Abstract
Red mark syndrome (RMS) is a fish disease caused by the infection of Rickettsial agents, especially affecting rainbow trout (Oncorhynchus mykiss). The disease is prevalent in many countries in Europe (France, Switzerland, Italy, and Slovenia), South America (Chile), North America (USA), and even Asia (Japan). However, it has not been reported in Korean aquaculture. In February 2019, rainbow trout presenting red spot lesions with swollen features on the lateral side of their body were observed at a hatchery in Korea. Fishes showing those clinical signs were fry weighing 25 ± 5 g. Moreover, the fish showing the red spot lesions were found dead, which suggests an outbreak of a mortality-causing disease. The symptoms were similar to those of RMS, and we identified the presence of Rickettsia-like organisms associated with this disease using polymerase chain reaction (PCR), sequencing, histopathologic examination, and transmission electron microscopy. The distinct features of this infection, compared to that in previous reports, were that RMS occurred in small-sized fish and accompanied mortality. Additionally, the presence of the Rickettsia agent was accompanied with outbreak of the disease. Therefore, this is the first report of RMS outbreak in rainbow trout fisheries in Korea.
Highlights
Red mark syndrome (RMS) is a skin condition related to Rickettsia-like organism (RLO) infection that can develop in rainbow trout under specific circumstances [1]
A phylogenetic study proved that the RLOs associated with RMS belong to the recently described Midichloriaceae family and order Rickettsiales [6]
Immunohistochemistry, transmission electron microscopy (TEM), polymerase chain reaction (PCR) of the 16s rRNA gene, and histopathology analyses can be conducted for diagnosis of this disease
Summary
Red mark syndrome (RMS) is a skin condition related to Rickettsia-like organism (RLO) infection that can develop in rainbow trout under specific circumstances [1]. RMS first appeared in a single rainbow trout farm located in Scotland, and spread simultaneously throughout the rest of the United Kingdom [5]. The main causes of the disease remain unclear, but two main pathogens have been studied to elucidate the etiology of RMS, namely RLOs and Flavobacterium psychrophilum [10]. Studies reported that F. psychrophilum was not isolated or detected in RMS-affected tissues [10]. Intracytoplasmic oval-shaped microorganisms were detected within macrophages and erythrocytes in the skin of RMS-affected fish, which indicates that these microorganisms are a possible cause of this disease [1,11]. The present study is the first report of RMS among a farmed rainbow trout population in Korea. The affected fish constituted 20% of the group and showed clinical signs resembling RMS. Our results suggest that the RMS outbreak had occurred in Korea
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