping open reading frames, one large and one small. Segment B is 2784 base pairs long and has one open reading frame.
Proteins Viral RNA encodes proteins VP1, VP2, VP3 and VP3a, VP4, VP5, and VP1-RNA-dependant RNA polymerase. VP1, the largest protein at 94 kDa, is responsible for the synthesis of viral mRNA. The 5' end of the RNA is bonded to a serine residue in the VPg via a phosphodiester bond, allowing the VPg to act as a cap substitute during translation. VP2 (54 kDa) is the main structural protein, and is responsible for cell attachment. VP3 and VP3a are the most abundant polypeptides, though due to their smaller size (31 kDa and 29 kDa, respectfully), they contribute less to the overall viral mass than VP2. VP3 and VP3a have 3 important roles in morphogenesis: it interacts with V2 (the capsid protein), associates with the viral genome, and recruits polymerase into capsids. VP4 converts pre-VP2 to capsid protein VP2, while VP5 is likely related to gene expression.
Replication A single cycle of replication of IPNV, which takes place in the cytoplasm, takes 16–20 hours at 22 degrees Celsius. This falls within the optimal temperature for replication, which is between 20 and 24 degrees Celsius. Replication is slow or completely halted in cells above 24 degrees Celsius.
History
Discovery In 1940, the first pancreatic necrosis symptom was found in Canada's farmed Atlantic salmon, Salvelinus fontinalis, resulting in significant losses of juvenile trout. In the 1950s, the same symptom was observed in Oncorhynchus mykiss in the United States, and the fish disease was named infectious pancreatic necrosis (IPN). Shortly after, the infection expanded to Europe.
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