The Basic Lifecycle of the Major Groups of the Digeneans
Most digeneans are hermaphroditic (the major exception being the schistosomes, and one other group). In the majority of these parasites self fertilization may occur, but cross fertilization between different individuals is more generally the rule. The sperm enter the female system, either via the Laurers canal or more commonly through the common genital atrium, which opens into the uterus.
1) The Digenean Trematode Egg
The formation of the digenean egg follows that described for the platyhelminthes as a group. Briefly, as the egg enters the öotype of the fluke it becomes surrounded by a predetermined number of vitelline cells, the number of which will be specific for different parasites, which form the food reserve of the egg. These vitelline cells produce globules of a mixture of proteins and phenols, which are extruded to the outer surface of the developing egg. Here the phenols oxidise to form quinone, which then coalesces with the protein, reacting to form scleratin, a hard inert yellowish substance, making up the egg shell. As the eggs of different species may vary in thickness, their colours may vary from yellow, to a dark brown.The digenean egg is usually operculate, in common with other platyhelminthes. Exceptions to this may occur however, the most important being with the schistosomes. Here the eggs are non-operculate, and are ornamented with spines, the appearance of which are characteristic for different species of schistosome.
The formation of the digenean egg follows that described for the platyhelminthes as a group. Briefly, as the egg enters the öotype of the fluke it becomes surrounded by a predetermined number of vitelline cells, the number of which will be specific for different parasites, which form the food reserve of the egg.
These vitelline cells produce globules of a mixture of proteins and phenols, which are extruded to the outer surface of the developing egg. Here the phenols oxidise to form quinone, which then coalesces with the protein, reacting to form scleratin, a hard inert yellowish substance, making up the egg shell. As the eggs of different species may vary in thickness, their colours may vary from yellow, to a dark brown.The digenean egg is usually operculate, in common with other platyhelminthes. Exceptions to this may occur however, the most important being with the schistosomes. Here the eggs are non-operculate, and are ornamented with spines, the appearance of which are characteristic for different species of schistosome. The eggs hatch of operculate eggs involves the release of the opercular cap. This takes place under a variety of conditions, modified according to the particular species of trematode. For example some trematode lifecycles involve the ingestion of the egg before hatching (e.g. Dicrocoelium dendriticum, the lancet fluke), whilst others such as those of Fasciola hepatica, (the liver fluke), hatch in water. For the eggs that hatch in the external environment a number of factors may be important, for example light, temperature and changes in osmotic pressure. Again the exact details of these environmental requirements will be optimised for the particular conditions which will maximise the chances of completion of the parasite lifecycle. In all cases the egg hatches to release the miracidium.
2) The Larval Digeneans - the Miracidium
The miracidium is the name of the ciliated larval stage that is hatched from the digenean egg. In comparison with the other larval platyhelminthes it is very similar to the larvae of the monogeneans, (the oncomiracidium) and the larval cestodarian, or lycophore. In most cases the miracidium is usually a free swimming stage,that seeks out the primary, and in some cases only, intermediate hosts of these parasites. In all cases these primary, or 1st intermediate hosts are molluscs. In the few examples where the miracidium is not a free swimming stage the eggs are ingested, as with the lancet fluke Dicrocoelium dendriticum. Here the eggs hatch in the intestine of the mollusc liberating the miracidium, from where it immediately penetrates the intestinal wall to invade the molluscan tissues. In the free swimming miracidia the larval parasite exhibits distinct behavioural responses that enable it to enter the environment of, detect the presence of its hosts. These behavioural responses have principally been studied in the case of the schistosome miracidium. Morphologically the surface of the miracidium is covered with a series of ciliated plates, which may be clearly seen using electron microscopy after removal of cilia. These ciliated epidermal plates (in some species the cilia being replaced by spines) are discontinuous, not being in contact with each other but being separated by extensions of the underlying subepidermal layer, the whole structure being illustrated below.
