Early Infection Dynamics of the Beko Disease in Seriola spp

The early infection dynamics of Microsporidium seriolae, the causative agent of beko disease were investigated in wild and hatchery-reared juvenile Seriola quinqueradiata and S. dumerili at several grow-out and farming locations in western Japan. ​The progression of M. seriolae infection in the somatic muscle of the fish and its DNA in seawater were monitored by quantitative PCR and macroscopic detection of the cysts. ​In sea areas where outbreaks of beko disease have been confirmed in the past, fish became infected by M. seriolae after placing in sea cages when 102–103 copies/L of M. seriolae gene was continuously detected from the seawater. ​The parasite DNA was detected from the somatic muscle within 2 wk after exposure to the contaminated water and their cysts became visible within next 1 wk. ​M. seriolae infections were confirmed in S. quinqueradiata and S. dumerili even during the winter months (November to January), indicating that its infection season is longer than previously thought. ​Although direct comparison was not made, S. dumerili appeared to be more susceptible to this disease than S. quinqueradiata.