The role of live fish trade in the translocation of parasites: the case of Cystidicola farionis in farmed rainbow trout (Oncorhynchus mykiss)

Cystidicola farionis is a nematode of the swim bladder that parasitizes Salmonidae and Osmeridae families, with a range of diffusion in Europe, Asia, and North America. During a fish health monitoring performed in July 2017 in a trout farm located in northwest Italy, a prevalence of 70% for C. farionis was recorded in rainbow trout from a unique tank. In order to clarify the first occurrence of this parasite in northwest Italy, a total of 180 specimens were analysed in different seasons to obtain prevalence, mean intensity, and mean abundance of infestation. Moreover, amphipods were also sampled during each fish sampling campaign. Only fish sampled in August 2017 were positive for C. farionis, since fish were then sold and replaced with a new batch. All isolated nematodes were identified by morphological criteria as subadults (L4), but not sexually matured. All examined amphipods (genus Gammarus) resulted negative for the presence of the parasite. The most probable explanation of this report was due to the movement of a single fish batch from northeast Italy, previously described as an endemic area for this parasite. The absence of the parasite in the amphipods and consequently in other fish batches from other tanks are due to the lifetime cycle of the parasite that had no time to infect new hosts.