Metagonimus yokogawai

From Fish
Black spots (arrows) on the skin of ayu.
Metacercaria of Metagonimus yokogawai

Metagonimus spp are a parasitic digenean fluke that causes 'black spot disease' in marine and freshwater fish. They are also parasitic to cats, pigs and sea birds in the Pacific region[1].

Belonging to the taxonomic class Plathyhelminthes, Trematoda, Opisthorchiida, these parasites are infective to Ayu fish (Plecoglossus altivelis), Japanese crucian carp (Carassius cuvieri), Freshwater minnow (Zacco platypus) and Japanese icefish (Salangichthys microdon).

Life cycle

The adult parasites release fully embryonated eggs (each with a fully developed miracidium), which are then passed in the host's feces. After ingestion by a suitable snail (first intermediate host), the eggs hatch and release miracidia, which penetrate the snail's intestine. Snails of the genus Semisulcospira are the most common intermediate host for Metagonimus yokogawai. The miracidia undergo several developmental stages in the snail (sporocysts, rediae, cercariae). Many cercariae are produced from each redia. The cercariae are released from the snail and encyst as metacercariae in the tissues of a suitable freshwater or brackish-water fish (second intermediate host). The definitive host becomes infected by ingesting undercooked or salted fish that contains metacercariae. After ingestion, the metacercariae excyst, attach to the mucosa of the small intestine, and mature into adults (measuring 1-2.5 mm X 0.4-0.75 mm). M yokogawai infects humans, fish-eating mammals (eg, cats, dogs), and birds[2].

Clinical signs

Metagonimus spp infect the muscle and scales of fish. The infection site is visually observed since there are melanin depositions surrounding a metacercaria infecting to the scale[3].

Pathological effects to fishes are low. Neither abnormal swimming nor mortalities are reported. Adults also infect the small intestine of human. In heavy infection, they cause the diarrhea and colic, but don’t cause severe disease.


Diagnosis is based on isolation of metacercariae in adult fish


Praziquantel is the drug of choice for treatment of this parasite.


  1. McCarthy J, Moore TA. (2000) Emerging helminth zoonoses. International Journal of Parasitology 30:1351 –1360
  2. Saito, S. (1999) Metagonimus research done after 1960 -. Progress of medical parasitology in Japan. Meguro Parasitological Museum 7:205-215
  3. Ogawa, K. (2004) Metazoan diseases. Infectious and parasitic diseases of fish and shellfish. (Wakabayashi, H. and K. Muroga), Koseisha koseikaku, pp:381-405