Cestodes

** Definition, Classification and Characteristics


Definition: What are Cestodes? 


Commonly known as tapeworms, cestodes are a type of parasitic worm that can be found in the gastrointestinal tract of their hosts (definitive hosts). Although most infections have been shown to occur in developing nations, these are widely distributed across the globe.

As compared to other parasitic worms, cestodes are characterized by a long, flat body (ribbon-like) that can grow to over 20 meters in length depending on the species. They are also characterized by a segmented body that plays an important role in their life cycle.

In addition to depriving their hosts of nutrients, tapeworms, if untreated, also cause damage to the intestinal walls and intestinal blockage which can have a serious impact on the host.

 


Some of the most common cestodes include:

 

  • Diphylobothrium latum
  • Taenia solium
  • Taenia saginata
  • Hymenolepis diminuta
  • Echinococcus granulosus
  • Hymenolepus nana

 

Scolex of the rodent tapeworm with rostellum at the tip by Magdalena ZZ [CC BY-SA 3.0 (https://creativecommons.org/licenses/by-sa/3.0)]Scolex of the rodent tapeworm with rostellum at the tip by Magdalena ZZ [CC BY-SA 3.0 (https://creativecommons.org/licenses/by-sa/3.0)]


Classification of Cestodes

·      Kingdom: Animalia - As members of kingdom Animalia, Cestodes are multicellular organisms. They are also heterotrophs that are capable of sexual reproduction. 

 

·      Domain: Eukaryota - Like other members of the Domain Eukaryota, Cestodes are characterized by cells that contain membrane-bound organelles

 

·      Phylum: Platyhelminthes - The phylum Platyhelminthes consists of acoelomate flatworms that may or may not be segmented. Some are free-living flatworms while a majority exist as parasites (internal parasites or ectoparasites) or various animals.

 

·      Class: Cestoda 



Life Cycle of Tapeworms


Depending on the species, cestodes can complete their life cycle in one or several hosts. This section gives focus to the life cycle of Taenia saginata which requires an intermediate and definitive host to complete its life cycle. 

 

The life cycle of T. saginata can be said to start when the eggs (contained in the proglottid) are released, along with feces of an infected human (the definitive host) into the environment. In the environment (soil, grass, water etc), the eggs can survive for several months due to the covering that protects the eggs within.

When the eggs are consumed by an intermediate host (e.g. cow) they hatch to form oncospheres (larvae) which then enter the bloodstream before migrating to the muscle tissue. Here, they encyst to form cysticerci which are essentially encysted larvae. This form allows the organism to remain/survive (and remain infective) for years in the intermediate host.

 

* An intermediate host refers to the that which serves to support the immature form of a parasite. As such, it does not hold the mature form. 

 

By properly cooking meat (beef), the tiny cysticerci can be destroyed. However, in the event that the infected meat is poorly cooked and ingested (e.g. rare steak), the cysticerci excysts in the small intestine of the definitive host (human being) and matures into an adult in a period of about 2 months.

Using four suckers located on its head, the tapeworm is able to attach itself to the small intestine and starts producing proglottids that make up the body (stronila). By absorbing nutrients through its membrane, the proglottids continue to grow with eggs being increasingly produced.

When the proglottids (which contain the eggs) break off and are excreted to the environment, the life cycle continues once the eggs are again ingested by an intermediate host.

 

* A definitive host refers to the host that supports the mature (sexually reproductive) form of the parasite (tapeworm in this case).


* In the environment (soil, grass etc), proglottids dry and rupture to release the eggs (which are embryonated and remain in a thick protective shell). Therefore, an intermediate host may either consume the eggs or proglottid depending on how long they have been in the environment.


Sexual and Asexual Reproduction

Asexual Reproduction


In some cestodes, asexual reproduction occurs through a process known as regeneration. Here, a separated portion of the parasite (proglottid) is capable of regenerating into a new individual that develops to resemble the parent. 

According to a study that was conducted in Morgridge Institute for Research at the University of Wisconsin, signals operating in the neck of the organism was shown to play an important role in activating stem cells located along the body of the organism during fragmentation. 


Sexual Reproduction


Given that tapeworms are hermaphroditic (in that each segment consists of both male and female reproductive organs) they can not only produce eggs, but also fertilize them (within each fragment). However, eggs from one fragment of a tapeworm can also be fertilized by gametes from another segment of the same individual.

On the other hand, sexual reproduction among cestodes (within species) can also result from physical contact between two mature individuals. In this case, gametes (sperm cells) of one individual are simply absorbed through the membrane of another individual allowing for fertilization to take place.

Following fertilization, the eggs are encased in a special cocoon within the fragments that are hatched at some point during the life cycle of the organism.

 

* While sperm cells can simply be absorbed through the membrane lining of a tapeworm, in some species, reproduction has been shown to occur through penis fencing. Here, flatworms can use a penile structure (an extended two-headed stylet) to inject sperm cells through the body surface of another individual.

In this type of insemination (known as hypodermic insemination) one partner (which becomes the father) to fertilize the eggs of another (the mother). 



