Development of the head collar and collar spines during the larval stages of Isthmiophora hortensis (Digenea: Echinostomatidae)

It is uncertain when the head collar and collar spines of Isthmiophora hortensis (Digenea: Echinostomatidae), a zoonotic echinostome species in Far Eastern Asia, develop during its larval stages. In this study, the appearance of the head collar and collar spines was studied using light and scanning electron microscopy in cercariae and metacercariae experimentally obtained from freshwater snails (Lymnaea pervia) and tadpoles (Rana nigromaculata), respectively. The cercariae were shed from the snail on day 30 after exposure to laboratory-hatched miracidia. Metacercariae were obtained from the experimental tadpoles at 3, 6, 12, 15, 20, 24, 26, and 30 h after exposure to the cercariae. The head collar was already visible in the cercarial stage, although its degree of development was weak. However, collar spines did not appear in the cercarial stage and even in the early metacercarial stage less than 24 h postinfection in tadpoles. Collar spines became visible in the metacercariae when they grew older than 24 h. It was concluded that the head collar of I. hortensis developed early in the cercarial stage, but the development of collar spines did not occur until the worms became 24-h-old metacercariae in our experimental setting. Counting the number of collar spines was concluded as an unfeasible diagnostic method for I. hortensis cercariae when they are shed from the snail host.