Beko disease of cultured fish in Japan

Abstract

Beko disease is derived from the concave body surface of fish infected with microsporidians in the myocytes. In this paper, several fish microsporidians including Microsporidium seriolae from Seriola spp. (S. quinqueradiata, S. dumerili and S. lalandi), Microsporidium spp. from three other cultured fish, and Heterosporis anguillarum from Japanese eel Anguilla japonica are reviewed. A shift of eel-farming method from open field ponds to house-type heating culture systems has emerged the beko disease in cultured Japanese eel. Considering the successful transmission of H. anguillarum to uninfected juveniles, frequent screening in eel size seems to reduce the incidence of the disease in eel farms. Previously, beko disease of yellowtail S. quinqueradiata was not been considered as a serious problem, because yellowtail juveniles infected with M. seriolae usually recover from the disease until the time of adult stage. However, serious condition of the disease in cultured yellowtail has recently received attention as a re-emerging disease. Unknown life-cycle of M. seriolae makes the control measures difficult. As diagnostic methods, microscopic examination with Uvitex 2B staining and molecular tools with PCR have been developed. Furthermore, other related microsporidians have been reported in cultured red sea bream Pagrus major, hatchery-bred spotted halibut Verasper variegatus and juvenile Pacific bluefin tuna Thunnus orientalis.