Histology, immunocytochemistry and qRT‐PCR analysis of Atlantic salmon, Salmo salar L., post‐smolts following infection with infectious pancreatic necrosis virus (IPNV)

Abstract

Infectious pancreatic necrosis (IPN) is a very serious viral disease in terms of its impact on production of Atlantic salmon, Salmo salar L., fry and post-smolts. Post-smolts of Atlantic salmon were injected with infectious pancreatic necrosis virus (IPNV) and cohabited with naive fish to produce natural infection. Cohabitant fish were sampled every 2 days, up to day 36 post-infection (p.i.). From 90 cohabitant fish, 11 (12.2%) were positive by immunohistochemistry (IHC). The first detection of IPNV by IHC occurred on day 16 p.i. which coincided with the onset of mortality in this group. Besides the pancreas, the liver was found to be a key target organ for IPNV. For the first time, the virus was observed in the islets of Langerhans and in the kidney corpuscles of Stannius which suggests that the virus could affect the fish's metabolism. The liver of two fish, which showed the most widespread presence of IPNV by IHC, had a pathology including focal necrosis and widespread presence of apoptotic hepatocytes, many of which did not stain for virus by IHC. Up-regulation of cytokine gene expression was found only in the IHC-positive (IHC+ve) fish and reflected the level of infection as determined by IHC positivity of the liver. In most fish, interferon (IFN), Mx, γIFN and γIP were up-regulated in liver and kidney, while only IFN and Mx were up-regulated in gill. IL1β and TNFα were not induced in any tissue. The gill showed variable levels of constitutive expression of IL1β and γIFN. The two fish with liver pathology had the highest level of IFN expression, especially relative to the level of Mx expression, in the liver compared with the other IHC+ve fish which did not have a liver pathology. The results suggest that following widespread infection of hepatocytes, the cells may over-produce IFN, resulting in apoptosis of neighbouring cells with subsequent death from liver failure.

IPNV modulation of pro and anti-inflammatory cytokine expression in Atlantic salmon might help the establishment of infection and persistence

“…Production of pro-inflammatory cytokines has been studied previously in Atlantic salmon infected with IPNV. Similarly, authors have reported that no increase of expression for IL-1b and TNFa (which was not analyzed here) was detected in any tissues with respect to controls [19,26] or at most, IL-1b was slightly up-regulated after 24 days after infection [36]. The absence of up-regulation of pro-inflammatory cytokines at early stages of IPNV immune stimulation was not due to the inability of the fish to respond because treatment with LPS or Poly I:C increased IL-1b and IL-8 cytokine levels 24 h post-injection.…”

“…inoculation of IPNV [18] and in Atlantic salmon (Salmon salar), IL-1b expression was not induced in kidney and gills between days 16 and 34 after challenge by cohabitation [19], although in gills, constitutive expression was observed. The induction of other pro-inflammatory cytokines after IPNV infection has not been explored, although IL-1b, IL-6 IL-8, IL-11, IL-12 expression were detected in six cDNA libraries from liver, kidney, spleen, peripheral blood and thymus of infected Atlantic halibut [20].…”

Molecular profiling of the gilthead sea bream (Sparus aurata L.) response to chronic exposure to the myxosporean parasite Enteromyxum leei

“…Interferons (IFNs) induce a state of antiviral activity by transcriptional regulation of several hundred IFN-stimulated genes and play a major role in the defence against virus infection in vertebrates, including fish (Samuel, 2001;Alonso and Leong, 2002;Shultz et al, 2004;Robertsen, 2006;Ellis et al, 2010). They are also known to play a role in some parasitic infections, including resistance (Lillehoj and Choi, 1998;Pollock et al, 2001;McCall and Sauerwein, 2010).…”

A comparative histopathological, immunohistochemical and nested-PCR study for diagnosis of infectious pancreatic necrosis in the naturally infected cultured rainbow trout (Oncorhynchus mykiss)

“…Stars show significant differences between different diagnostic methods in the same weight (P \ 0.05) within the pancreatic acinar cells (Ferguson 1989;Smail and Munro 2001). Ellis et al (2010) reported that in addition to the pancreatic tissue, the liver parenchyma could also be a key target organ for IPNV. It has also been stated that kidney in some instances could be another target organ and is affected as a carrier tissue with areas of histopathological lesions (Johansen and Sommer 1995;Smail and Munro 2001).…”

“…Histopathology as a conventional detection method is still commonly used for the diagnosis of viral diseases such as IPN (Ellis et al 2010). It has the difficulties of storing the pathologic specimens correctly in order to preserve microscopic lesions especially when large numbers of samples need to be detected, and thus more accurate and sensitive diagnostic methods are needed.…”