| Parasite | Cardicola sp. |
|---|---|
| Taxonomy | Platyhelminthes, Trematoda, Strigeidida |
| Host | Northern bluefin tuna (Thunnus thynnus) |
| Disease name | Blood fluke disease |
| Infection site | Branchial vessel |
| Clincal sign | No external signs are evident. Parasite eggs are microscopically observed to be fully packed in the capillaries of gill filament. |
| Parasitology | Adult worm parasitizes the
artery of the gill lamellae. The body is 2-3 mm long, leaf-like and elongated.
This parasite is apparently different species from the blood flukes
parasitizing greater amberjack, yellowtail or tiger puffer and from Cardicola forsteri, a parasite of
Sorthern bluefin tuna (Thunnus maccoyii) reported in Australia. Intermediate host could be
involved in the life cycle. |
| Pathology | Affected fish exhibits
circulatory impairment due to occlusion of the gill capillaries with parasite
eggs. It is likely that this parasite is highly pathogenic to host fish. |
| Health hazard | Since this parasite is not infectious to human, it is harmless in food hygiene. |
| Diagnosis | Parasites and parasite eggs are readily observed in the gill by light microscopy. |
| Other information | As growing interests on aquaculture of Northern bluefin tuna, this parasite has been recently found in cultured tuna. The extent of damage, distribution, seasonality and life cycle of the parasite are not well studied yet. In case of Cardicola forsteri in Southern bluefin tuna, just a single worm out of 10 fish was observed in March, when the fish were introduced to the sea cages. However, within the initial 2 months, the prevalences and intensities increased rapidly, peaking in May (prevalences: 100 %; intensities: an average of 27 flukes / fish) followed by declines (prevalences: 35 %; an average of 3 flukes / fish) in August. It was considered that the pathogenicity to Sourthern bluefin tuna was low because no mortalities attributed to C. forsteri were observed during the investigation (Aiken et al., 2006). Further investigations on the parasite of Northern bluefin tuna are warranted. |
| 参考文献 | Aiken, H. M., C. J. Hayward and B. F. Nowak (2006): An epizootic and its decline of a blood fluke, Cardicola forsteri, in farmed southern bluefin tuna, Thunnus maccoyii. Aquaculture, 254, 40-45. |
(Photos by I. Takami)
Fig. 2. An adult worm of Cardicola sp.
1.JPG)
Fig. 1. Many parasite eggs are accumulated in the gill capillary of
Northern bluefin tuna.
2.JPG)