General Parasitology

Platyhelminthes

Trematodes

Cestodes

Nematodes

Introduction to Digenean Flukes
Adult Fluke
Adult Bodyplan
Digenean Tegument
Reproductive System
Lifecycle
Other Flukes

Other Flukes

1) Paragonimus westermani

Paragonimus egg

Digenean eggs are operculate, and contain a miracidium larvae. This egg, of the lung fluke, is morphologically a typical example of a Digenean egg. However, it should be noted that in this particular parasite, the eggs are found in the sputum as well as the faeces (as a consequence of the location of the adult flukes in the lungs), those in the faeces having been swallowed. Most Digenean eggs are found only in the faeces.
Paragonimus westermani is the most well known species infecting man (in addition to zoonotic infections in cats, dogs, pigs and a large number of other animals). However the genus includes a large number of species, all of which also appear to be capable of infecting man as well. These include;

P. kellicotti
P. africanus
P. uterobilateralis
P. ohirai
P. iloktsuensis
P. caliensis
P. peruvianus
P. mexicanus
many others, (may be synonyms).

The lifecycles of the parasites are all similar, with different animals acting as definitive hosts, depending on the geographical region the parasites are found.

Morphology

This fluke is reddish-brown in colour, oval in shape and is 8 - 16mm x 4 - 8mm in size. It's tegument is covered in many small spines. The suckers are approximately the same in size, the ventral sucker being slightly anterior to the midline of the parasite. The bifurcated gut is not branched, unlike, for example, Fasciola.
For the sexual organs, the testes are lobed, and are in parallel in the posterior portion of the body, whilst the ovary is also lobed, and is located above the testis, usually on the right hand side of the body, opposite a tightly coiled uterus. The vitellaria are located laterally, running from the anterior to the posterior of the parasite. The eggs a typical of the Digeneans (see illustration above), and are ~ 80 - 120 µm in length, with a flattened operculum.

Lifecycle

The adult parasites normally live in the lungs of the definitive host, though ectopic parasites found in other locations are also frequently seen, suggesting this parasite is not well adapted to living in man. Unembryonated eggs are laid by the parasites, and are passed either directly in the sputum, or may be swallowed then passed in the faeces. The miracidium develops over 17 - 21 days at 30°C, then hatches in water to infect the snail intermediate host by penetration. These snail hosts belong to a number of genera depending on the geographical location of the parasite, but the principle host are species of the genus Semisulcuspira (e.g. S. libertina). The parasite then undergoes a series of asexual divisions from sporocyst, to two generations of redia, then finally the cercaria. Morphologically these are unusual having only a small (microcercus) tail, a spiny tegument and a stylet. These cercaria, which can survive up to 48 hours, do not swim to find their next host, but either crawl in a leech like manner or are carried by currents in the water. The second intermediate hosts for this parasite are fresh-water crustaceans, particularly crabs. Once the cercaria locates these it penetrates them, and migrates to the muscles or heart where they encyst as metacercaria, where they are infective after ~ 6 weeks. Infection of the definitive hosts takes place by eating raw or undercooked fresh-water crustaceans. the metacercaria excyst in the duodenum, penetrate through the intestinal wall into the body cavity, from whence they migrate to the diaphragm. This they penetrate to enter the thoracic cavity, from where they enter the lungs. The migration through the body takes ~ 15 - 20 days, becoming mature parasites in 5 to 6 weeks and living up to 20 years (although 6 is more normal).

Pathology of Infection. This may be divided into two areas:

Pulmonary Paragonimiasis - Here fibrotic tissue forms a cyst around the parasites in the lungs, the parasites usually pairing even though they are hermaphroditic. This cyst is surrounded by a cellular infiltrate. Symptoms, which may be severe in infections with high worm densities, include pleurisy, pneumothorax, a bad cough, bronchitis, blood in the sputum, mild anaemia and weariness and may be confused with tuberculosis or pneumonia. This may be treated with drugs such as Praziquantel or Bithional.

