CONFIRM INFOZ

  BASIC BODY PLAN OF A CESTODE

INTRODUCTION

The cestodes consist of two separate subclasses, the Cestodarians, parasites of fish and other cold-blooded vertebrates. These are non-segmented parasites, with only a single set of sexual organs. In contrast, the more well-known members of the Subclass Eucestoda are parasites of both warm and cold-blooded vertebrates, including mammals such as man. They resemble a colony of individual animals in that their bodies are divided into a series of segments (the proglottids), each with
their own complete set of internal organs.

 There may be many hundreds of these proglottids, resulting in the complete parasite having a long, ribbon-like body. The appearance of this long body is the origin for the common name for these parasites, the tapeworms. The common names of these parasites are often derived from their intermediate hosts, ingestion of which results in their infection, e.g. the Fish, Beef and Pork Tapeworms.

  Alternatively, they may be named after the definitive host that the adult parasites are usually found in. For example, the Rat tapeworm H. diminuta and the Dog Tapeworm Dipylidium caninum. The study of the morphology of the cestode body may be divided into two distinct areas. Firstly, the morphology of the adult cestode (the tapeworm) and secondly the morphology of the cestode larvae, or Metacestode.

The Scolex

This is the "head" and attachment organ of the parasite. There are four main types of the scolex, by which the tapeworm may be taxonomically classified.

a) No particular attachment organs The scolices of some tapeworms of the order Caryophyllidea (parasites of freshwater fish) have no individual attachment organs. (NB. Some authors do not
recognise this taxonomic order, placing these parasites within the Pseudophyllidea).


b) A Bothria - This is composed of a pair of shallow, elongated, weakly muscular
grooves. Tapeworms of the order Pseudophyllidea are equipped with bothria on
their scolices.

The bothria of Pseudophyllidea

c) A Bothridia - These are large, leaflike muscular structure, exhibiting a significant
degree of variation. Some bothridia are sessile, some are stalked, while others are
hooked with accessory suckers. Tapeworms of the order Tetraphyllidea and others
are equipped with bothridia.

d) Acetabular Suckers - Tapeworms of the order Cyclophyllidea are equipped with four acetabular suckers. Parasites in this order may also have additional features at the apex of the scolex such as;
 - Glandular areas
 - Protrusible suckers
 - Suckers armed with hooks
 - Hooks (e.g. Taenia)
 - A rostellum, an eversible muscular proboscis, often covered
 with hooks (e.g. Hymenolepis, Echinococcus, Dipylidium)
 - A Myzorhynchus (a protrusible muscular mass).

The Neck
 
This is the area of the proliferation of the parasite, from which the proglottids of the strobila grow.


The Strobila
 
This is composed of a series of proglottids. Each proglottid contains a complete set of male and female reproductive organs, although these organs usually mature at different rates. Often the male organs develop before the female organs and degenerate before the female organs age. The large, gravid proglottids at the posterior end of the tapeworm are full of developing, or in the extreme terminal proglottids, mature eggs.

The Cestode Tegument

The related custodians that also belong within the cestodes have a tegument that appears to be intermediate with that of the cestodes and monogeneans. This is another piece of evolutionary evidence that indicates a monogenean origin for the tapeworms. In this case, the surface of the custodian tegument is covered with numerous microvilli, similar in form to the cestode microfiche (see below), but lacking the dense electron cap seen in these parasites.
The code tegument is a syncytial layer, showing many features typical of that found in other parasitic Platyhelminthes.

  There are, however, several distinguishing features present in these parasites. On the very outer surface of the tegument, a surface glycocalyx is seen to cover the outer plasma membrane. Below this glycocalyx, a characteristic feature of the cestode tegument is the presence of numerous microfiches (Mt), long spine-like processes that are in fact a highly modified form of microvilli. Each microthrix has a hard, pointed, electron-dense cap which is seperated from the rest of the microstrip by a crescent shaped membranous cap.

  The mirotriches are thought to serve two functions. Firstly, the tapeworms do not possess a gut and must absorb all of their nutrients across the surface tegument. The microfiches significantly increase the surface area of the parasite and can be seen as an adaptation to maximise the amounts of nutrients available to the parasite. This is supported by the finding of microtubles in the shaft of the microfiches. Secondly, the spine, like the character of the microfiches probably help the parasite maintain its position in the gut. This can be more clearly seen by comparing the microfiches found in different regions of the parasite’s body.