Male and Female Reproductive Systems

As already mentioned, tapeworms are hermaphroditic in nature with each segment carrying both male and female reproductive organs. In a mature segment, the testes are connected to small sperm ductules to form the sperm duct. This duct extends and ends up in the genital pore where it forms the muscular cirrus.

The female reproductive system consists of a bilobulated ovary that is connected to the oviduct. In addition, it consists of a sinuous tube (which acts as the vagina) that extends from the genital atrium to the oviduct (which is the site of fertilization).

 

Apart from hooks and suckers that contribute to the parasitic nature of cestodes, tapeworms also have the following adaptations:

 

·      Transmission from one host to another - By relying on one or several intermediary hosts, cestodes are able to ultimately infect the definite host.


·      Resistance - Cestodes are resistant to both the acidic nature of the intestine as well as various enzymes present in this part of the host's body. This is commonly attributed to the glycocalyx layer.


·      The degeneration of non-essential systems - Given that cestodes live in the small intestine of their respective hosts, their bodies only consist of systems they need to survive in this region. 


Morphological/Structural Characteristics


Generally, the body of Cestodes is divided into three distinct parts. 


These include:

 

This micrograph reveals the morphology of a Taenia solium tapeworm scolex by Content Providers(s): CDC Creation Date: 1986 in public domain.This micrograph reveals the morphology of a Taenia solium tapeworm scolex by Content Providers(s): CDC Creation Date: 1986 in public domain.


The Scolex


The scolex is the anterior part of cestodes. It's a specialized segment that consists of hooks and suckers that anchor the parasite to the walls of the small intestine. On the scolex, the hooks (which vary in number from one species to another) protrude from a muscular structure known as the rostellum.

This structure has been shown to be retractable, capable of protruding and retracting into a small pouch known as the rostellar pouch. 

While some species have both hooks and suckers on their scolex (e.g. Taenia solium which has between 22 and 32 hooks and four suckers), others do not have a rostellum (and thus hooks) and therefore have to rely on suckers to attach onto the walls of the small intestine in order to continue absorbing nutrients. 

For other species, like catfish tapeworms, the ovoid suckers are also used to cling onto fish given that they do not have hooks. 


* By comparing the structure of the scolex, presence or absence of hooks etc, it's possible to distinguish different types of cestodes.


Neck


The neck of a cestode is the thin segment that is immediately attached to the scolex. According to recent findings, fragmentation of cestodes is influenced by signals operating in the neck region. Through these signals, stem cells in the body of the organism are activated resulting in fragmentation. 


Proglottid


Proglottids are individual segments of the cestodes attached to the neck region. As the parasite grows, the number of proglottids increase allowing the organism as a whole to grow to several meters in length (the length is dependent on the species). 

Together, these segments make up the strobilus. A mature proglottid contains both male and female reproductive organs and is therefore capable of reproduction. Those closest to the scolex are the youngest segments (and thus immature) while those located further away from the scolex are the mature ones (ready for fragmentation). Also known as gravid segments, these proglottids are filled with eggs. 

As previously mentioned, cestodes do not have a gut (as well as a mouth). They also lack a body cavity and are therefore acoelomate. While they lack a mouth and digestive system, cestodes simply absorb nutrients through their surface membrane. 

Based on close microscopic studies, the entire surface of the strobilus was shown to be covered with tiny wrinkles as well as projections that increase the surface area for nutrients absorption. 



The body wall of cestodes has been shown to consist of:

 

·      Cuticle - Outermost layer of the body wall and is produced by the underlying cells. The cuticle is itself divided into three layers that include: the scary outer fringe, the homogeneous middle hair-like/spiny layer as well as basement membrane. 


·      Inner plasma membrane - The layer that underlies the basement membrane matrix. It serves to support the protoplasmic layer.


·      Protoplasmic layer - The layer of the body wall which is coated with cytoplasmic spine as well as microtriches.


·      Outer/surface membrane - This membrane is porous and consists of a base and cap which is filled with microtubules.

 

* In cestodes, the spaces between the organs are filled with mesenchyme cells. 


* Cestodes have been shown to produce their own enzymes and increase the rate of starch hydrolysis thus indicating membrane digestion. 


* By secreting cGMP (cyclic guanosine monophosphate), tapeworms cause the contraction of intestinal smooth muscle (of the host) which significantly decreases the flow of material through the host's intestine.


This has been shown to be beneficial to the parasite in that it allows tapeworms more time to absorb nutrients while at the same time decreasing forces that would otherwise flush them out of the body. 

 

 

Some of the other important cestode structures include:

 

Protonephridia - While cestodes lack a mouth and gut, they possess an osmoregulatory structure known as protonephridia. Scattered within the mesenchyme, this system drains in to the main longitudinal canals located on each side of the parasite. The protonephridia has also been suggested to play an important role in the excretion of waste material. 

 

Nervous system - The nervous system of cestodes consists of a pair of lateral longitudinal nerves that run through the entire length of the organism. Some of the species have also been shown to possess several additional pairs of these nerves as well as one or more ring nerve where the longitudinal nerves connect.