Extrapulmonary Paragonimiasis - In this case disease is caused by ectopic parasites in aberrant locations. These may vary from parasites encysted in the abdominal wall, to more serious cerebral infections. Depending on the location of these ectopic parasites symptoms will vary. If in the intestinal wall there may be diarrhoea and abdominal pain. Cerebral involvement may occur in many cases, with symptoms such as visual disturbances, headaches, and in more serious cases embolisms and epilepsy.

2) Opisthorchis sinensis (The Chinese Liver Fluke)

Opisthorchis sinensis egg

Opisthorchis sinensis (syn. Chlonorchis sinensis) is the main representative of this genus that infects man. O. sinensis is found throughout South-East Asia, Japan, Korea, Taiwan and most of China (except the North-East) the incidence of this parasite being estimated at between 20 and 30 million cases.

In addition a number of other species normally found in other animals may cause accidental infections in man. These include;

These both have similar lifecycles to O. sinensis, with cyprinid fish second intermediate hosts.

Morphology

The elongated adult parasites are opalescent grey in colour and vary in size from 10 - 25mm in length by 3 - 5mm in width. The tegument in the larval fluke is spiny, but in the adult parasite is smooth, in contrast to other flukes such as Fasciola or Paragonimus. The gut is bifurcated as with all flukes, but not branched and run down the body to the posterior end. The ventral sucker is smaller than the oral sucker and is situated approximately a quarter of the way down the parasite from the anterior end

For the sexual organs, the testis are situated at the posterior end of the parasite are arranged in tandem and are highly branched (thus differing from the related species O. viverrini and O. tenuicolis). In contrast the ovary is small and slightly lobed, being situated just above the testes and below a long loosely coiled uterus which ends in the genital pore just above the ventral sucker. The vitellaria are situated laterally in the mid third of the body. The eggs are ~30 x 15 microns in size, and in O. sinensis the operculum is situated in a rim with distinct shoulders.

Lifecycle

The embryonated eggs pass out in the faeces are ingested by the snail intermediate hosts, which may below to a number of genera belonging to the subfamily Buliminae (e.g. Parafossarulus manchouricus, Bulimus fuchsiana, Alocima longicornis) and Hua ningpoensis of the Thiaridae. Here the eggs hatch in the rectum of the snail to release the miracidium, which penetrates the gut wall to invade the snail tissues. There is one generation of sporocysts in this parasite, followed by numerous redia (of a single generation)appearing ~ 17 days post infection, then numerous cercaria 21 to 30 days post infection. Morphologically the cercaria lophocercous (i.e. have "keeled" tails). The cercaria leave the snail hosts then penetrate the cyprinid fish intermediate hosts, where they migrate to the subcutaneous connective tissues and muscle (often near the caudal fin). The body of the cercaria then round up to form the highly resistant metacercaria, which may be seen as black spots on the fins of the fish. These metacercaria are infective after ~ 23 days. To continue the lifecycle the raw or undercooked fish must the be eaten, and the most effective means of control of this parasite is to cook the fish, although for cultural reasons this may not be feasible. As well as man most fish eating mammals may also be infected, acting as reservoir hosts for the parasite. Of these reservoir hosts dogs are probably the most important, but cats and rats are also commonly infected as well. In these definitive hosts the metacercaria excyst in the duodenum, from where they crawl into the bile duct where they mature in 3 to 4 weeks to complete the lifecycle. The parasites themselves live up to 25 years.
Note that as well as the bile duct, the parasites may also be present in the pancreatic duct and gall bladder as well, particularly in heavy infections.

