Effect of temperature on transfer of Midichloria-like organism and development of red mark syndrome in rainbow trout (Oncorhynchus mykiss)

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

“…Thus, to date, a Rickettsia -like organism (RLO), belonging to the Midichloriaceae family and the order of Rickettsiales, and named Midichloria-like organism (MLO) is believed to be the cause of RMS [ 1 , 23 , 24 ]. This has been supported by many studies where MLOs were visualized both in splenic impression smears as morula-like structures, typical of Rickettsia -like organisms, in the cytoplasm of splenic macrophages of RMS-affected rainbow trout [ 18 , 25 ], and by TEM as intracytoplasmic microorganisms resembling Rickettsiales within macrophages, fibroblasts, and erythrocytes both in the field and experimental cases [ 3 , 24 ]. Furthermore, the application of immunohistochemistry showed positive staining of what was considered MLO-related antigens in the internal organs of most RMS-affected fish [ 22 ].…”

“…It causes bright red skin lesions leading to marked economic losses on behalf of the fish farmer with high morbidity (up to 90%) without mortality, with no effects on weight, appetite, or fish behavior [ 1 ]. Its development and spontaneous resolution seem markedly dependent upon water temperature [ 2 , 3 ]. RMS condition in rainbow trout has been widely reported in at least three continents [ 4 , 5 , 6 , 7 , 8 , 9 , 10 , 11 , 12 , 13 , 14 , 15 , 16 , 17 , 18 ].…”

“…RMS condition in rainbow trout has been widely reported in at least three continents [ 4 , 5 , 6 , 7 , 8 , 9 , 10 , 11 , 12 , 13 , 14 , 15 , 16 , 17 , 18 ]. This disease seems transmissible, and current literature suggests a bacterial infection as the likely etiology [ 2 , 3 , 19 , 20 , 21 , 22 ], with two agents, Flavobacterium psychrophilum and a Rickettsia-like organism, considered the most likely candidates. Further investigation demonstrated a robust and favorable correlation between MLO and RMS-affected fish [ 22 ].…”

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

“…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 ). Even though no aetiological agent has been proven through Koch's postulates, RMS has been consistently associated with the presence of Midichloria‐like organisms which have been visualized in situ by transmission electron microscopy both in field and experimental cases (Cafiso et al, 2016 ; Galeotti, Ronza, & Beraldo, 2017 ; Metselaar et al, 2020 ; Orioles et al, 2022b ).…”

“…Skin lesions were classified as moderate or severe as reported recently in the literature (Orioles, Galeotti, et al, 2022 ). Results of the molecular biology analysis are reported in Table 1 .…”