CP = Ciliated Plate
The plates themselves show a definite arrangement, being placed in four to five transverse rows, the exact arrangement of which may vary between different trematodes. Beneath the plates are layers of muscle fibres. At the anterior end of the larvae is a non-ciliated conical projection, the terebratorium, (or anterior papillae), bearing apertures of the apical and penetration glands. These themselves are found at the anterior end of the body. Miracidia possess a number of sensory organ, the most important of which are the dorsaly situated eye spots, beneath which is found the cerebral mass. Other sensory organs are situated within folds of the terebratorium. Below all of the structures is found the miracidium's large rounded germinal cells, which often are often grouped in clusters called germ balls. Finally the miracidia possess a protonephridial excretory system, basically similar to that found in the adult parasites. On examination of eggs containing mature miracidia it is the clearly seen that flame cell activity that is the first sign of the initiation of hatching of the egg.
On invasion of the molluscan tissue the miracidium sheds its ciliated plates, in almost all cases rapidly transforming into an endoparasitic form, the sporocyst, although in a few unusual groups the miracidium may contain a fully developed redia.
3a) The Larval Digeneans - the Sporocyst
The sporocyst develops within the molluscan host as a hollow fluid filled germinal sac, into which protrude germinal masses. At the conical anterior of the sporocyst body a birth pore is located, from which subsequent generations of larvae emerge. The germinal masses develop internally into either daughter sporocysts, which are essentially the same as their parent sporocysts, or into a second larval stage, the redia described below.
Different species of trematode will undergo different patterns of larval development, although the miracidium will always develop into a sporocyst to start with, and if daughter sporocysts are formed, redia do not develop. For example, in the schistosomes there are two generations of sporocyst in the snail host, but the redia stage is absent. In contrast in the lung fluke Paragonimus the parent sporocyst produces two generations of redia. In the organisms where there are two generations of sporocyst, these may be found in different locations within the body of their host, the locations differing between different species of trematode. For example in the trematode Schistosomatium douthitti mother sporocysts are found near the molluscan hosts oesophagus and cerebral ganglia. The daughter sporocysts when they emerge from the parental sporocyst migrate through the host tissues, localising near the molluscs digestive diverticulum. These cycles of asexual division within the mollusc result in an enormous increase in the reproductive potential of these organisms, unsurpassed within Metazoan organisms, whereby a single miracidium is capable of giving rise to many hundreds of thousands of cercariae. The sporocyst stage obtains nutrients by passage of soluble material across the sporocyst tegument.
3b) The Larval Digeneans - the Redia
The redia are the second larval form to develop within the molluscan host (but may be absent in some groups, such as the schistosomes). They are similar to sporocysts, containing germinal masses within a fluid filled sac, which may develop into either second generation daughter redia, or more commonly into the final larval stage within the mollusc, the cercaria.
They differ from the sporocysts however, in that they are a much more active form, and importantly they possess simple gut. The tissue they feed on is predominantly molluscan in origin, but the redia of some groups (e.g. those of the echinostomes) may actively seek out the developmental stages of other trematodes (e.g. schistosome sporocysts) within the same intermediate host. This was observed in a series of experiments carried out in the 1960's by Lie et. al.
The gut itself consists of a mouth, opening into a large muscular pharynx, which in turn opens into a simple rhabdocoel like intestine. Externally, behind the mouth many redia have a ridge-like collar, below which the birth canal opens and from which either cercariae or daughter redia emerge. Further along the body there a lobe like extensions of the body, which are thought to aid the movement of the parasite within its host's tissues.
An interesting exception to the general rule that cercaria are produced by the redia is found in a few tremadodes where the redia produce progenetic metacercaria, fully capable of producing viable eggs. In these few very unusual cases the trematode may only have a single molluscan host, although the metacercaria may still be capable of developing in a second host as well. Exceptions such as these, and those described above involving miracidia containing fully developed redia is evidence of the evolutionary past of these organisms. It has been noted that the redia bears some resemblance to some of the more advanced turbellarians, and as described above, this stage is a very active form of the parasite, fully capable of actively ingesting host material, and in some cases even predation of competing parasites within their hosts. It has been postulated that the group as a whole emerged from an ancestral parasitic turbellarian, with a single molluscan host, after the development of internal division and asexual reproduction, later developing specialised forms to exploit the varying environments that these organisms have to cope with.
4) The Larval Digeneans - the Cercaria
Some of the Types of Cercariae
(e.g. Fasciola sp.)
