CERCARIAE: YOU AND THE TREMATODES (A crash course)

Human relationship with other organisms ranges from highly beneficial to extremely detrimental with humans suffering in case of unhealthy relationship with these much smaller organisms. Beneficial microorganisms such as the lactobacilus in the membrane of the vagina metabolizes carbohydrates anaerobically, producing lactic acid which is bactericidal and protects the host from invasion of pathogens; the normal flora of the human body serves various purposes which are beneficial to the human host, these organisms also benefits from this relationship as they obtain their meal and other requirements for existence from these hosts.

Microbes and some macro-organisms that inhabits the internal organs and systems of humans are also understood to be dependent on its host for their respiration and excretion, this is a quintessential case of mutualism. In some other relationships however, the organisms benefits from the host while the host neither gains nor loses, this is seen in cases where the organism lives on substances which are not needed by the host and are unable to attract the recognition of the immune system.

These organisms usually lives on substances such as the excretory products of the host or substances which the host produces in abundance to an extent that depletion of these substances will not be noticed by the hosts' system and the body do not react to them as they are not immunogenic or the immune system is capable enough to subdue their effects.

This sort of relationship is very rare and in case of immunosupression, these organisms may turn harmful to the body as the system which keeps them unnoticed have been greatly incapacitated, this is referred to as an opportunistic infection as these organisms only attacks the body when oppurtuned, even organisms which are normally beneficial to the body can also turn harmful when ‘oppurtuned'.

In contrast ; some organisms benefits from it’s host by feeding from the host’s very useful products while these hosts suffers severely as these substances which the organisms feed on are highly needed by the hosts' body systems, the host suffers more as these organisms are able to trigger immune reaction with intensity ranging from mild to highly discomforting and fatal if not controlled by medical and nutritional means, this relationship is known as parasitism and the organisms in question is a parasite.

Parasites are relatively useless to it’s host but depends on these hosts for most vital aspects of its existence and this host hence suffers discomfort resulting from the organisms' high dependence on it’s very vital products, from the immune reaction geared towards the destruction of the organisms and from the lifestyle and habits of these organisms. Immune reaction geared towards the parasites may include hypersensitivity, inflammation, allergy and traumatic destruction of cells infected by these organisms.

Parasites vary in size, some parasites are microscopic and so tiny as to be detected only by the use of a microscope, some parasites are also large enough to be observed by the naked eye. Some are visible by the normal human eye but can only be appreciated by the use of a microscope; many parasites are also large enough to be studied to a very reasonable extent without the aid of a microscope.Human parasites are mainly protozoa and helminthes.

Protozoa are unicellular microscopic organisms of about 1-150um in size with a high rate of reproduction, they thus have a very short generation time, this explains their high infectivity as they can multiply into several pathogenic daughters in a very short while. A protozoan cell has the ability to perform most actions of a living cell, they posses locomotive organelles such as flagella, cilia and pseudopodia which confers motility on them, these are extensions of the cell membrane which are mobile and produces general movement of the whole cell.

Protozoa may be free living, existing independent of any vertebrate or invertebrate host, pathogenic protozoa however are the cause of many common diseases such as malaria, sleeping sickness, river blindness and leshmaniasis.

THE HELMINTHES : THE WIDESPREAD HANGMEN!

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Helminthes are another class of human parasites also known as parasitic worms. Their definitive host is man and their intermediate host are other lower vertebrates and invertebrate where they complete some stage of their life cycle. Helminthes are relatively of large size with some bigger than 1cm in size, they are bilaterally symmetrical multicellular organisms with varying morphology and over one million species of relatively contrasting forms and characteristics.

Helminthes differ in shape with some possessing round or flat shape and some elongated; they differ in mode and extent of pathogenicity, presenting varying symptoms and effects in case of an unfortunate infection.
Helminthes are classified into phyla and classes with numerous species, but common to all helminthes are features such as:

• Oviparity:

Helminthes are oviparous, they lay eggs which marks the onset of their life cycle, eggs of helminthes are complex, with its shell having three layers, an outer proteinic layer, middle chitinous layer and an inner lipoidal layer; this is to protect the eggs from the harsh environmental condition and ensure that these eggs remain viable for a long period of time amidst unfavorable conditions. Parasitic worms lay large number of eggs, ranging from several hundreds to thousands of eggs which matures into the larval stage of their life cycle.

• Life cycle:

Eggs of helminths develops from eggs to the larva, the larva matures from eggs which remains viable throughout the required time of maturation, eggs which lose their viability during the period of maturation does not mature into larvae, the larval stage of the helminths are also very important as this precedes their development into adults, larval maturation takes about three weeks and some times a month depending on the specie and the serenity of the environment, larvae of helminths undergoes many developmental stages as it acquires pathogenic abilities and matures into it’s most developed form, the Adult.


