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

Abstract

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.

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

“…Red mark syndrome represents a serious challenge for freshwater aquaculture rearing systems [ 1 ]. It can threaten land-based rearing sites in Southern Europe during cooler months [ 36 ], and the outbreaks described here exemplify a typical scenario. Various PCR methods have been developed in recent years to detect RMS-causing Rickettsia-like organisms (RLO) [ 19 , 24 , 37 , 38 ].…”

Dedicated Protocol for Ultrastructural Analysis of Farmed Rainbow Trout (Oncorhynchus mykiss) Tissues with Red Mark Syndrome: The Skin—Part One

“…These procedures led to the detection of the causative agent of this disease in the investigated samples, as well as providing an ultrastructural overview of the investigated MLO. Because the pathogenesis of RMS in rainbow trout is a subject of ongoing debate among the scientific community [ 10 , 17 ], and previous studies have faced elucidating this issue [ 18 , 19 ], mainly shedding light on skin tissue, but sometimes failing to examine other organs, like spleen, and kidney [ 13 ], to advance our understanding of the pathogenic mechanisms underlying RMS, this protocol provides a detailed description of the main procedures, requested for TEM analysis, performed on samples of skin-derived from rainbow trout, affected by RMS disease. The ultrastructural observations provided by applying this TEM preparative may be informative of the shape and distribution of the investigated microorganism within the target tissue.…”