(e.g. Donax sp.)
(e.g. Schistosoma sp.)
(e.g. Bithynia sp.)
In almost all species of trematode it is the cercarial stage that emerges from the mollusc, and is the infective form for the vertebrate host, although there may be exceptions to this general rule. For example in some cases a sporocyst, modified to have a thickened internal wall resistant to the environment, emerges, to be ingested by a second intermediate host, (e.g. as is the case in the trematode Dicrocoeloides petiolatum) Other exceptions, involving redia producing progenetic metacercaria, have already been described above.
The trematode cercaria exhibits considerable variations in structure, which is very important taxonomically, and reflects in many cases adaptations to the specific lifecycle of the parasite involved. Because of this great diversity of form, a system of cercarial classification has evolved, based on the gross morphology of these larval forms. Firstly cercariae may be divided into three major groups;
i) Monostome Cercariae - These lack a ventral sucker, and have simple tails. These forms develop within rediae
ii) Amphistome Cercariae - In these the large ventral sucker is situated at the base of a slender unbranched tail. These forms develop within rediae.
iii) Distome Cercariae - This is the commonest cercarial form, with the ventral sucker lying some distance from the posterior end, in roughly the anterior third of the body. These distome cercariae may themselves be divided into a large number of subgroups, based on other morphological features, particularly the form that the cercarial tail takes. Some of these forms are described below;a) Leptocercous Cercariae - These cercariae have straight slender tails, which are much narrower than the cercarial body. This form is further subdivided into;i) Gymnocephalous Cercariae - In these the suckers are equal in size. This is a common form, represented within such species as Fasciola hepatica, and develop within rediaeb) Trichocercous Cercariae - These forms have long tails, equiped with rings of fine bristles. They are usually found in marine trematodes.
ii) Xiphidiocercariae - These are similar to the gymnocephalous forms, but in these the oral sucker is equiped with a stylet, used in penetration of their next hosts, and they generally develop within sporocysts. iii) Echinostome Cercariae - In these there is a ring of spines at the anterior end of the larvae, as in adult forms of these parasites. These are found within trematodes of the genus Echinostoma, and develop within rediae.
c) Cystocercous Cercariae - In these the end of the tail is highly enlarged, with a cavity into which the larval body may be retracted. These usually develop within sporocysts.
d) Microcercous Cercariae - Cercaria with vestigial tails, and which may develop within both rediae and sporocysts.
e) Cercariaea Cercariae - Cercaria with no tails, where the cercaria is not a free swimming form, and may develop within both rediae and sporocysts.
f) Furcocercous Cercariae - In these the tails are forked at the end. The cercaria of the most important group of trematodes, the schistosomes, have cercariae of this form. This form develops within sporocysts.
Otherwise, both externally and internally the structure of the body of the cercaria resembles that of the adult trematode into which they will grow. For example, the ring of spines found at the anterior end of echinostome cercariae are also present in the adult flukes.
The outer surface of the cercaria is a tegument, which may however differ from that found in the adult form in a number of ways. For example in the schistosomes the tegument is covered with a trilaminate plasma membrane, (as opposed to the two bi-lipid membranes found in the adult), on the outer surface of which there is a glycocalyx, (absent in the adult). However many other features of this tegument appear similar to that of the adult, the differences almost certainly being adaptations due to the differing environments that these two lifecycle stages experience. For example, spines are found on the surface of both forms of tegument, and the overall structure of a syncytium conected to subtegumental cells is the same. For more details on the structure of the tegument, go to the page devoted to the digenean tegument. Within the cercarial body a number of different types of gland cells may be found, including cystogenous gland cells, used by the larvae to secrete a cyst wall during formation of the metacercarial stage, and penetration gland cells, used by the cercaria to penetrate its next host, either a second intermediate host, or in some groups the definitive host, (such as the schistosomes), where the cercaria is the final larval stage.
The cercaria released from their molluscan intermediate host are usually a free swimming form. These must then locate either their next, and usually final intermediate host, their definitive host which they actively penetrate (e.g. in members of the family Schistosomatidae), or locate a suitable solid substrate to encyst upon, or be ingested by their definitive host (members of the family Azygiidae).
