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ABSTRACT: The attachment and penetration of Centrocestus armatus cercariae into the fish host Zacco temmincki are described in this study. Light and scanning electron microscopy (SEM) were used to examine the topographical features and behavior of cercariae. Histochemistry and transmission electron microscopy (TEM) were employed to trace glandular products and secretions released by cercariae during penetration. Cercariae are first carried into the fish gill chambers via the respiratory currents. The frequency of respiratory-current reversals of fish increased when infected with cercariae. The behavior of cercariae, during breaks in current flow that preceded each current reversal, was observed using a specially devised apparatus. Cercariae produce a mucuslike secretion upon attachment, shed their tail, and employ a brief period of leech-like creeping behavior before penetration. In all cases, the site of penetration was via the surface of the primary gill lamellae. SEM revealed a well-developed anterior penetration apparatus, and a highly contractile body region, that created a driving force for penetration. TEM and histochemistry showed that the mucus observed on the surface tegument of cercariae during attachment were glandular secretions from the parasite. The significance of fish respiratory current reversals to the success of cercariae penetration nicely illustrates the exploitation by the parasite of a host response to a stimulus.
Centrocestus armatus is a heterophyid trematode. The morphological characteristics of the adult worm was described by Woo et al. (1998), and additional detail was provided by Kimura et al. (2007), who also documented the life cycle in the laboratory. The adult worms are usually found in the intestines of fish-eating birds such as Nycticorax nycticorax; sporocysts, rediae, and cercariae develop in freshwater snails, such as Semisulcospira libertina. Metacercariae commonly occur in freshwater cyprinid fishes such as Zacco temmincki and Zacco platypus.
Centrocestus armatus is widely distributed in Japan, Taiwan, the Korean Peninsula, and China, where it has been reported in a wide range of wild and domestic animals (Tanabe, 1922). A number of isolated cases of human infections have also been reported from these same countries (Kurokawa, 1935; Waikagul, 1977; Hong et al., 1988). The epidemiology (Kimura and Uga, 2003, 2005) and the transmission dynamics between the 2 intermediate hosts have recently been described (Palier et al., 2007). In earlier experiments, we also observed that the...