Digenea
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 Helicometra, an opecoelid digeneanfrom the intestine of a fish
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Strigeidida |
Digenea (Gr. Dis - double, Genos - race) is a subclass within the Platyhelminthes consisting of parasitic flatworms with a syncytial tegument and, usually, two suckers, one ventral and one oral. They are particularly common in the digestive tract, but occur throughout the organ systems of all classes of vertebrates. Once thought to be related to the Monogenea, it is now recognised that they are closest to the Aspidogastrea and that the Monogenea are more closely allied with the Cestoda. Around 6000 species have been described to date.
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[edit] Morphology
[edit] Key features
Characteristic features of the digenea include a tegument. They possess a vermiform, unsegmented body-plan.
There are typically 2 suckers, an anterior oral sucker surrounding the mouth, and a ventral sucker sometimes termed the acetabulum, on the ventral surface.
Monostome is a term used to describe worms with one sucker (oral). Flukes with an oral sucker and an acetabulum at the posterior end of the body are called Amphistomes. Distomes are flukes with an oral sucker and a ventral sucker, but the ventral sucker if somewhere other than posterior.
[edit] Reproductive system
Adult digeneans are commonly hermaphroditic. This is likely to be an adaptation to rarity, allowing the life cycle to continue when only one individual successfully infects the final host. Fertilisation is internal, with sperm being transferred via the cirrus to the Laurer's Canal or genital aperture. A key group of digeneans which are dioecious are the schistosomes. Asexual reproduction is ubiquitous in the first larval stage. While the formation of the digenean eggs and of asexual reproduction in the first larval stage (miracidium) is well reported, the developmental biology remains a problem which is very complex and far from being solved. Electron microscopic studies have shown that the light microscopically visible germ balls consist of mitotically dividing cells which give rise to embryos and to a line of new germ cells that become included in these embryonic stages. Since the absence of meiotic processes is not proven, the exact definition remains doubtful.
[edit] Male organs
Protandry is the general rule among the Digenea. Usually 2 testes are present, but some flukes can have more than 100. Also present are vasa efferentia, a vas deferens, seminal vesicle, ejaculatory duct and a cirrus (analogous to a penis) enclosed is a cirrus sac.
[edit] Female organs
Usually there is a single ovary with an oviduct, a seminal receptacle, a pair of vitelline glands (involved in yolk and egg-shell production) with ducts, the ootype (a chamber where eggs are formed), a complex collection of glands cells called Mehlis’ gland, which is believed to lubricate the uterus for egg passage. In addition, they possess a canal called Laurer's Canal, which leads from the oviduct to the dorsal surface of the body. Most trematodes possess an ovicapt, an enlarged portion of the oviduct where it joins the ovary. It probably controls the release of ova and spaces out their descent down the uterus.
[edit] Digestive system
The great majority of digenetic trematodes are inhabitants of the vertebrate alimentary canal or its associated organs, especially the liver, bile duct, gallbladder, lungs, pancreatic duct, ureter and bladder. These are organs containing cavities rich in potential semi-solid food materials such as blood, bile, mucous and intestinal debris. Most species possess a mouth and forked, blind ended digestive system and feed actively. They are also capable of direct nutrient uptake through the tegument.
[edit] Nervous system
Paired ganglia at the anterior end of the body serve as the brain. From this nerves extend anteriorly and posteriorly. Sensory receptors are, for the most part, lacking among the adults, although they do have tangoreceptor cells. Larval stages have many kinds of sensory receptors, including light receptors and chemoreceptors. Chemoreception plays an important role in the free-living miracidial larvae recognising and locating its host.
[edit] Life cycles
There is a bewildering array of variations on the complex digenean life cycle, and plasticity in this trait is almost certainly a key to the group's success. In general, the life-cycles may have two, three, or four obligate (necessary) hosts, sometimes with transport or paratenic hosts in between. The three host life-cycle is probably the most common. In all species bar a couple of odd exceptions, the first host in the life-cycle is a mollusc. This has lead to the inference that the ancestral digenean was a mollusc parasite and that subsequent hosts have been added by terminal addition. In all digenean life cycles, alternation of generations is an important feature. This phenomenon involves the presence of several discrete generations in one life-cycle.
A typical digenean trematode life-cycle is as follows. Eggs leave the vertebrate host in faeces and use various strategies to infect the first intermediate host, in which sexual reproduction does not occur. Digeneans may infect the first intermediate host (usually a snail) by either passive or active means. The eggs of some digeneans, for example, are (passively) eaten by snails (or, rarely, by an annelid worm) in which they proceed to hatch. Alternatively, eggs may hatch in water to release an actively swimming, ciliated larva, the miracidium, which must locate and penetrate the body wall of the snail host.