Pathology of Infection

For this parasite, as for many other helminth diseases, the severity of disease is dependent on the number flukes the individual host harbors. Most infections consist of less than 100 parasites and the majority of these are asymptomatic. When infections consist of up to 1000 flukes non-specific symptoms such as diarrhoea, abdominal pain and splenomegaly may be seen. More severe symptoms may be seen in heavier infection, which may consist of many thousands of parasites (up to 21 000 flukes have been observed in a single infection). Here symptoms may include fever acute pain in the upper right quadrant of the liver and enlargement of the liver (which may become cirrhotic resulting in portal hypertension). There may also be jaundice, tachycardia and loss of weight, and in the circulatory system eosinophilia is often seen. The presence of the parasites in the bile duct leads to inflammation and fibrosis of the bile duct, this may be followed by metaplasia often leading to cancer. The disease itself is rarely fatal. However changes in the bile duct favour highly pathogenic bacteria such as Salmonella, which flourish in bile ducts containing Opisthorchis.

3) The Echinostomes

Adult Echinostome

The Echinostomes are a large group of flukes, which are distinguished by a horseshoe-shape collar of spines on the lateral and dorsal sides of the oral sucker. In addition they generally have elongated bodies of moderate size, with a tegument bearing many minute spine-like scales. The lifecycle is similar to that of Opisthorchis, with a snail first intermediate host (as with all the Digenean flukes), usually followed by a fish second intermediate host, although in some species this may be an amphibian of another mollusc. A number of these parasites have been reported in man, either as a natural parasite, (as with the three species detailed below), or more commonly as accidental infections.

Echinostoma ilocanum
Echinostoma jassyenese
Echinostoma malayanum

3) Fasciola hepatica - The Liver Fluke

This parasite is a typical fluke, and is very similar to the related species F. gigantica, F. hepatica is the causative agent of Fascioliasis, or 'liver rot' in ruminants, where it may be an important pathogen. However a wide variety of other mammals including man may also be infected, ingestion of contaminated vegetation such as watercress being a common mode of infection here. It is very cosmopolitan in distribution being found throughout all regions of the world, both temperate and tropical. In addition to F. hepatica and F. gigantica there are a number of more minor species.

 Morphology

The adult parasite has a flat leaflike body, typical of flukes, and measures from 20 to 30 mm long by 8 to 15 mm wide. It has an anterior elongation (a cephalic cone) on which the oral and ventral suckers are located, these being on approximately equal size. The intestine of the adult parasite is very highly branched, with numerous diverticulae extending from the extreme anterior to the extreme posterior of the parasites body. The two testes, also highly branched, are located in tandem in the posterior half of the body. The smaller branched ovary is located just above the testes and is linked to a short convoluted uterus opening to a genital pore above the ventral sucker. The vitellaria are highly diffuse and branched in the lateral and posterior region of the body

Lifecycle

The lifecycle of F. hepatica is a typical example of a Digenean lifecycle, with miracidial, and cercarial stages. In addition there is a metacercarial stage, not seen in the schistosomes, consisting of an encysted, resistant cercaria. The lifecycle of the parasite proceeds as detailed below.
Eggs (measuring approximately 170 by 100µm) laid by the adult parasite pass into the duodenum with the bile, leaving the body in the faeces of the definitive host. At this stage these eggs are still unembryonated, further development to maturation taking approximately two weeks. The eggs then hatch to release the motile miracidium. This then locates, and then actively penetrates the intermediate snail host. The are amphibious snails belonging to the genus Lymnaea, different species having different affinities for the parasite. In North America L. bulimoides is the principle intermediate host, whilst in Europe and most other regions the most important species is probably L. truncatula.
Within this snail intermediate host a primary sporocyst is formed, similarly to the case with the schistosomes. However this sporocyst stage is succeeded by one (or occasionally two) generations of a lifecycle stage called a redia. The redia differ from sporocysts in having an oral sucker, a rudimentary, sac-like, gut, and other appendages. Secondary (daughter) redia are sometimes formed from these when environmental conditions are adverse, but either these, or more commonly the primary (mother) redia, then give rise to the cercarial stage. These cercaria, which take ~ five to seven weeks to develop from the initial infection, then leave the snail when environmental conditions are optimal. This is during moist conditions when a critical temperature of 10°C is exceeded. The cercaria of Fasciola have rounded bodies measuring between 0.25 and 0.35mm long, and long thin unbranched tails measuring ~0.5mm long. On emerging from the snail the cercaria attaches to submerged blades of grass or other vegetation (e.g. watercress), the tail falls away and the cercarial body secretes a four-layered cyst covering from cystogenous glands present on the lateral regions of its body. The formation of this cyst wall may take up to two days, after which the metacercaria is fully infective to the definitive host. The metacercarial cyst is only moderately resistant, not being able to survive dry conditions. If however they are maintained in conditions of high humidity and cool temperatures they may survive for up to a year.
To continue the lifecycle these metacercarial cysts must however be ingested, accidentally along with the contaminated vegetation, by the definitive host. This is most commonly a ruminant such as cows and sheep. The parasite is infective to almost all mammals, although deer are an exception here*, the only constraint being the dietary habits of the animal. However in some of these hosts, such as man or the horse the parasite is much less well adapted, and ectopic infections may occur, with adult flukes being found in unusual locations such as the lungs or under the skin. In the normal infection however the development continues as described below.
After ingestion the metacercaria excysts in the small intestine, releasing the young parasite. This then penetrates the gut wall, entering the peritoneal cavity. From here it migrates over a period of ~ seven days, directly to the liver. The young fluke then penetrates the liver tissues, through which it migrates, feeding on mainly on blood, for about six weeks. After this period the fluke enters the bile duct, maturing to the fully adult parasite after about 3 months after initial infection. Egg production then commences, completing the lifecycle.