Cestodes and Trematodes


Like cestodes, trematodes also fall under the phylum Platyhelminthes. While they can infect various invertebrates, trematodes are also human parasites responsible for such diseases as Schistosomiasis. Like cestodes, trematodes also possess suckers and hooks with which they anchor themselves to the host. 

Although trematodes are flattened like cestodes, they are shorter and only grow to be a few centimeters in length. They are also unsegmented and thus do not fragment.

One of the other big differences between cestodes and trematodes is the fact that unlike cestodes, trematodes have a digestive tract that runs from the oral cavity to the esophagus where it's then divided into the bilateral ceca. As such, it's able to consume food material through the oral cavity and digest them. 

With regards to reproduction, trematodes are divided into two major groups that include hermaphrodites and schistosomes. Whereas hermaphroditic trematodes possess both the male and female reproductive organs, schistosomes are divided into male and female sexes. 

 

* Unlike cestodes, trematodes are capable of moving through tissue and vasculature using inchworm locomotion.

 

 

Some of the other similarities between cestodes and trematodes include:

 

 


Cestodes and Nematodes


Also known as roundworms, nematodes, and members of phylum Nematoda that can be found in various environments across the globe (various marine and terrestrial habitats). Given that nematodes can be found in diverse habitats, a majority are free-living organisms that feed on various such single-celled organisms as bacteria as well as fungi among other organisms.

As a result, they have been shown to play an important role in pest control. However, some, which exist as parasite, are harmful to human beings as well as plants. 

Like cestodes, nematodes are also bilaterally symmetrical and multicellular organisms. However, as compared to cestodes, they exhibit many differences.

As compared to cestodes, nematodes are sexually dimorphic organisms that only reproduce sexually. They are also cylindrical with a body cavity (pseudocoelom) and exhibit tissue-level organization. In addition, they are unsegmented and thus do not fragment as is the case with cestodes.

However, they have been shown to molt their cuticle periodically. Like trematodes, nematodes also have a digestive system. This is composed of the stomodeum (consisting of the mouth opening, esophagus as well as the buccal cavity), the intestine as well as the proctodeum where waste material is expelled. 



Diseases


Although a majority of infected people exhibit no symptoms, cestode infections can result in life-threatening infections. As already mentioned, some untreated cases can result in significant damage to the small intestine. This is due to the fact that the parasites' hooks can gradually cause tissue damage particularly in cases where they increase in numbers.

On the other hand, the growth of the parasite can result in digestive blockage. Here, the parasite has been shown to cause blockage to a number of organs/tissues such as the appendix, bile ducts as well as pancreatic ducts thus affecting the normal functions of the digestive system.



Apart from such infections as human taeniasis caused by T. solium and T. saginata in human beings, tapeworm infections can also result in the following complications:

 

·      Cyticercosis - Lesions and cysts caused by larvae infections. Here, the larvae spreads to other tissues and organs in the body having migrated from the intestine. 

 

·      Neurocysticercosis - This type of infection affects the brain and nervous system and can result in meningitis and seizures etc. Due to its nature, Neurocysticercosis can be fatal. 

 

·      Hydatid disease - Also known as echinococcosis, hydatid diseases occur when cestode larvae infect such organs as the liver. This has been shown to cause large cysts which in turn affect normal blood circulation. 



Prevention


Prevention is one of the best ways to control the spread of cestode infections. It's also less complicated and much cheaper as compared to treatment.


Some of the best strategies to prevent these infections include:

 

  • Properly washing hands with water and soap before eating 
  • Thoroughly washing fruits before eating 
  • Properly cooking vegetables and meat in order to kill the eggs and larval stages of the parasites (cooking meat at temperatures above 145 F is recommended)
  • Properly discarding human and animal feces
  • Avoiding raw meat
  • Treating infected animals (pets and domestic animals)

 


Treatment


Oral medication - To get rid of intestinal tapeworms, oral medicine (such as praziquantel) and laxatives may be prescribed. Whereas oral medicine dissolve or kill the parasite, laxatives help excrete them from the small intestine. 

 

Steroids - Anti-inflammatory steroids are used to mitigate the symptoms caused by infections outside the intestines. 

 

Surgery - Surgery is recommended in the case of life-threatening cysts. In particular, surgeries are carried out when vital organs are compromised to prevent further complications. 



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References


Ellen J. Lesh and Mark F. Brady. (2019). Tapeworm (Taenia Solium, Taenia Saginata, Diphyllobothrium, Cysticercosis, Neurocysticercosis). ncbi. 

 

K. D. Murrell. (2005). WHO/FAO/OIE Guidelines for the surveillance, prevention and control of taeniosis/cysticercosis. 

 

Leon Jacobs. (1941). Tapeworm Diseases. he American Journal of Nursing Vol. 41, No. 7 (Jul., 1941), pp. 754-758.

 

 

 

Links 

 

https://www.microbiologybook.org/parasitology/cestodes.htm

https://micro.magnet.fsu.edu/optics/olympusmicd/galleries/darkfield/tapewormscolex.html

https://www.mayoclinic.org/diseases-conditions/tapeworm/symptoms-causes/syc-20378174



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