Abstract
Inodosporus fujiokai is a pathogenic microsporidian causing cyst formation within the muscles of salmonids. Confirmed infections exist in Oncorhynchus mykiss, O. masou ishikawae, and O. masou subsp. (Biwa trout), yet its full host range and differences in susceptibility among salmonid species remain undefined. This study assessed the susceptibility of various freshwater fish species to I. fujiokai, providing a risk assessment for local freshwater aquaculture in Shiga Prefecture, Japan. Salmonids (O. mykiss, O. masou ishikawae, O. masou subsp., and Salvelinus leucomaenis) and the cyprinid honmoroko Gnathopogon caerulescens were collectively exposed to infective spores of I. fujiokai from common prawn Palaemon paucidens, and the mortality and the infection levels were compared. Additionally, the susceptibility of ayu Plecoglossus altivelis (Plecoglossidae) was investigated using spore immersion and intubation methods. All tested salmonids exhibited high susceptibility, with S. leucomaenis being particularly vulnerable. Some fish died from the infection without evident cyst formation, suggesting the involvement of multiple pathogenic factors. Infection levels in G. caerulescens and P. altivelis were low, suggesting that I. fujiokai does not pose a significant risk to their aquaculture. These findings expand the understanding of the host range of I. fujiokai and offer valuable insights for risk assessment in freshwater aquaculture.