_{life cycle of Echinococcus which is of the helminths family. Source Wikimedia. Creative Commons Attribution-Share Alike 4.0 International, 3.0 Unported, 2.5 Generic, 2.0 Generic and 1.0 Generic license. Contributed by David Richfield}

Adult helminthes are equipped with muscular cup-shaped structures known as suckers, this enables them to obtain nutrient from the hosts' digestive system, they can offset the immune system and if uncontrolled can last for years and even a life time, they can reproduce and this ensures the continuation of another generation.

According to structural and other characteristic features, helminthes are grouped into two phyla:

1. Phylum Nemahelminthes

Members of this phylum are known as Nematodes, phylum nemahelminthes contains the most abundant species of worms. Many worms belonging to this phylum are free living and can be found in fresh water and soil.


^{a Soybean cyst and its egg. Source: Wikimedia Creative commons license. Contributed by the Agricultural Research Service}

Others are parasitic worms infecting both plants and animals. Nematodes are unsegmented bilaterally symmetrical ‘thread-like’ worms with rudimentary excretory and nervous system, they lack a developed circulatory system and hence depends on their host for the actions performed by the circulatory and excretory systems. Nematodes are normally macroscopic; however, their sizes varies as some measures about one meter (1m) in length and some do not exceed 5mm in length.

2. Phylum Platyhelminthes (Flat worms)

Platyhelminthes consists of two classes of parasitic worms, the cestodes and the trematodes.


^{Adult tapeworm. Source: Flickr CC BY 2.0 license. Author: unknown]}

Cestodes are popularly known as Tape worms due to their structural resemblance to a tape, tape worms attach to the walls of the intestines where they obtain their food from the hosts' own small intestine; they lack a digestive cavity and hence the high dependence on its host for food, the head or scolex acts as a means by which these worms attaches to the intestine, they are mainly hermaphrodites, having the ability of self-fertilization, thus they can procreate more easily as there is no need for the presence and union of separate sexes

You and the Trematodes

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Trematodes also known as flukes are digenetic worms belonging to the class DIGENEA. They are leaf-like unsegmented flat worms With size ranging from few millimeters to several centimeters. Trematodes have a well developed reproductive system and nervous system which consists of a group of paired ganglion cells and nerve trunks. They have a high reproductive ability with an adult fluke being able to lay thousands of eggs per day which matures into hundreds or thousands of larvae depending on the specie.

_{Trematodes at different developmental stages. Source Wikimedia. Creative commons license. Contributed by Jack}

Trematodes have a functional but incomplete alimentary canal, with an adult fluke possessing two muscular cup-shaped suckers located orally and ventrally. Trematodes lacks an anus; however, their excretory system consist of collecting tubules and flame cells, the collecting tubules leads to a median bladder opening at the posterior end of its body, the excretory system is bilaterally symmetrical and serves the purpose of metabolic waste removal.
The class trematoda consists of over 10,000 species with only about 9,000 (nine thousand) registered species. These are grouped into two based on characteristic differences.

The blood flukes (SCHISTOSOMES):

Theodor Bilharz in 1851 discovered a fluke in the mesenteric veins of a dead patient during an autopsy in Cairo, Egypt. This fluke was named Bilharzia after its discoverer, this was subsequently renamed to Schistosoma. Schistosomes or blood flukes are very important group of Trematodes, the diseases caused by this group of Trematodes is referred to as Schistosomiasis. They are dioecious flukes having both male and female sexes which are both essential for reproduction. They reside in blood vessels around the bladder and the intestines.

Schistosomes have three major invective species; Schistosoma japonicum, Schistosoma Masoni and Schistosoma hematobium(the actual fluke discovered by Bilharz in Cairo). Regionally important species includes Schistosoma Mekongi and Schistosoma Intercalatum. These species varies a bit from each other in terms of pathogenicity and areas of infection, but are relatively similar in morphology and life cycle.

Life cycle of SCHISTOSOMES

Both sexes of the schistosomes are involved in the process of reproduction, the life cycle of a schistosome commences after the fertilization of a female schistosome by the male counterpart. The females are held in the gynaecophoric canal of the male from where they extend their anterior end into the small venues of their host (as they reside in the blood vessels) and lay their eggs one at a time.

_{Life cycle of schistosoma. Source Wikimedia. Creative Commons CC0 1.0 Universal Public Domain Dedication license. Contributed by the CDC.}

Eggs of Schistosoma hematobium finds their way into the urinary bladder of their definitive host, the eggs of Schistosoma japonicum and Schistosoma Masoni enters the lumen of the rectum and ileum respectively, here they are passed out in urine (Schistosoma hematobium) and faces (Schistosoma japonicum and Schistosoma Masoni). Eggs of schistosomes are microscopic, when examined under a microscope, they are differentiated from the eggs of other Trematodes by the presence of spines on their eggs.

