An overview of cercariae from the Egyptian inland water snails

Abstract

In digenetic trematodes, cercariae represent a juvenile stage of the vertebrate-inhabiting adult worms. In addition, the name cercaria can be used properly in a generic sense for a species in which the adult stage is unknown, as is done with the term cysticercus and microfilaria among some cestodes and nematodes, respectively[1,2]. Generally, the cercaria has an oval or elongated body, flattened in the dorsoventral plane, and a tail. An oral sucker is localized subterminally at the anterior extremity of the body. There may be also a ventral sucker which is located in the middle or posterior part of the body. In addition, there are digestive and protonephridial excretory systems, different types of glands, genital primordium and sensory organs (external sensory papillae and eyespots). Morphology of the tail varies considerably in the different species. It may be considerably shortened or completely lost. The range of variation in cercarial morphology is considerable, and most have specializations that enable them to survive a brief free-living existence and make themselves available to next host[2,3]. A classification system has been created by Lühe in 1909 for grouping of cercariae into several types based on their morphological variations[4]. In this system, salient morphological characteristics are used for grouping cercariae into various major groups, and each major cercarial group may be divided into subgroups on the basis of minor morphological differences. In this classification, Lühe recognized five major groups: lophocercariae, gasterostome, monostome, amphistome and distome cercariae[4]. The classification of Lühe was modified and supplemented by many subsequent workers. Probably the most complete version of this classification is available in the review by Dawes in 1946[5]. Species identification based on morphology of cercariae is usually difficult and unreliable. However, by using detailed morphological criteria alone, identification of cercariae to the family level, and occasionally to the genus level is possible[1]. In Egypt, since the pioneering work of Sonsino in 1892 and Looss in 1896 on the role of molluscs as intermediate hosts of digenetic trematodes[6,7], only fragmentary publications are available for identification of cercariae present in the country. In addition, some of such publications may not be accessible to some researchers. This is because they are either theses or published in old volumes of non-international journals. The present work was carried out with the aim of preserving our heritage through reviewing the different types of cercariae known from the Egyptian inland water snails. ARTICLE INFO ABSTRACT