6.3.2: Trematodes (Flukes)

Trematodes are one type of platyhelminth (flatworms). Trematodes are commonly referred to as flukes. The typical trematode has a flat-leaf shaped body that exhibits bilateral symmetry.

They lack circulatory systems; thus, their flat body pattern is important for the diffusion of nutrients throughout the organism. Typical trematodes are also monoecious. Monoecious organisms are organisms that have male and female reproductive structures in the same individual; hermaphroditic. Trematodes are also characterized by having incomplete digestive systems. Their digestive systems are composed of an oral sucker, a pharynx, and intestines that end in blind pockets. Parasitic trematodes have structures that they utilize to aid in remaining attached to their host organism. Such structures include the oral sucker and ventral sucker (acetabulum).

Trematodes can be grouped based on the type of tissues in which they reside within their hosts. Blood trematodes reside within the blood vessels of the digestive or urinary tracts and feed on blood. Tissue trematodes live within tissues and organs such as the epithelium of the small intestine, bile ducts, gallbladder, liver, or lungs. Examples of trematodes that are considered blood flukes include Schistosoma mansoni, Schistosoma japonicum, and Schistosoma haematobium. Tissue flukes include species such as Clonorchis sinensis, Fasciola hepatica, and Fasciolopsis buski.

Most trematodes have life cycles that include an intermediate host and a definitive host. The intermediate hosts are typically freshwater snails. Definitive hosts are vertebrate animals, including humans, domestic animals, birds, fish, and others. Definitive hosts harbor the adult trematode, which reproduces sexually and releases eggs. When the eggs exit the definitive host in fecal material and are introduced to freshwater snails (the intermediate host), they enter the intermediate host and reproduce asexually. From the intermediate host the trematodes can be transmitted to a new definitive host by several different mechanisms, depending on the species of trematode involved. Some species such as Clonorchis sinensis and Fasciola hepatica are transmitted via food-borne transmission. Trematodes such as Schistosoma mansoni, Schistosoma haematobium, and Schistosoma japonicum are acquired via direct penetration of cercariae through the host’s skin when the host is in contaminated water.

In this exercise you will survey two trematodes that demonstrate the typical body pattern and reproductive organization (Clonorchis sinensis and Fasciola hepatica), and two trematodes that do not follow the typical body pattern or reproductive organization (Schistosoma mansoni and Schistosoma japonicum).

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