Eggs of Schistosoma hematobium has a terminally placed spine, spines of a Schistosoma Masoni egg are placed laterally. These eggs matures and releases the Miracidium (which is the first larval stage of the Trematodes) under favourable environmental conditions such as in water bodies. The miracidia are swimming organisms and can move freely in water bodies, they meet the specific intermediate host (which is always of the molluscan family) via chemoattractants secreted by these hosts and enter through the mantle or pseudopodia of the snail as their shell can not be penetrated.

In the intermediate host, they mature into sporocysts, this phase takes about 5weeks. The sporocysts mature into daughter sporocysts which matures into the cercaria, the cercaria breaks off from the sporocysts into the water body.

ORGAN FLUKES: NO ONE IS SAFE!

Fasciola hepatica (liver fluke) is the first scientifically known Trematode, it was discovered by de Brie in 1379 and named by Linnaeus in 1758. Fasciola hepatica is one of the species of organ flukes. Organ flukes are hermaphroditic Trematodes which resides in body organs such as liver, lungs, and bile ducts where they can reproduce by self-fertilization. The organ flukes have more numerous species than the blood flukes and are larger in size.

Their morphology are similar as the adult organ flukes bears two suckers in the oral and ventral positions, but the gynaecophoric canal is absent in the organ flukes as only one fluke is enough for reproduction and egg laying to take place. Species of organ fluke includes Fasciolopsis buski, Fasciolopsis gigantica, fasciola hepatica, paragonimius westermani e.t.c, these worms inhabit different organs of the body and hence differs in pathogenicity and symptoms of infection.

Life cycle of organ flukes

Organ flukes have a very long life span, fasciola hepatica can remain viable for over eight years (8years). The life cycle of the organ fluke is a bit different from that of schistosomes but there are reasonable similarity and their life cycles are in essence more similar than they are different. The life cycle of the organ fluke starts when an adult organ fluke lays it’s eggs. Eggs of an organ fluke is distinct from that of schistosomes due to the presence of a lid-like structure known as the Operculum.

The Operculum is a mobile structure and further development of the eggs is only possible when the Operculum flips open to release the eggs. Like the eggs of the schistosomes, the Miracidium (the first larval stage of development) stretches out of the egg without leaving a clear space and swims on the water body. The Miracidium penetrates the intermediate host which is usually a snail where it matures into a Sporocyst. The Sporocyst develops into several Radiae, the first generation of Radiae gives rise to the second generation of Radiae which precedes the Cercaria. The Cercaria of the organ flukes encyst and forms the Metacercaria.

SWAMPS AND RIVERINE SCARE

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Trematodes are very destructive worms, causing several diseases in man. Parasites may inhabit different organisms during the development stages; the Trematodes are not an exception to this, this makes their life cycle more complex and their control more tedious. The definitive host of the Trematodes is man and ruminant animals such as goats, cow and sheep, these ruminant animals are actually reservoir hosts as Trematodes do not undergo any further development in these hosts.

Schistosomes and organ flukes completes a greater part of their larval stage in snails, the miracidia enters the snails and develops here into the cercaria. The cercaria leaves the snail, in organ flukes, these cercaria possesses the ability to secrete substances which clothes them in a cyst, this is the Metacercaria. In the schistosomes however, this ability is not present, but the cercaria of schistosomes are able to burrow into broken skin of humans, they thus continue to swim uncysted in search of their definitive host.

This causes a major scare for people living in riverine areas as water bodies such as lakes and rivers and swamps are very good breeding ground for the Trematodes. These areas are also conducive for the snails. When the cercaria of schistosomes escapes from the snails, they swim in water, and are able to burrow into the human skin when one swims or wades in the infected water body, when these cercaria penetrates the skin, they mature into Schistosomule, the Schistosomule matures into an adult schistosome. They travel to their desired blood vessels where their harm continues.

The encysted cercaria of the organ flukes rests on water plants such as the water cress, they also penetrates the Cyclops and water vertebrates such as fishes, consumption of these foods introduces the Metacercaria into the body, where they excyst and continue their development into adult flukes and travel into their desired organs.

Temperate and tropical scare

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Apart from the inhabitants of swampy and riverine areas, people living in temperate and tropical areas also runs the risk of infection by Trematodes. People infected by Trematodes excretes the eggs in their feceases, these eggs can mature on humid soils and grasses. Snails are also found in these regions and when the snails feed on these grasses or when moving on them, the Miracidium may also infect these snails and continue their maturation, the cercaria form exits the snails and encysts on the moist grasses which also includes vegetables. On consumption of these vegetables, the Metacercaria of the organ flukes enters the gastrointestine.

