Parasite	Microsporidium sp.
Taxonomy	Microspora, Microsporea
Host	Red sea bream (Pagrus major)
Disease name	Beko disease ('Beko' means uneven , concave surface in Japanese.)
Infection site	Trunk muscle
Clinical signs	Infected fish exhibits uneven and concave body surface. White cysts are observedin trunk muscle (Fig. 1).
Parasitology	Many spores are produced inside the cysts. The spore of this parasite (length 2.9-3.9mm; width 1.9-2.6 mm) is slightly larger than that of Microsporidium seriolae, infecting to yellowtail (Egusa et al., 1988).
Pathology	Histopathological characteristics and the development of the parasite are similar to those in beko disease in yellowtail. Cysts are encapsulated by the host connective tissues and the parasite proliferates and sporulates inside the cysts.
Health hazard	Since this parasite is not infectious to human, it is harmless in food hygiene.
Diagnosis	Check the spores by the wet-mount of cysts. The sample should be smeared and stained by Uvitex 2B followed by a fluorescent microscopic observation. The stained spores emit blue fluorescence under UV radiation.
Other information	This disease occurs only in juvenile stage of red sea bream. Usually, fish recovers after release of spores outside the host. However, cysts in trunk muscle sometimes remain even in commercial-sized fish.
References	Egusa, S., K. Hatai and Y. Fujimaki (1988): Notes on Microsporidium species, the etiological agent of “Beko” disease in red sea bream juveniles, Pagrus major. Fish Pathol., 23, 263-267. (In Japanese)

Fig. 1. Microsporidium cysts in the muscle of red sea bream fry.