Red mark syndrome of trout (Oncorhynchus mykiss; Walbaum, 1792): Histopathological scoring and correlation with gross lesions

Abstract

Red mark syndrome (RMS) is a skin disorder affecting rainbow trout (Oncorhynchus mykiss). The present work aimed to correlate the gross skin lesions affecting 46 fish sampled from farms surveyed for RMS with their microscopic features, identifying histological parameters that may be suggestive of disease progression. Skin lesions were grossly included in one of three categories (types I, II and III) according to the progressive degree of severity. Histological parameters and anti‐proliferating cell nuclear antigen (PCNA) tissue immunoreactivity were semi‐quantitatively assessed. In the dermis, PCNA‐positive lymphocytes, fibroblasts and endothelial cells were indicative of active phlogosis. A significant increase in PCNA‐immunoreactive lymphocytes, from gross type I to type III cases, was found only in the hypodermis. The histological parameters significantly associated with the gross lesion severity were progressive loss of the epithelium and scales, recruitment of inflammatory cells in the stratum compactum, loss of architecture of the stratum compactum, perivascular and perineural granulomatous inflammation and increase in lymphocyte infiltration of the muscular layer. In the type II and type III categories, inflammation in the hypodermis and muscle displayed a granulomatous pattern, reinforcing the hypothesis of an immunopathological mechanism. The morphological diagnosis of “deep chronic dermatitis associated to panniculitis and myositis, characterised by lympho‐histiocytic and granulomatous reaction” is suggested.

Observations on Red Mark Syndrome in juvenile rainbow trout farmed in RAS system

“…As reported by the results of the 25th Annual Workshop of the European Reference Laboratories for Fish Diseases (DTU‐AQUA, Denmark), RMS remains to date a major concern in rainbow trout farming. This syndrome has been studied extensively both in field and experimental cases, and diagnostic features have been established (Galeotti, Sarli, et al, 2021 ; Jorgensen et al, 2019 ; Oidtmann et al, 2013 ; Verner‐Jeffreys et al, 2008 ). Slovenia and Bosnia‐Herzegovina were the most recent European areas where RMS was reported to outbreak in intensive aquaculture farms (Galeotti, Manzano, et al, 2017 ; Galeotti, Volpatti, et al, 2021 ).…”

A questionnaire-based survey on the presence of red mark syndrome in Italian rainbow trout (Oncorhynchus mykiss) farms

“…Cohabitation challenge models established under experimental conditions have also allowed the condition to be reproduced in an experimental setting (Erickson 1969;Oman 1990;Verner-Jeffreys et al 2008;Jørgensen et al 2019). Presumptive diagnosis is mainly based on clinical features, while confirmatory diagnosis is based on skin histology and molecular biology techniques such as quantitative and digital droplet PCR (Oidtmann et al 2013;Galeotti et al 2016;Galeotti, Sarli, et al 2021;. Unfortunately, a confirmatory diagnosis is rarely performed.…”

Understanding the Pathogenesis of Red Mark Syndrome in Rainbow Trout (Oncorhynchus mykiss) through an Integrated Morphological and Molecular Approach

“…To date, the diagnosis of RMS is typically based on clinical features and histopathology, complemented by molecular biology techniques, such as PCR for MLO DNA [ 1 , 26 ]. A recent study proposed by the authors provides a histological scoring system based on three categories of severity of microscopic lesions [ 26 ].…”

“…To date, the diagnosis of RMS is typically based on clinical features and histopathology, complemented by molecular biology techniques, such as PCR for MLO DNA [ 1 , 26 ]. A recent study proposed by the authors provides a histological scoring system based on three categories of severity of microscopic lesions [ 26 ]. The loss of epithelium and scales, the presence of inflammatory cells in the stratum compactum, disruption of the architecture of the stratum compactum, granulomatous inflammation around blood vessels and nerves, and an increase in lymphocytes in the muscular layer were all significant histological parameters that correlated with the severity of gross lesions.…”

“…In addition to those conclusions, the appearance of microthrombosis and necrosis of the epidermis in advanced severe lesions was comparable to that found in skin inflammation reactions caused by the buildup of immune complexes (type III). Furthermore, these immune complexes can coincide with delayed-type hypersensitivity (type IV), as seen in canine scabies, where both type III and type IV mechanisms are present [ 26 ].…”