  It has been noted in many species that the microfiches found covering the scolex, the attachment organ of the parasite, were much longer than those covering the strobila, and in some species show particular adaptations. For example, the microfiches covering the strobila of E. granulosus have been found to show curved hooks or sometimes even barbs. Below the layer of microtriches the main syncytial layer of the tegument is found. This has been seen to contain numerous vesicles and membrane bound, electron dense rod-like structures,refered to as disc-shaped bodies (Db). Finally, many mitochondria, mainly in the distal region of the tegument, may be seen. These are unusual in that they do not have many cristae, reflecting the anaerobic metabolism of the organism.

   The tegumental nuclei are however not located in this outer layer but are found within subsegmental cell bodies (StC), situated beneath the circular (Cm) and longitudinal muscle (Lm) layers, embedded within the parenchymal tissues (P) and mesenchymal musculature (M). These subsegmental cell bodies also contain other cellular elements such as Golgi apparatus and lipid inclusion bodies (L) which are connected to the outer syncytium and areas of glycogen storage (Gs) by long protoplasmic extensions (Pe). The location of these critical cellular elements away from the outer surface of the parasite, exposed to immunological attack by the parasite's host, is an essential adaptation to a parasitic lifecycle adopted by all of the parasitic Platyhelminthes.

  The parenchymal tissues are similar to those of the trematodes and fill the spaces between the internal parasites organs (all cestodes and other Platyhelminthes being acoelomate organisms).
These tissues are a syncytial network formed by anastomosis of mesenchymal cells, with spaces filled with carbohydrate-rich parenchymal fluid.

Lung flukes (Paragonimus species)

 

   The genus Paragonimus contains more than 30 species that have been reported to cause infections in animals and humans. Among these, approximately 10 species have been reported to cause infection in humans, of which P. westermani is the most important. P. westermani, also known as the Oriental lung fluke is most common in China, Korea, Thailand, Philippines, and Laos. Isolated endemic foci
have also been reported from the states of Manipur, Nagaland, and Arunachal Pradesh in India.

  A low prevalence has been reported from African countries of Cameroon and Nigeria, where infections with Paragonimus africanus and Paragonimus uterobilateralis were reported. Humans are infected by eating raw or partially cooked crab or crayfish or crabs soaked in wine as a food delicacy or by drinking juice from raw crabs or crayfish as a part of a food habit. It inhabits parenchyma of the lung close to bronchioles in humans, foxes, wolves, and various feline hosts (e.g, lions, leopards, tigers, cats). Paragonimus species belong to the family ‘Troglotrematidae’ and they possess the

following striking features:

- The adult worm is reddish brown fluke.

- The body of adult worm is thick, fleshy, and ovoid in shape

- The tegument is spiny or scaly

- Have weak suckers

- Testes lie side by side

- Uterus is short with a few tight uterine coils forming a ‘rosette’

- Extensive vitellaria in lateral fields



- Cercariae are microcercous xiphidiocercariae

- The eggs are ovoid, brownish yellow, thick shelled and operculated.

 Life cycle and Transmission

  The infection is typically transmitted via ingestion of metacercariae contained in raw freshwater crabs or crayfish. Additionally, consumption of the raw meat of paratenic hosts (e.g, omnivorous mammals) may also contribute to human  infection. Freshwater snails and crabs are first and second intermediate hosts of Paragonimus species, respectively. In the duodenum, the cyst wall is dissolved, and the metacercariae are released.

  The metacercariae migrate by penetrating through the intestinal wall, peritoneal cavity, and, finally, through the abdominal wall and diaphragm into the lungs. There, the immature worms finally settle close to the bronchi, grow, and develop to become sexually mature hermaphrodite worms.


    Adult worms begin to lay the eggs, which are unembryonated and are passed out in the sputum. However, if they are swallowed, they are excreted in the feces. The eggs develop further in the water. In each egg, a ciliated miracidium develops during a period of 2-3 weeks. The miracidium escapes from the egg and penetrates a suitable species of snail (first intermediate host), in which it goes through a generation of sporocysts and 2 generations of rediae to form the cercariae. The cercariae come out of the snail, invade a freshwater crustacean (crayfish or crab), and encyst to form metacercariae. When ingested, these cause the infection, and the cycle is repeated.

