Natural mass infection by heterophyid metacercariae in aquacultured Japanese eel in Taiwan

A natural mass infection of heterophyid metacercariae in aquacultured Japan ese eel Anguilla japonica in Taiwan was observed. Of the 28 000 adult eels in 2 ponds, about 25 000 (90 %) showed swollen, cloudy and white eyes. Alth ough morbidity was about 90%, there was no mortality among the affected eel s. Histopathological sections showed edema and hemorrhage of the eye. Numer ous metacercariae were observed in the muscle tissues around the eyeball, t he subcutaneous tissue and even in the cartilage. Of the 6 eels digested wi th artificial gastric juice, all were found to contain metacercariae in the ir muscle tissues.-The average number of metacercariae recovered from the 6 eels was 1219, with a range of 50 to 3762. These metacercariae, when fed o rally to immunodeficient (scid) mice, developed into adult worms which were identified as Procerovum cheni Hsu 1950. The naturally infected eels were transferred to a new pond without snails and their eye lesions were not app arent anymore after 2 wk. In a follow-up investigation, 19 of 20 apparently healthy eels in a nearby aquaculture farm were found to harbour metacercar iae in their muscles. However, the number of the metacercariae ranged from 1 to 14, with an average of 4.21. This is the first report of heterophyid m etacercariae causing mass morbidity iri aquacultured eels.