Helminth Taxonomy - The Digenean Flukes (Flatworms)

Class Trematoda

Sub Class Digenea

These are usually endoparasites of man and many other animals. These have intestines, and are active feeders. Lifecycles are more complicated compared to the monogenean flukes, with two or three hosts in their lifecycles. For example, some, such as the schistosomes, have only one intermediate snail host, whose free-swimming cercaria on being shed from the snail can directly infect the definitive host. With some, (such as Fasciola hepatica), the cercaria encyst on vegetation, and must be ingested to infect their definitive hosts, or for others (such as Paragonimus sp.), the cercaria invade a second intermediate host such as a crustacean and encyst as a metacercaria within the hosts tissues. The definitive host in these cases is infected on ingesting the raw or undercooked second intermediate host. In almost all cases however, the first intermediate host is a mollusc, and it is here that a series of asexual divisions occur, though the details of these vary from species to species. These asexually dividing molluscan forms are either simple sac like sporocysts, or may be equipped with a rudimentary gut, oral sucker and other appendages, in which case they are referred to as redia. In some such as the schistosomes and Dicrocoelium dendriticum there are two generations of sporocysts, mother sporocysts giving rise to daughter sporocyst, which in turn divide to form the cercaria. In others, such as F. hepatica, the miracidium gives rise to a single generation of sporocysts, followed by a generation of redia before the cercaria are produced. Common human parasites include the schistosomes, the lung fluke Paragonimus, Fasciola hepatica and the Chinese liver fluke Opisthorchis sinensis, as well as a number of more minor species.

digean lifecycle

The digeneans are mostly hermaphroditic, except for a few examples such as the schistosomes, with either asexual self fertilisation, or cross-fertilisation with other individuals if they are available. Most digeneans are oviparous. Ova on leaving the single ovary enter an organ called an ootype before reaching the uterus. Here cells from the vitelline glands cover the ovum, these vitelline cells producing both the egg yolk and the hard outer egg shell. The eggs are usually operculate (except for the schistosome eggs), and contain a miracidium, the infective form for the snail intermediate host.

This miracidium is usually a short lived free swimming form that is invasive for the snail, an exception here being D. dendriticum, where the eggs must be swallowed by the snail, miracidia hatching in the snails gut. In general, the larvae are very host specific, the adults less so in terms of their hosts. Most digeneans are located in the intestinal tract, although different species may be found in many other locations.
Their teguments are covered on the external surface with spines or scales, and is an active syncytium. They have usually two suckers, an oral sucker, within which is situated the opening to the gut, and a ventral sucker (acetabulum), although some species may have a third (e.g. members of the family Heterophyidae). There is a simple digestive system, with a pharynx and oesophagus leading to a blind ended gut, which usually bifurcates and may be highly branched). They have a primitive protonephridial excretory system with waste products impelled along excretory canals by means of flame cells. The nervous system is very simple and sense organs as such do not appear to be present in the adult, although they may be in some of the larval forms where they aid in host location.

The pathology due to infection with these parasites is usually very density dependent, only in cases of very high worm burdens is severe pathology present, whilst at lower densities symptoms may be unpleasant, but are not necessarily life threatening. Pathology may be due to feeding activity by the adults (for example with F. hepatica), or due to immune responses to eggs trapped in the tissues (for example the schistosomes).

Examples (including all major human infecting species):

Super order Anepitheliocystida

Order Strigeatida
- Cercaria with forked tails

Family Diplostomatidae. Metacercaria encyst in fish and amphibian intermediate hosts.
e.g. Alaria alata - Parasitic in cat, dog, fox and mink. Here cercaria penetrate amphibians, acting as second intermediate hosts, where they encyst as mesocercaria (a "resting" form of the parasite, see the digenean biology pages within this site). The definitive host may then either be infected by eating the amphibian, or other animals may eat the amphibian, the mesocercaria undergoing a partial, arrested, development, accumulating in various tissues. These third paratenic intermediate hosts can include snakes, birds and most mammals including accidental infections in humans. The definitive host may then also become infected by eating these paratenic hosts.
Family Schistosomatidae - These are relatively unusual Digeneans, as they have separate sexes, and adult forms are located within the circulatory system. The cercaria are directly infective to the definitive hosts.

Subfamily Schistosomatinae - With well developed gynacophoric canal
Genus Schistosoma *- Parasites of man and other mammals.
Order Echinostomatida - Cercaria with simple tails, which encyst as metacercaria (an encysted dormant stage) on vegetation.

Family Fasciolidae - Large, flattened, leaf-like parasites of mammals
e.g. Fasciola hepatica - Liver Fluke. In the bile duct and liver of humans and ruminant mammals. Metacercaria encyst on vegetation, infection of the definitive host is by ingestion of contaminated vegetation.

e.g. Fasciolopsis buski - Parasite of the human small intestine. Metacercaria encyst on vegetation, infection of the definitive host is by ingestion of contaminated vegetation.

Family Paramphistomatidae - Large, thick bodied, gut parasites, with an anterior oral sucker, and posterior ventral sucker.
e.g. Gastrodiscoides hominis - Parasite of the human caecum. Metacercaria encyst on vegetation, infection of the definitive host is by ingestion of contaminated vegetation.

Family Echinostomatidae
- The Echinostomes, intestinal parasites of birds, reptiles and some mammals. Elongated flukes with raised collars, equipped with backward directed spines, behind the oral sucker.
e.g. Echinostoma sp. Some of these species may infect man
Super order Epitheliocystida

Order Plagiorchiida
- Cercaria with unforked tails which lack excretory vessels. Cercaria have an oral stylet in the oral sucker

Family Dicrocoeliidae - Small flukes, parasitic in the gut, liver, gall bladder and pancreas of most vertebrates. Cercaria encyst in arthropods.

e.g. Dicrocoelium dendriticum - Parasitic in the bile duct of sheep, cattle, pigs and other mammals (rarely man). Cercaria are ingested by ants of the genus Formica as second intermediate hosts, where they encyst as metacercaria. The definitive host becomes infected by accidental ingestion of ants.

Family Troglotrematidae - Flukes found in lungs, intestine, nasal cavity and subcutaneous tissues in birds and mammals.

e.g. Paragonimus westermani - Lung Fluke. In man and other carnivores (particularly cats). Cercaria invade crustaceans (crabs) as second intermediate hosts, where they encyst as metacercaria.

Order Opisthorchiida - Cercaria with unforked tails which have excretory vessels. Cercaria do not have an oral stylet.

Family Heterophyidae - Small flukes in birds and mammals. May have a small genital sucker associated with the ventral sucker.

e.g. Heterophyes heterophyes - Parasitic in the small intestine of humans and other fish eating mammals. Cercaria invade fish as secondary intermediate hosts, where they encyst as metacercaria.

e.g. Metagonimus yokogawai - Parasitic in the small intestine of humans and other fish eating mammals . Cercaria invade fish as secondary intermediate hosts, where they encyst as metacercaria.
Family Opisthorchiidae - Weakly developed suckers. Found in gall bladder and bile duct of mammals, birds and reptiles.
e.g. Opisthorchis sinensis - Parasitic in the bile duct of humans and fish eating mammals (used to be called Chlonorchis sinensis). Cercaria invade fish as secondary intermediate hosts, where they encyst as metacercaria.

N. B. The trematodes also include the Aspidogastreans.

Class Cestoda (tapeworms)
Introduction to Platyhelminthes