Development and applications of cestode and trematode laboratory models.

The phylum Platyhelminthes comprises important endoparasitic organisms, some of which can cause serious diseases in humans and other mammalian hosts. Cestodes and trematodes are characterized by, sometimes complex, life cycles with distinct larval and adult stages and obligatory changes between intermediate and final hosts. Most adult cestodes parasitise the intestine of a definitive host, where the sexual development takes place. Selfor cross-fertilisation results in the production of eggs, which contain a zygote eventually forming a first larval stage (oncosphere), and which is then taken up by an intermediate host. Within the intermediate host, the parasite is usually targeted to distinct compartments or organs, and develops into a second larval organism, within which the pre-adult parasites develop. The life cycle is completed when the infected intermediate host tissue is consumed by a definitive host. Depending on the species, cestode larvae can localise in a variety of organs, but often they are targeted to a specific site, causing diverse pathologies which depend on the localisation, the reproductive potential and the size of parasites. For many years, researchers have been studying medically and economically important members of this class. They are represented by several species, most of them included in the family Taeniidae, which can cause severe diseases such as echinococcosis and cysticercosis. Others such as Hymenolepis, Mesocestoides, Schistocephalus and Ligula have less medical importance, but can still be economically relevant. Due to the fact that they could be relatively easily maintained in the laboratory either in vitro or in a laboratory animal host, they represent important model systems for investigating various aspects of cestode biology, some of which are covered in this special issue. Digenean trematodes are similar to cestodes, since they also exhibit an amazing variety of life cycles, which include at least one (sometimes more) intermediate and one final host. The former is usually a mollusc, the latter is almost always a vertebrate. As with cestodes, most trematodes are monoecious, with schistosomes being the important exception from that rule. Although over 20,000 trematode species have been described, only few cause important diseases, particularly in the tropics. Clearly the genus Schistosoma is foremost among those. Schistososomes have separate sexes, and the final host is infected percutanously by an actively swimming cercaria. Infections are acquired in freshwater and may lead to serious disease and death by worms located in the blood vessels, urinary tract or intestine, depending on the species. Infections with liver flukes such as Clonorchis sinensis, Opisthorchis felineus and O. viverrini are acquired by eating raw or insufficiently cooked freshwater fish of many species. Serious symptoms such as liver cirrhosis and even death occur. Another liver fluke, Fasciola hepatica, is rare in humans and common in sheep and cattle, and symptoms of infection can be serious. Infection is acquired by ingesting metacercariae encysted on vegetation. Metacercariae, attached to aquatic plants, are also the source of infection by the giant intestinal fluke Fasciolopsis buski of man and pigs, which is common in many parts of Eastern, Southeastern and Southern Asia, and on Pacific islands.