Subclinical Edwardsiella ictaluri Infection of Wild Ayu Plecoglossus altivelis

Edwardsiella ictaluri infection is a newly emerging disease in ayu Plecoglossus altivelis in Japanese rivers, and has been continuously observed since the first outbreak in 2007. The present field study was performed in a river in Hiroshima Prefecture, Japan, where mortality due to E. ictaluri infection was recorded in 2007. Investigations over the course of 3 years (2008-2010) revealed the existence of constant subclinical E. ictaluri infection in apparently healthy ayu in the river, with higher rates (average 45.4%) in September and October, when water temperature drops and fish mature sexually. Among 11 other wild fish species examined at the same time, only one forktail bullhead Pelteobagrus nudiceps was positive for E. ictaluri. The bacterium was not isolated from groups of juvenile ayu just before the release into the river for stock enhancement of this species. On the other hand, the ubiquitous presence of E. ictaluri-specific phages in the river water and sediments suggested the existence of E. ictaluri in the river; that represents an environmental reservoir of the pathogen and may serve as a potential infection source for ayu. The current subclinical status of wild ayu infected with E. ictaluri might turn to overt infec­ tion and thus cause mortality under unidentified stress conditions.