Low Pathogenic Strain of Infectious Pancreatic Necrosis Virus (IPNV) Associated with Recent Outbreaks in Iranian Trout Farms

Abstract

Infectious pancreatic necrosis (IPN), first described as acute viral catarrhal enteritis, is a highly contagious disease with variable pathogenicity that has been linked to genetic variation in the viral VP2 gene encoding the capsid protein. In this study, the IPN virus (IPNV) is isolated from the moribund fish from five of fourteen Iranian trout farms from 2015 to 2017. The affected fish showed mortality rates ranging from 20% to 60%, with the main clinical signs of exophthalmia, darkened skin, and mild abdominal distension, as well as yellow mucoid fluid in the intestine. Histopathological examination of intestinal sections confirmed acute catarrhal enteritis in all samples. RT-PCR assay of the kidney tissue and cell culture (CHSE-214) samples consistently confirmed the presence of the virus. The phylogenetic analysis of the partial VP2 sequence revealed that the detected isolates belong to genogroup 5, and are closely related to the Sp serotype strains of European origin. Characterization of VP2 of all isolates revealed the P₂₁₇T₂₂₁ motif that previously was associated with avirulence or low virulence, while all IPNV-positive fish in this study were clinically affected with moderate mortality. The IPNV isolates from Iran are associated with two lineages that appear to have originated from Europe, possibly via imported eggs.

Keywords: Sp serotype; VP2; pathology; rainbow trout; virulence.

Figure 1

Histopathological lesions in tissue samples of trout infected with the IPNV. (A) Photomicrograph of the intestine with acute catarrhal enteritis. Notice blunting of villi. Excess mucous (M) is seen in the lumen. (B) High magnification of intestine with acute catarrhal enteritis.

Figure 2

The cytopathic effect is caused by IPNV in the CHSE-214 cell line. (A) Control uninfected cells; (B) infected cells showing the typical cytopathic effect (CPE) for IPNV (spindle-shaped cells) at 7 dpi (40×).

Figure 3

Phylogenetic analysis of Iranian isolate of IPNV detected in farmed trout (O.mykiss) based on the partial nucleotide sequences of the VP2 gene (405 bp). The phylogenetic trees were constructed using the Geneious Prime (neighbor-joining with the Hasegawa–Kishino–Yano (HKY) model of nucleotide substitution). The Iranian isolates of IPNV detected in this study (Acc No. KX665156-9 and MK748210) were classified as genogroup 5, serotype Sp (A2) with close identity to Turkish, European, and other Iranian isolates.

Figure 4

The neighbor network (A) and its derived Parsimony network (B) of non-homologous amino acid of VP2 of 15 IPNV sequences are listed in Table 2, and 57 IPNV sequences are analyzed in Ulrich et al. [26]. The highlighted branches indicate two clades of IPNV isolates in Turkey and Iran (A): Isolates IPNV 01-111 from Scotland [26], (B): IPNV (37) indicates 37 IPNV isolates. Yellow nodes: IPNV isolates of Atlantic salmon, Blue nodes: IPNV isolates of rainbow trout.