Myxozoans are metazoan parasites which are highly pathogenic in commercially importantfish, particularly in aquaculture. Enteromyxosis is an economically significant diseasecaused by the myxosporean parasite Enteromyxum leei.Presently, early diagnosis of enteromyxosis in sharpsnout seabream was achieved bymonitoring fish on a time-scale scheduled basis in combination with a validated moleculardiagnostic technique. Two different successive fish batches of a commercial fish farm weremonitored starting from the first month of on-growing in sea cages until fish reachedcommercial size. Fish weight and water temperature differentially ranged between them.E. leei detection was performed both by classical microscopicobservation of fresh smears and by quantitative polymerase chain reaction (qPCR) whileprevalence, mean intensity and mortality rate were calculated. In total, 520 fish werescreened; 320 from the first batch and 200 from the second one. Parasitosis illustrated aseasonal occurrence pattern (spring and summer). When qPCR was used as a diagnosticmethod, enteromyxosis was diagnosed earlier with more increased prevalence compared to themicroscopic observation. In this case, the parasite was detected at the lowest watertemperature that has ever been recorded (14.7 °C), during the first month of on-growing in sea cages. The two-factorcombination of water temperature and fish weight proved to be the most significant factorthat could determine the emergence and development of parasitosis and smaller fish provedto be more susceptible. Mortality was influenced by both prevalence and intensity ofparasitosis. An earlier diagnosis and identification of all developmental stages of theparasite throughout its life cycle was verified with qPCR, significantly before anyoutbreak occurred. The latter could become a key diagnostic tool for the control ofenteromyxosis, allowing stakeholders and fish farmers to adopt all necessary precautionarymeasures minimizing economic loss in sharpsnout seabream intensive farming.

An epidemiological cohort study of Enteromyxum scophthalmi in cultured turbot was performed on a farm in North Western Spain. Four different ongrowing stocks (A, B, C, D) were monitored monthly until market size. Fish from stocks C and D were divided into 2 subgroups, receiving filtered (CF and DF) or unfiltered (CUF and DUF) water. The lack of water filtration was positively associated with infection prevalence, as all fish kept in filtered water remained uninfected. Parasite abundance varied seasonally (P<0·05) in stock B and subgroup CUF. Infection was also associated (P<0·05) with host weight, and the highest prevalences and intensities were detected in 101–200 g and 201–300 g fish. Distribution pattern of E. scophthalmi in subgroups CUF and DUF had a variance higher than the mean, indicating overdispersion. The minimum period necessary for the first detection of the parasite and for the appearance of disease symptoms and mortality, varied depending on the stock and introduction date, although a long pre-patent period was always observed. Several factors, such as host density, parasite recruitment and parasite-induced fish mortality can contribute to the observed distribution pattern. Risk factors found to be associated with E. scophthalmi infection, including water quality and accumulation of infective stages in the culture tanks, should be considered when designing control strategies to prevent the introduction and spread of infective stages in the facilities.