Analysis of the situation regarding the spread of red mark syndrome (RMS) among Salmonids (Salmonidae) in European countries (a review)

Purpose. The purpose of the presented review is to analyse the spread of the red mark syndrome (RMS) in rainbow trout (Oncorhynchus mykissWalbaum, 1792) from countries neighbouring Ukraine and summarise the ways and mechanisms of the spread of this disease in order to prevent the potential spread of this disease in specialized farms in Ukraine. Findings.The article provides data on the spread of red mark syndrome in rainbow trout from Europe, and methods of its prevention at fish farms. Special attention is paid to the spread of the disease in countries neighbouring Ukraine, which causes concern about the possible penetration of the pathogen into the territory of Ukraine. Geographic proximity to countries with confirmed RMS outbreaks, as well as the presence of developed aquaculture, in particular rainbow trout farming, increase the risk of introducing this disease. The cause-and-effect relationship of RMS is still unknown. A midichlorine-like organism (MLO) is considered the most likely causative agent, as all other possible agents lack sufficient evidence to associate them with RMS. Future studies should focus on confirming the causal relationship ofMLO with RMS and developing prevention strategies. The latent period of the disease is an important aspect that needs to be studied in detail.The use of new highly sensitive methods, such as polymerase chain reaction (PCR), will allow more accurate control of the dynamics of infection and determine the optimal time frame for taking fish samples for the purpose of isolating the pathogen.The hypothesis of the host's hypersensitivity reaction to MLO as one of the reasons for the development of RMS requires further investigation. Understanding the factors that contribute to the development of RMS and efforts to isolate MLO are important for the development of targeted solutions to control the disease. Strategies may focus on helping to build up the host's natural immunity against RMS, thereby avoiding the use of antibiotics. The risk of developing RMS in fish is thought to be multifactorial, with certain farming practices and environmental factors making some fish populations more susceptible to the condition.The role of water, eggs, environment, invertebrates and parasites as possible vectors of MLO requires additional research. Collection of up-to-date data on the incidence of RMS is important for a more accurate determination of its economic impact on trout aquaculture. Disease surveillance and broader data on the prevalence of RMS in Europe and the world should be implemented by conducting large-scale studies combining data from different institutions.This will increase the level of biosecurity and help develop effective risk management strategies, which will contribute to maintaining the stability of aquaculture in Ukraine. The potential economic consequences of RMS can be significant for Ukraine. Despite the low mortality, treatment and management costs can significantly affect the economics of aquaculture by increasing production costs. It is important to implement biosecurity measures, including control of live fish imports, regulation of aquaculture systems and increased biosecurity on farms. It is also necessary to conduct a study to assess the potential economic impact of RMS on the industry, which will help to develop effective risk management strategies and maintain the stability of aquaculture in Ukraine. Practical Value. Monitoring of salmon diseases in fish farming enterprises of Ukraine will allow determining the ecological niches of these pathogens and establish the potential risks of epizootics, primarily in the conditions of aquaculture.