Pathology and clinical symptoms

a. Acute manifestations: Acute pulmonary infection is characterized by low-grade fever, cough, night sweats, chest pain, and blood-stained rusty-brown sputum.

b. Chronic manifestations: Lung abscess or pleural effusion develops in individuals with chronic infections. Fever, haemoptysis, pleurisy pain, dyspnea and recurren attacks of bacterial pneumonia are the common symptoms. The condition mimics pulmonary tuberculosis.

c. Fever, headache, nausea, vomiting, visual disturbances, motor weakness, and localized or generalized paralysis are the symptoms of cerebral paragonimiasis.

Diagnosis

Diagnosis is based on finding the characteristic eggs in brown sputum. The eggs can also be found in the faeces due to swallowing sputum. A chest x-ray may show cystic shadows and calcification. Serological tests, in particular, the ELISA method, are useful diagnostic tests.

Epidemiology

The epidemiology of the disease (Paragonimiasis) depends on one of the following

- Presence of appropriate snail, crab and mammalian reservoir hosts in the area

- Pollution of snail habitats with sputum and faeces of man as well as natural mammalian reservoir hosts infected with the parasite

- Consumption of metacercariae through eating of raw or undercooked crabs or through contamination of the fingers and cooking utensils with metacercariae while cleaning

Control

- Proper cooking of crabs before consumptions

- Proper waste disposal

Life Cycles and Transmission and Control  of Schistosomes

  Once the eggs are laid by the adult female worms, the majority of them first pass through the veins of the blood vessel in which the worm is living, and then into the lumen of the intestine and are passed in the faeces (S. mansoni and S. japonicum) or into the lumen of the bladder, and are then passed in the urine (S.haematobium).

  Those eggs that reach fresh water hatch, releasing a miracidium which, to develop further must infect a specific snail species within 24 hours. The eggs of each species are markedly different but each produce virtually identical miracidium. A single miracidium can multiply in the snail to produce nearly 100,000 cercariae.

  Asexual multiplication takes place in the snail, and results in the release of cercariae (minute in size with forked tails, 200mm long) into the water about 3 – 6 weeks later. Cercariae actively swim around and when they have located, or come into contact with a definitive host, they actively penetrate the skin. They can stay  active looking for a host for 24-48 hours after which if they don’t find a host they will die.

   The head of the cercariae migrates to the liver and develops into either adult male or female worms (flukes), where they pair up and then migrate to their region of the venous blood system (species specific sites). The females leave the males and moves to smaller venules closer to the lumen of the intestine or bladder to lay her eggs (about 6 weeks after infection). The majority of adult worms live from 2-4 years, but some can live considerably longer.

Acute manifestations

a. Cercarial dermatitis, also known as swimmer's itch, is an allergic reaction caused by the penetration of cercariae in persons who have been exposed to cercariae in salt water or fresh water. Cercarial dermatitis manifests as petechial haemorrhages with oedema and pruritus, followed by maculopapular rash, which may become vesicular. The process is usually related to avian
schistosomal species of the genera Trichobilharzia, Gigantobilharzia, and Orientobilharzia, which do not develop further in humans.

b. Katayama syndrome corresponds to maturation of the fluke and the beginning of oviposition.
 This syndrome is caused by high worm load and egg antigen stimuli that result from immune complex formation and leads to a serum sickness –like illness. This is the most severe form and is most common in persons with S mansoni and S japonicum infections. Symptoms include high fever, chills, headache, hepatosplenomegaly, lymphadenopathy, eosinophilia, and dysentery. A history of travel in an endemic area provides a clue to the diagnosis.

• Chronic manifestations

a. Symptoms depend on the Schistosoma species that causes the infection, the duration and severity of the infection, and the immune response of the host to the egg antigens.

b. Terminal haematuria, dysuria, and frequent urination are the main clinical symptoms of urinary schistosomiasis.

c. The earliest bladder sign is pseudotubercle, but, in long-standing infection, radiography reveals nests of calcified ova (sandy patches) surrounded by fibrous tissue in the submucosa.

d. Dysentery, diarrhoea, weakness, and abdominal pain are the major symptoms of intestinal schistosomiasis.

e. A reaction to schistosomal eggs in the liver causes a periportal fibrotic reaction termed Symmers clay pipestem fibrosis.

f. Haemoptysis, palpitation, and dyspnea upon exertion are the symptoms
of schistosomal cor pulmonale that develops as a complication of hepatic schistosomiasis.

g. Headache, seizures (both generalized and focal), myeloradiculopathy with lower limb and back pain, paresthesia, and urinary bladder dysfunction are the noted symptoms of CNS schistosomiasis due to S.
japonicum infection.