Ruminant animals will also harbour the Metacercaria when they feed on the infected grasses and on consumption of the meat from these animals, man gets infected too. This is mainly for organ flukes as the cercaria of schistosomes which is not encysts may not be able to withstand the adverse weather conditions. Different species of Trematodes inhabits different species of molluscs, hence the type of snail found in a region plays a major role in the sort of trematode infection in such area.

General scare

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Anyone can be infected by Trematodes, no matter the region, as commercial vegetables and meats provides a good means of infection. Fishes, cyclops and aquatic plants are also commercialized, the Metacercaria is very viable and can withstand a given level of adversity. Manure and fertilizers can also be a route of infection as the Metacercaria which can mature in animal dung can remain quiescent even in dried dungs, when these dungs are used in improving the productivity of the plants, the Metacercaria continues it’s life on the surface of the grasses and is also invective when these plants especially vegetables are eaten raw or under cooked .

Pathogenicity of Trematodes

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Trematodes like other parasitic helminthes and protozoa are harmful to the body system. Their rate and intensity of infection varies, however, as some are less pathogenic than others when compared. Trematodes obtains it’s food from the host, this results in malnutrition despite adequate nutrition and normal feeding habits as the host struggles for these nutrients with these flukes.

The Metacercaria of the organ flukes and cercaria of schistosomes are normally not pathogenic, but on maturation into adult flukes, they cause intense reactions in the body, especially in the area where they inhabit, the fasciola hepatica which inhabits the human liver may cause liver abscess and gastrointestinal diseases.

The immune system also recognize these helminthes and react accordingly to them, eosinophils are the cells of the immune system in charge of fighting parasites, helminthes included, thus acute schistosomiasis and other sickness from helminthes is accompanied by a rise in the number of eosinophils (eosinophils), this is also a major factor in the diagnosis of helminthic infections. Professional diagnosis however are beat made by the medical laboratory scientist and the biomedical scientist as various kinds of diseases caused by trematodes shows symptoms which are different from each other.

Wrong diagnosis eventually leads to misinterpretation and erroneous treatment as the trematodes continues multiplying, hence patients who show signs of helminthic infection such as dysentery, jaundice and cholelithiasis should visit the hospital immediately.

Everyone can be safe

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Helminthes are hard to tame, this is because of their various efficient route of infection, unfortunately, these routes are important aspects of human nutrition, this makes their control very tedious and almost impossible. Complete safety from trematodes would involve the eradication of snails (their intermediate host) which vital members of the ecosystem, abstinence from aquatic foods and meats of ruminant animals and limitation of vegetable consumption are the only way to tame the trematode infection, but as a matter of fact, adherence to these practices may never bring an end to trematodes and these practices are very hard to adhere to as it automatically means malnutrition.

Everyone can be safe however, but this only a tentative statement with hopes they proper preparation and cooking of these foods before consumption will at least reduce the rate of helminthic infection , snails and other agents of helminthes survival and spread should be processed properly and cooked well to destroy the cyst of these flukes.
The metacercaria is however a very tough organism and may still persist even after proper cooking. Wading and swimming in unclean water bodies should be avoided and humans should desist from defecating and urinating inside water bodies and other actions which pollutes the water bodies.

Proper diagnosis and treatment of infected persons is also advised to ensure that the eggs of the trematodes produced in infected hosts do not continue the life cycle of the trematodes. These measures if properly followed will ‘tame’ the rate of spread of not only trematode-caused helminthic infections, but also the spread of other helminthes and parasites as a whole.

Conclusion

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Parasitoids, pests, predators, pathogens and parasites constitutes the ‘nature’s hangmen’, we face strong opposition from these organisms and they account for the greatest percentage of the natural problems faced by man. The struggle we face against these hangman is complexed by the fact that they are part of our ecosystem and their eradication destabilizes the ecosystem, apart from being part of the ecosystem, they also are part if man’s requirements for survival and also serves medical and industrial purposes.

In fighting these organisms, there arises the need to put many things into consideration, total eradication of some these organisms should come as a last resort and should be avoided if possible. Worms though harmful are essential in human medicine for the production of immunoregulatory drugs, their ability to reduce the intensity of the immune reaction geared towards their destruction is harnessed by the medical engineers for the production of immunoregulatory drugs. Amidst their pathogenicity, they also serve a purpose.

REFERENCES

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1. Helminths ~wikipedia
2. Medical parasitology 4th edition by Dr. Arora and B. B Arora (pg 155 - 178)
3. The parasite's foundation of youths ~Nationalgeographic
4. Trematoda infections ~Medscape

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