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

Red mark syndrome (RMS) is a widespread skin disease of rainbow trout (Oncorhynchus mykiss). Its aetiological agent is believed to be a Midichloria-like organism, an intracellular bacteria of the order Rickettsiales. RMS has been reported on several occasions, however, a comprehensive analysis of the epidemiology of this disease and its actual economic impact on the trout aquaculture industry is missing. Through a questionnaire-based survey, we estimated the prevalence of RMS and the implications of this infection on trout farms in Italy in 2020-2021. One hundred and nine farms, representing 64% (around 22,000 tons) of the total national production, were surveyed. In short, the results show that RMS in Italian farms is consistently and persistently present on Italian territory, affecting about 30% of farms. RMS is observed to have similar features to those generally described in the literature. Clinical disease is observed more often in springtime; size-wise, the most commonly affected groups are fish close to market size and fish weighing less than 100g (22% of cases). Most interestingly, the recurrence of clinical signs in the same batch of fish was observed in 64% of cases. Despite its diffusion, the disease is not perceived as a major concern in the national trout sector, mainly due to its intermittent nature, low mortality and spontaneous resolution. The main costs associated with RMS were related to antibiotic treatment and commercial downgrading; however, it was impossible to estimate the impact of these on production costs.