| Parasite | Metagonimus spp. |
|---|---|
| Taxonomy | Plathyhelminthes, Trematoda, Opisthorchiida |
| Hosts | Ayu (Plecoglossus altivelis), Japanese crucian carp (Carassius cuvieri), Freshwater minnow (Zacco platypus), Japanese icefish (Salangichthys microdon) |
| Disease name | Black spot disease |
| Infection site | Muscle, scale |
| Clinical signs | Infection site is visually observed since there are melanin depositions surrounding a metacercaria infecting to the scale (Fig. 1). |
| Parasitology | The metacercaria is 0.15-0.18 mm in size (Fig. 2). Metagonimus spp. infect to mammals and birds (final host), passing through snails (first intermediate host) and fishes (second intermediate host) (Ogawa, 2004). M. yokogawai and M. miyatai infect to ayu, while M. takahashii infect to common carp and crucian carp. |
| Pathology | Pathological effects to fishes are low. Neither abnormal swimming nor mortalities are reported. |
| Health hazard | Adults infect to the small intestine of human. In heavy infection, they cause the diarrhea and the stomachache, but don’t cause the severe disease. An antihelminthic is effective for treatment. |
| Diagnosis | Parasites from common carp and crucian carp are M. takahashi. A parasite from the muscle of ayu is M. yokogawai, but one in the scale of ayu is M. yokogawai or M. miyatai. It is difficult to distinguish these two species by the morphology. |
| Other information | Infection to ayu started in April and peaked in August in Lake Biwa (Saito, 1999). This result indicates that release of cercariae is influenced by the water temperature. Similar tendencies (on release of cercariae) were observed in the river. Infection rate in wild ayu was high, and cultured ayu reared by the river water were infected. Meanwhile, those reared by the well water weren’t infected. |
| References | Ogawa, K. (2004): Metazoan
diseases. Infectious and parasitic diseases of fish and shellfish.
(Wakabayashi, H. and K. Muroga), Koseisha
koseikaku, pp. 381-405. (In Japanese) Saito, S. (1999): Metagonimus ? research done after 1960 -. Progress of medical parasitology in Japan. Meguro Parasitological Museum, 7, pp.205-215 (In Japanese). |
Fig. 1. Black spots (arrows) on the skin of ayu.
(Photos by K. Momoyama)
Fig. 2. Metacercaria of Metagonimus yokogawai
