Microsporidium seriolae

From Fish
External appearrance of diseased yellowtail fry
Mass of Microsporidium cysts in the trunk muscle
Fresh spores of Microsporidium seriolae

Microsporidium seriolae, the causative organism of Beko disease, is a microsporidian parasite of fish, affecting the trunk muscles of marine fishes such as Yellowtail (Seriola quinqueradiata), Greater amberjack (Seriola dumerili) and Yellowtail amberjack (Seriola lalandi).

Life cycle

Many spores are formed in the cyst. Spore length 2.9-3.7 mm; spore width 1.9-2.4 mm. The life cycle is unknown. This parasite doesn't transmit from fish to fish, suggesting that an intermediate host is involved in the life cycle.

Clinical signs

Affected fish have small cyst-like bodies (a few mm-1 cm) in the muscle and show the concave body surface. The white ‘cysts’ exhibit the variable shape.

The cyst, in which the parasites proliferate and sporulate, is encapsulated by the thin connective tissue of host (Fig. 5). Few host reactions are observed during the cyst development. After cyst's degeneration, the neighboring muscle tissue shows diffuse colliquative necrosis, causing the concave body surface. Heavy infection often cause death in affected fish[1]. After spores are ingested by phagocytes and shed into outside of the host, the fish may recover[2].

Since this parasite is not infectious to human, it is harmless in food hygiene.


Diagnosis is ascertained by visual inspection of fish for spores via wet-mount. The sample should be smeared and stained by Uvitex 2B followed by a fluorescent microscopic observation[3]. The stained spores emit blue fluorescence under UV radiation (Fig. 4). The high sensitive detection method by PCR has been developed[4], although its specificity is uncertain.


  1. Ogawa, K. (2004) Protozoan diseases. Infectious and parasitic diseases of fish and shellfish. (ed. by Wakabayashi, H. and K. Muroga), Koseisha koseikaku, pp:285-338
  2. Sano, M., J. Sato and H. Yokoyama (1998) Occurrence of beko disease caused by Microsporidium seriolae (Microspora) in hatchery-reared juvenile yellowtail. Fish Pathol 33:11-16
  3. Yokoyama, H., J.-H. Kim, J. Sato, M. Sano and K. Hirano (1996) Fluorochrome Uvitex 2B stain for detection of the microsporidian causing beko disease of yellowtail and goldstriped amberjack juveniles. Fish Pathol 31:99-104
  4. Bell. A. S., H. Yokoyama, T. Aoki, M. Takahashi and K. Maruyama (1999) Single and nested polymerase chain reaction assays for the detection of Microsporidium seriolae (Microspora), the causative agent of ‘Beko’ disease in yellowtail Seriola quinqueradiata. Dis Aquat Org 37:127-134