To locate these various targets the cercariae are equiped with a variety of sensory organs. These commonly include two or more eye spots, as well as touch receptors, and allow specialised cercarial behaviour, designed to bring the cercariae into an environment giving the maximum probability of infecting their next hosts. For example the cercariae of the schistosomes exhibit negative phototrophy (swimming to the surface of the water), and positive thermotrophy and thigmotrophy, being attracted to warm objects moving in the water. As well as these behavioural responses within the free swimming cercariae, the parasite exhibits definite circadian rhythms in terms of shedding from the molluscan host, again being shed at times optimal for bringing them into contact with their next host. For example the schistosome cercariae are generally shed during daylight, in the morning, whilst those of other species emerge only at night. For a more full treatment on the subject or circadian rhythms exhibited by parasites.
In a few groups, such as Alaria spp. However, the parasite employs three intermediate hosts. In these cases the cercaria penetrates the second intermediate host to form a resting stage, the mesocercaria described below. In these cases this second intermediate host is in turn ingested by a third intermediate host, where it encysts to form a metacercaria.
5) The Larval Digeneans - the Mesocercaria
The mesocercaria is essentially a resting stage within the parasite lifecycle, employing a second intermediate host in a parasite lifecycle utilising four hosts. It is defined as follows (by Pearson 1956);
The mesocercaria is a definite prolonged stage in the adult generation of strigeate trematodes, which closely resembles the cercarial body, from which it develops in the second intermediate host, and which does not possess metacercarial features; it develops in turn into the metacercaria in another host.
In parasites having this larval stage the mesocercaria are capable of infecting and surviving within a very wide range of paratenic hosts which may ingest the second intermediate host, thus in effect increasing the number of hosts which the parasite may use in its lifecycle. For example amphibians infected with mesocercaria of Alaria may themselves infect a wide variety of other amphibians, reptiles, birds and mammals if they are ingested by these animals.
6) The Larval Digeneans - the Metacercaria
This is a much more common "resting" larval stage of the trematode parasitic lifecycle, formed either in a final intermediate host (when a mesocercaria, or more commonly a cercaria enters its body), or on a solid substrate in the external environment. The final intermediate host may be a fish (e.g. Opisthorchis sinensis), an arthropod (e.g.Dicrocoelium dendtriticum, employing an ant second intermediate host, and Paragonimus westermani employing a crustacean), or another mollusc, as with some of the echinostomes. As stated above, some trematodes however do not have second intermediate hosts, but either encyst as metacercariae on solid substrate's, such as aquatic vegetation or on shells of aquatic organisms, which will in turn be ingested by the parasites definitive host, or in some groups such as the schistosomes, as already described, the cercariae directly penetrate the skin of, and infect, the parasites definitive host. Although generally the metacercariae are inactive encysted forms, the metacercaria of some species do remain free and active. For example the metacercariae of trematodes belonging to the genus Diplostomulum where the larvae are found lens, humours and cranial ventricles of a wide range of hosts. In most other metacercariae however encystment does occur. The structure of the cyst wall itself varies considerably, though generally it is a complex mixture of tanned proteins, lipids and polysaccharides. Within the cyst wall the morphology of the larva usually closely resembles that of the cercarial body, although as described above, in some groups sexual maturation may occur either fully or partially. To continue further the metacercaria must be ingested, either along with the body of the intermediate host it inhabits by a carnivorous definitive host, or along with the vegetation it has encysted on by a herbivorous or omnivorous host.
7) The Larval Digeneans - the Juvenile Adult Stages
On ingestion the metacercaria (or cercaria) must transform into the adult form. The precise details of this process will vary considerably, depending on how the definitive host was infected. For example in some species the adult flukes are found within the alimentary tract. In these cases the metacercarial cyst wall is broken down to release what is essentially a young fluke, which only has to migrate a short distance to reach their prefered site within the hosts body. In other groups however the adult forms are located in other sites within the body. In these cases the liberated young fluke must penetrate the gut wall, or in the case of the schistosomes penetrate the hosts skin (see the pages devoted to this process in schistosomes within this site by selecting this link). Then they must undergo a migration through the hosts body. This is usually via the circulatory system, but again the precise details of the migratory path will vary considerably.