After post-ingestion hatching or penetration of the snail, the miracidium metamorphoses into a simple, sac-like mother sporocyst. The first generation is thus the Egg-Miracidium-Mother sporocyst sequence. The mother sporocyst undergoes a round of internal asexual reproduction, giving rise to either rediae (sing. redia) or daughter sporocysts. The second generation is thus the daughter parthenita sequence. These in turn undergoe further asexual reproduction, ultimately yielding large numbers of the second free-living stage, the cercaria (pl. cercariae).
Free-swimming cercariae leave the snail host and move through the aquatic or marine environment, often using a whip-like tail, though a tremendous diversity of tail morphology is seen. Cercariae are infective to the second host in the life cycle, and infection may occur passively (e.g., a fish consumes a cercaria) or actively (the cercaria penetrates the fish).
The life cycles of some digeneans include only two hosts, the second being a vertebrate. In these groups, sexual maturity occurs after the cercaria penetrates the second host, which is in this case also the definitive host. Two host life-cycles can be primary (there never was a third host) as in the Bivesiculidae, or secondary (there was at one time in evolutionary history a third host but it has been lost).
In three host life cycles, cercariae of these taxa develop into a resting stage in the second intermediate host,, the metacercaria, which is usually encysted in a cyst of host and parasite origin, or encapsulated in a layer of tissue derived from the host only. This stage is infective to the definitive host. Because the body of the cercaria ultimately becomes the body of the adult, so the final generation is the Cercaria-Metacercaria-Adult sequence. Infection of the definitive host occurs when the intermediate host is eaten by the definitive host. Metacercariae excyst in the definitive host’s gut in response to a variety of mechanisms, such as gut pH levels, digestive enzymes, temperature, etc. Once excysted, adult digeneans migrate to more or less specific sites in the definitive host and the life cycle repeats.
[edit] Human digenean infections
Only about 12 of the 6,000 known species are infectious to mankind, but some of these species are important diseases with of 200 million people infected world wide. The species that infect humans can be divided into groups, the Schistosomiasomes and the non-Schistosomiasomes.
[edit] Schistosomiasomes
The Schistosomiasomes are all parasites of the circulatory system of their primary host, meaning they live and feed inside the blood vessels. Because of this they are all very thin animals, ranging in size from 10 to 30 mm (0.43 to 1.26 ins) in length to 0.2 to 1.0 mm in diametre. The males are shorter and thicker than the females. Females only reach sexual maturity after they have been mated by a male, the male has a long groove along one entire side of his body that he uses to clasp the female. After mating the two remain locked together for the rest of their lives. they can live for several years and produce many thousands of eggs.
Four species of Schistosomiasis are found to effect human beings, they are all members of the genus Schistosoma and all have snails as the intermediate host, S. mansoni is the most common while S. japonicum represents the greatest problems in control because it infects a large number of non-human mammals such as Cattle, Dogs and Rats.
| Scientific Name | Snail Genera | Endemic Area |
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| Schistosoma mansoni | Biomphalaria spp. | Africa, South America, Caribbean, Middle East |
| Schistosoma haematobium | Bulinus spp. | Africa, Middle East |
| Schistosoma japonicun | Oncomelania spp. | China, East Asia, Philippines |
| Schistosoma intercalatum | Bulinus spp | Africa |
[edit] non-Schistosomiasomes
There seven major species of non-Schistosomiasomes which infect mankind, all infect people when they are eaten as metacercarial cysts - all leave in human faeces. One species, Paragonimus westermani lives in the lungs, and so can also have its eggs passed out in saliva. The other six species live in the human digestive system. Normally infection with these parasites is mildly to (occasionally) seriously unpleasant, only very rarely is it life-threatening.
| Scientific Name | Snail Genera | Mode of Infection | Endemic Area |
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| Fasciolopsis buski | Segmentina sp. | Plants | Asia, India |
| Heterophyes heterophyes | Pirinella | Mullet, Tilapia | Asia, Eastern Europe, Egypt, Middle East |
| Metagonimus yokogawaii | Semisulcospira sp. | Carp, Trout | Siberia |
| Gastrodiscoides hominis | Helicorbis sp. | Plants | India, Vietnam, Philippines |
| Clonorchis sinensis | Bulinus sp. | Fish | East Asia, North America |
| Fasciola hepatica | Lymnea sp. | Plants | Central America, North America, South America |
| Paragonimus westermani | Oncomelania sp. | Crabs, crayfish | Asia |
[edit] Important publications
Key to the Trematoda, vol.1 Gibson, D.I., Jones, A., and Bray, R.A. (2002) ISBN 0-85199-547-0