*In the deer a related species, Fascioloides magna, is a common parasite. This fluke is also a common parasite of cattle and sheep. It does not infect man.

Pathology of Infection.

This may be divided into two major forms;

i) Acute Fascioliasis
As a pathological is more commonly seen in sheep, and is not usually a feature of fascioliasis in man or other animals. It occurs when very large numbers of metacercaria (>10 000) have been ingested at once. In these cases as the resulting large numbers of migrating larvae invade the liver they cause a traumatic hepatitis, frequently resulting in death. In some cases the liver capsule may rupture into the peritoneal cavity causing death due to peritonitis.
More commonly, on ingestion of fewer metacercaria, a period of fever and eosinophilia is seen.

ii) Chronic Fascioliasis
This form of the disease is much more common in all of its host, particularly man. Here the infection is only rarely fatal, but, at least in domesticated animals, is of economic importance. In man the presence of the flukes causes a number of fairly non-specific symptoms including malaise, an intermittent fever, mild jaundice and anaemia, eosinophilia and, frequently, pain under the right costal margin. In addition, as described above, as Fasciola does not appear to be fully adapted to using man as a definitive host, the flukes may often give rise to ectopic infections, particularly in the lungs and subcutaneous tissues, where they may be found encysted.

iii) 'Black Disease'
In addition to these two major forms of fascioliasis, in some cases a condition known as 'black disease' is a complication, usually fatal, of the infection. Here a secondary infection due to the bacterium Chlostridium oedematiens, proliferating in necrotic lesions produced by the young larvae migrating in the liver.

In addition in some regions, particularly the Middle East, Fasciola may give rise to a condition known there as halzoun. This is characterised by an acute irritation of the throat, and is due to the transient attachment of flukes ingested in raw liver.

Fasciola gigantica

This is a very similar parasite to F. hepatica, with which it may mate to produce hybrid strains when simultaneous infection of these two parasites occur. It is found more commonly in tropical regions of the world where it is quite cosmopolitan in distribution, areas affected including Africa, Asia, many Pacific islands including Hawaii (where it has been reported in man), the Middle East and Southern Europe and the south of the USA.

Morphology

Similar to F. hepatica, but much larger, reaching from 25 to 75 mm long by 15 mm wide. In addition the cephalic cone is proportionally shorter than that of F. hepatica, the body is even more leaflike in shape. The egg is also very similar to that of F. hepatica, but again is larger in size (~200 by 100 µm)

Lifecycle

Similar to F. hepatica, but requires warmer temperatures and is more associated with permanent areas of water.

Pathology of Infection.

Similar to F. hepatica.