| Parasite | Neoheterobothrium hirame |
|---|---|
| Taxonomy | Plathyhelminthes, Monogenea, Polypisthocotylea |
| Host | Japanese flounder (Paralichthys olivaceus) |
| Infection site | Gill filament, buccal cavity wall |
| Clinical signs | The gills become pale (even whitish) and anaemic by blood-feeding of the parasite in heavy infection. |
| Parasitology | The body of an adult parasite is max. 33 mm long and elongated. A larva
attaches the gill filament, and then migrates to the branchial cavity followed
by the maturation (Fig. 1). The parasite produces eggs on about 1-2 months
at 15-20 C after infection (Tsutsumi et al., 2003). The adult grasps the buccal cavity wall by a haptor with 4 pairs
of clamps at the posterior end (Fig. 2). The hermaphroditic adult produces
several hundreds eggs (per 1 day) with long filaments at both sides. The
eggs hatch on 1-3 weeks at 10-25 C., but do not hatch at 30 C. (Yoshinaga
et al.,
2000a). |
| Pathology | The anaemia is a hypochromic
microcytic anaemia accompanied with hypoglobulia, the decrease of hemoglobin
contents and the appearance of immature erythrocytes (Nakayasu et al., 2002). There is concern that Neoheterobothrium hirame contributes to the decrease
of wild population of Japanese flounder. |
| Health hazard | Since this parasite is not infectious to human, it is harmless in food hygiene. |
| Diagnosis | Adults attaching the buccal cavity can be visually seen. Immature parasites attaching the gill filament can be observed under the light microscope. Identification is easy because no similar monogenean parasitizing Japanese flounder has been known. |
| Other information | Anaemic Japanese flounder has been reported since about 1995. A causative agent of the disease was first suspected to be virus, but now demonstrated to be Neoheterobothrium hiram. The parasite might have been introduced to Japan from U.S.A. A bath treatment for 60 min. with 3 % NaCl-supplemented seawater is effective for removing immature parasites on the gills. However, this treatment has only a limited effect on the adult attaching the buccal cavity (Yoshinaga et al., 2000b). |
| References | Nakayasu, T., T.
Yoshinaga and A. Kumagai (2002): Hematological characterization of anemia
recently prevailing in Japanese flounder. Fish
Pathol., 37, 38-40. Tsutsumi, N., T. Yoshinaga, T. Kamaishi, C. Nakayasu and K. Ogawa (2003): Effects of water temperature on the development of the monogenean Neoheterobothrium hirame on Japanese flounder Paralychthys olivaceus. Fish Pathol., 38, 41-47. Yoshinaga, T., I. Segawa, T. Kamaishi and M. Sorimachi (2000a): Effects of temperature, salinity and chlorine treatment on egg hatching of the monogenean Neoheterobothrium hirame infecting Japanese flounder. Fish Pathol., 35, 85-88. Yoshinaga, T., T. Kamaishi, I. Segawa and E. Yamamoto (2000b): Effects of NaCl-supplemented seawater on the monogenean, Neoheterobothrium hirame, infecting the Japanese flounder. Fish Pathol., 35, 97-98. |
Fig. 2. A stained specimen of N. hirame
(Photos by K. Ogawa)
Fig. 1. A mass of N. hirame (arrow) attaching on the buccal cavity wall of Japanese flounder