Intestinal schistosomes

• Laboratory confirmation of S. mansoni and S. japonicum infection can be
made by finding the eggs in the faeces. When eggs cannot be found in the faeces, a rectal biopsy can be examined.

• Serological tests are of value in the diagnosis of schistosomiasis when eggs cannot be found. An enzyme linked immunosorbent assay (ELISA) using soluble egg antigen, is employed at HTD.

Urinary schistosome

• The definitive diagnosis of urinary schistosomiasis is made by finding the characteristic ova of S. haematobium in urine. Terminal urine should be collected as the terminal drops contain a large proportion of the eggs. The urine can then be centrifuged and the deposit examined microscopically for ova. Eggs can sometimes be found in seminal fluid in males.

• A bladder biopsy is seldom necessary to make the diagnosis. A rectal snip may show the presence of ova as they sometimes pass into the rectal mucosa.

  Serological tests can be of value when eggs cannot be found in clinical samples. An enzyme linked immunosorbent assay using soluble egg antigen todetect antischistosome antibody is most sensitive.


Note: There is a marked periodicity associated with the time when most eggs are passed out. Higher numbers of eggs are encountered in urine specimens passed between 10 am and 2pm, presumably as a result of changes in the host’s metabolic and physical activities.

Epidemiology of Schistosomiasis

The following factors are of epidemiological importance in the transmission of schistosmiasis:

- The presence of water bodies such as rivers, streams, lakes, dams suitable for the breeding of the snail intermediate hosts

- Presence of appropriate snail hosts necessary for the developments of the asexual stages and transmission of the infective stage to the human definitive host

- Contamination of natural water bodies with infected human urine and faeces

- Human water contact activities including swimming, laundry and fetching

- Factors that promote intramolluscan development of parasite and subsequent transmission to man

- Socio-economic status of the people such as good sanitary system and water supply

Control

- Reduction of human-water contact

- Improved sanitation by proper waste disposal

- Attacking the adult forms of parasite through chemotherapy to reduce theworm burden or egg production

- Eradication or reduction of snail population through the use of molluscicides

- Development of vaccine to induce immunity

- Modification of the ecology of the snail habitat

- Biological control through the introduction of competitors snails into the snail habitat

CLASSIFICATION OF DIGENETIC TREMATODES ACCORDING TO THEIR HABITAT

   The digeneans are a group of specialised endoparasitic platyhelminthes. A common feature is that all have complex lifecycles, involving one or more intermediate hosts, the first of which is always a mollusc, which is usually aquatic. As adults they are found in most vertebrates groups, including fish, amphibians, reptiles, birds, and mammals, acting as definitive hosts, where they may be highly pathogenic.

  They may be located in most of the internal organs of these definitive hosts, including the lungs, bladder and blood stream, although the majority are found in the gastrointestinal tract, or closely associated organs such as the bile
duct and liver. They exhibit a flattened leaf-like body, structurally similar to many of the free living turbellarians. The digeneans are classified below based on their locations in the definitive hosts;

• Blood flukes - Schistosoma haematobium, S. mansoni, S. japonicum, S. mekongi,
and S. intercalatum

• Liver flukes - Fasciola hepatica, F. gigantica, C sinensis, Opisthorchis felineus,
O viverrini, Dicrocoelium dendriticum, and D. hospes Pancreatic flukes - Eurytrema pacreaticum, E. coelomaticum, and E. ovis

• Lung flukes -Paragonimus westermani, P. mexicana, and P. skrjabini

• Intestinal flukes – Fasciolopsis buski, Metagonimus yokogawai, Echinostoma
ilocanum, Watsonius watsoni, Heterophyes heterophyes, and Gastrodiscoides
hominis

Blood flukes (Schistosoma species)

