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

Abstract

Red mark syndrome (RMS) is a widespread skin disorder of rainbow trout in freshwater aquaculture, believed to be caused by a Midichloria-like organism (MLO). Here, we aimed to study the pathologic mechanisms at the origin of RMS by analyzing field samples from a recent outbreak through gene expression, MLO PCR, quantitative PCR, and a histopathological scoring system proposed for RMS lesions. Statistical analyses included a One-Way Analysis of Variance (ANOVA) with a Dunnett’s multiple comparisons test to assess differences among gene expression groups and a nonparametric Spearman correlation between various categories of skin lesions and PCR results. In short, the results confirmed the presence of a high quantity of 16S gene copy numbers of Midichloria-like organisms in diseased skin tissues. However, the number of Midichloria-like organisms detected was not correlated to the degree of severity of skin disease. Midichloria-like organism DNA was found in the spleen and head kidney. The spleen showed pathologic changes mainly of hyperplastic type, reflecting its direct involvement during infection. The most severe skin lesions were characterized by a high level of inflammatory cytokines sustaining and modulating the severe inflammatory process. IL-1 β, IL-6, IL-10, MHC-II, and TCR were upregulated in severe skin lesions, while IL-10 was highly expressed in moderate to severe ones. In the moderate form, the response was driven to produce immunoglobulins, which appeared crucial in controlling the skin disease’s severity. Altogether our results illustrated a complex immune interaction between the host and Midichloria-like organism.

Skin Microbial Community Associated to Strawberry Disease in Farmed Rainbow Trout (Oncorhynchus mykiss Walbaum, 1792)

“…With regard to the histological characterization of wounds, our findings agree with previous works ( Figure 1 and Figure S1 ). An inflammatory response was observed, which, depending on the degree of progression of the lesion, was extended from the dermis to the hypodermis, accompanied by a significant loss of scales and leukocyte infiltration [ 4 , 5 , 26 , 27 ]. The observed inflammatory response indicates the activation of an innate immune response in the lesions [ 27 , 28 , 29 ].…”

“…The condition of SD or RMS in rainbow trout appears to be horizontally transmissible, and according to the current literature [ 4 , 5 , 45 ], a bacterial infection is considered the potential etiology, with Rickettsia-like organisms deemed the most probable candidate. Additional research revealed a strong and positive correlation between fish affected by RMS and an RLO identified as a Midichloria-like organism (MLO), belonging to the Midichloriaceae family and the order Rickettsiales [ 4 , 46 ].…”

“…Additional research revealed a strong and positive correlation between fish affected by RMS and an RLO identified as a Midichloria-like organism (MLO), belonging to the Midichloriaceae family and the order Rickettsiales [ 4 , 46 ]. Hence, an MLO is considered the potential cause of RMS or SD [ 4 , 5 , 44 , 45 , 46 , 47 ]. MLOs were identified in splenic impression smears, presenting morula-like structures characteristic of RLOs.…”

“…These structures were observed in the cytoplasm of splenic macrophages in rainbow trout affected by RMS [ 48 , 49 , 50 ]. Transmission electron microscopy (TEM) further validated the presence of intracytoplasmic microorganisms resembling Rickettsiales within macrophages, fibroblasts, and erythrocytes [ 5 , 26 , 48 , 49 , 50 ]. Therefore, considering the observed inflammatory response, which has the potential to induce alterations in the skin layers, our findings point towards a disruption in the stability of the protective barrier constituted by mucus and skin-associated microbiota.…”

“…This condition is observed in trout close to harvest (300–600 g). Histopathological, microbiological, and virological analyses were carried out by Chilean researchers, and they have determined that these lesions are characteristic of SD, first described in the United States [ 4 , 5 , 6 ]…”