REVIEW One hundred years of research on the natural infection of freshwater snails by trematode larvae in Europe Elżbieta Żbikowska & Anna Nowak Received: 9 April 2009 / Accepted: 24 April 2009 / Published online: 13 May 2009 # Springer-Verlag 2009 Abstract Research on the infection of snails by trematodes has been conducted in Europe for over a hundred years. The initial poor knowledge of the intra-molluscan stages of these parasites together with the difficulty of classifying them constituted a serious obstacle to the undertaking of integrated parasitological and malacological efforts to gain a better understanding of the phenomenon. The compilation of morphological and anatomical results of research on trematode larvae resulted in the publication of keys to designate species of parasites, but was not sufficient to encourage malacologists to collaborate with parasitologists. This paper undertakes to collect data published over the last hundred years on the natural infection of European popula- tions of freshwater snails by trematode larvae. The aim of this undertaking is to make researchers of malacofauna and, above all, experts on freshwater snails aware of the scale of the problem of molluscs being exploited as intermediate hosts of trematodes and, consequently, to encourage parasitologists and malacologists to collaborate on this phenomenon that is crucial for both parasites and hosts. Introduction Freshwater snails play a key role in the life cycle of trema- todes. They serve not only as a source of food and a place of reproduction for these parasites but also as a means of transport, thanks to which the parasite can get to its next host (Lockyer et al. 2004). The uniqueness of the relationship between these organisms derives from the fact that the majority of the 18,000 known species of trematodes exploit snails as the mandatory first intermediate host (Littlewood and Bray 2001). Many researchers have no doubt that these parasites were originally linked only with molluscs, and only as a consequence of evolutionary development did they include in their life cycle a further group of hosts (Pojmańska and Grabda-Kazubska 1985; Cribb et al. 2001). The intra-molluscan development of trematodes begins with the swallowing of the eggs of the parasite or the active penetration by the miracidium. In both cases, the ciliated larva casts off the ciliated epithelium in the body of the snail, metamorphosing into the next stadium—mother sporocyst or redia. The remaining larvae can generate daughter stage sporocysts in which tens or even hundreds of thousands of cercariae are ultimately formed, succes- sively leaving the snail’ s body, with the duration of the release of the cercariae depending not only on the species of parasite but also the health of the host (Wright 1971). Cercariae, depending on the species, transform into the next stage, metacercariae, or penetrate directly into the definitive host. Metacercariae encyst in the external environment or inside the second intermediate host, which can be either vertebrates or invertebrates—including snails (Furmaga 1968; Wright 1971). Cercariae of some species of trema- todes can transform into metacercariae in the snails in which they were formed (Styczyńska-Jurewicz 1962) or after penetrating into other individuals of the same or another species, usually locating themselves on the lining of the mantle cavity, pericardial cavity and kidney lumen (Adam and Lewis 1992; Skirnisson et al. 2004). Host snails in which cercariae or metacercariae appear are an important link in the distribution of trematodes in vertebrate populations, including farm animals and humans. This fact was decisive in initiating research on the occurrence of Parasitol Res (2009) 105:301–311 DOI 10.1007/s00436-009-1462-5 E. Żbikowska (*) : A. Nowak Department of Invertebrate Zoology, Institute of General and Molecular Biology, Nicholas Copernicus University, Toruń, Poland e-mail: ezbikow@biol.uni.torun.pl