Notes on Microsporidium species, the Etiological Agent of “Beko” Disease in Red Sea Bream Juveniles, Pagrus major

Abstract

“Beko” disease in red sea bream juveniles was characterized by formation of masses of a microsporidian parasite and muscle liquefaction around the masses in the lateral muscle in a similar manner to “Beko” disease in yellowtail juveniles, Seriola quinqueradiata caused by Microsporidium seriolae. In most of the diseased fish studied masses of the parasite were composed of multi-nucleated cells in the outer layer, sporoblast mother cells in the middle layer and sporoblasts and spores in the inner layer. Neither pansporoblast membrane nor sporophorous vesicle was observed. These cells were much the same in arrangement and morphology as the cells in sporulation stages of M. seriolae; this fact indicated that the present organism is closely related to M. seriolae. Although the present organism differed in spore size and host species from M. seriolae, the distinction between the two must wait the results of cross-infection experiments. For this reason the present organism was tentatively designated as Microsporidium sp.