Abstract
A recently described microsporidian Inodosporus fujiokai demonstrated trophic transmission from common prawn Palaemon paucidens to rainbow trout Oncorhynchus mykiss. Some of the infected trout showed petechiae-like red spots containing microsporidian spores in the trunk muscle beneath the skin and many died with hypoxia symptoms. However, it remained unclear whether the red spots are the typical sign of the disease, and the causal link between the infection and host death has not been determined. Furthermore, the identity of a similar microsporidian observed in Biwa trout Oncorhynchus masou subsp. in our previous study was not conclusive. We conducted two prawn-feeding trials using the two trout species, O. mykiss and O. masou subsp., in order to determine the pathology of I. fujiokai infection and for deeper understanding of the disease. The infections of I. fujiokai associated with high mortalities with hypoxia symptoms (opened mouth and operculum) were confirmed in both trout species that have been fed with P. paucidens. Whitish microsporidian cysts were detected in the heart as well as trunk muscle at around 20 days after feeding the infected prawns. Red spots beneath the skin were rarely observed and considered resulting from fish processing, or a post-mortem symptom. Histological analyses suggested that the cyst formation in the heart caused myocarditis which reduced cardiac function, leading to hypoxia.