  Schistosomiasis, or bilharzia, is a tropical parasitic disease caused by blooddwelling fluke worms of the genus Schistosoma. Over 200 million people are infected in at least 75 countries with 600 million or more people at risk of infection. The main schistosomes that infect human beings include S haematobium (transmitted by Bulinus snails and causing urinary schistosomiasis in Africa and
the Arabian peninsula), S mansoni (transmitted by Biomphalaria snails and causing intestinal and hepatic schistosomiasis in Africa, the Arabian peninsula, and South America), and S japonicum (transmitted by the amphibious snail Oncomelania and causing intestinal and hepatosplenic schistosomiasis in China, the Philippines, and Indonesia).

  S. intercalatum and S. mekongi are only of local importance. S. japonicum is a zoonotic parasite that infects a wide range of animals, including cattle, dogs, pigs, and rodents. S. mansoni also infects rodents and primates, but human beings are
the main host. A dozen other schistosome species are animal parasites, some of which occasionally infect humans.

  Unlike other trematodes, schistosomes have separate sexes, but males and females are found together. The male is short and stout and holds the relatively long female worm in its gynaecophoric canal, a groove like structure. With S. haematobium, both male and female live together in the veins that drain the urinary bladder, pelvis, and ureter, whereas S. japonicum and S. mansoni live in the inferior and superior mesenteric veins, respectively.

  Hence, these flukes are known as blood flukes. These species are distinguished from the other Schistosoma species based on the morphology of their eggs and their adult and cercarial forms. S. haematobium eggs have a terminal spine, whereas S. mansoni
and S. japonicum eggs have lateral spines and central spines, respectively

Morphology

  The adult males measure up to 15 millimetres in length and females up to 10 mm. The schistosomes remain in copula throughout their life span, the uxorious male surrounding the female with his gynaecophoric canal. The male is actually flat but the sides roll up forming the groove. The cuticle of the male is covered with minute papillae.

The female only possess these at the anterior and posterior end as the middle section being covered by the male body. Oral and ventral suckers are present, with the ventral one being lager serving to hold the worms in place, preventing them from being carried away by the circulatory current.

The ova of S. mansoni are 114-175 µm long by 45-68 µm wide. They are light yellowish brown, elongate and possess a lateral spine. The shell is acid fast when stained with modified Ziehl-Neelsen Stain.

The mesocercaria

   The mesocercaria is essentially a resting stage within the parasitic life cycle, employing a second intermediate host in a parasite lifecycle utilising four hosts. 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, birdsand mammals if they are ingested by these animals.


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 action. 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.

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 short distance to reach their preferred 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. 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.

Features of digenetic trematodes

• Digenetic trematodes are unsegmented leaf-shaped worms that are flattened
dorsoventrally.
• They bear 2 suckers, one surrounding the mouth (oral sucker) and another on the
ventral surface of the body (ventral sucker). These serve as the organs of
attachment.
• The sexes of the parasites are not separate (monoecious). An exception is
schistosomes, which are diecious (unisexual).
• The alimentary canal is incomplete, and no anus is present.
• The excretory system is bilaterally symmetrical. It consists of flame cells and
collecting tubes. These flame cells provide the basis for the identification of the
species.
• The reproductive system consists of male and female reproductive organs and is
complete in each fluke.
• The flukes are oviparous. They lay operculated eggs. An exception is schistosome
eggs, which are not operculated.
• All have complicated life cycles, with alternating asexual and sexual
developments in different hosts.

The cercaria

Some of the types of Cercariae 

1. Gymnocephalus (e.g. Fasciola sp.)
2. Trichocercous(e.g. Donax sp.)
3. Furcocercous(e.g. Schistosoma sp.)
4. Microcercous(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 ventralsuckerlying 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 rediae

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.

b) Trichocercous Cercariae - These forms have long tails, equiped with rings of fine bristles. They are usually found in marine trematodes.

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 grow. For example, the ring of spines found at the anterior end of echinostome cercariae is 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 found on the surface of both forms of tegument, and the overall structures of a syncytium conected to subtegumental cells are the same.

  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 the 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 cercariae 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 equipped 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. 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.

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 or 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 the 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 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 organs, 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 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 clearly seen that flame cell activity 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.

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

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.


  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 are lobelike 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.