The Kingdom Animalia contains approximately 40 different phyla. One of the largest and most elusive is the phlyum platyhelminths. platyhelminths is a phylum of primitive "worm like" invertebrates.The most commonly known members,due to the afflictions they cause, are tape worms, flukes and other parasitic and non parasitic tribloblasts. The phylum typically (although it is under debate) comprises four classes and is made of around 20,000 species. Of these four classes only one encompasses non parasitic species which would be an indication of their tendency towards invasions with pathogenic potential.The structure of these worms varies with class but they all are bilaterian, unsegmented and minus a colem . In comparison to other worms like the annelid Lumbricus terristris which has a complete cardiovascular system and a two ended digestive system, platyhelminths is quite basic. Although simple in evolutionary terms, they are cephalised with a functional central nervous system and are very effective at flourishing within or on a host unnoticed. Two of the three parasitic classes of platyhelminths attach to the surface of the host ectoparasitically while the third operates from within. Many are flat to minimise water resistance and some can suckers, hooks and enzymes used to degrade the hosts skin. With such highly tailored equipment it is clear to see that platyhelminths have evolved to be very effective as parasitic invaders.
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Depending on their class, platyhelminths can range in size any where from 1mm to several metres in the case of tapeworms. platyhelminths are usually known as flatworms and not without good reason. They have a dorsoventrally flattened body form. However, the worms are without a proper circulatory or respiratory system and their specialised flattened structure allows for thin cell regions to allow diffusion of O2 and other nutrients. There is a blind gut in some species but this only leads down to a Y shaped tract that leads to two cecum dead ends. All platyhelminths are unsegmented and without an internal body cavity and in this respect, among others, they are very different to other worm like organisms eg certain annelids and nematodes. In fact the New-Zealand non parasitic Planarian is known to prey on earthworms.
The enviroment in which platyhelminths' live and feed depends upon the nature of their feeding. Turbellaria, the only class of platyhelminths which is not entirely parasitic are free living aquatic metazoans which scavenge on dead animals or prey on other smaller organsims. Turbellaria is a heterogeneous group (1) mostly found in fresh water or damp earth. They are equipped with cilia on the ventral surface and move by gliding along the film of mucus they secrete.(2) Monogenea and Trematodes live on outer surfaces of other animals or internal organs of animals. (2)Monogeans are monogenetic and so can mature fully on or in their host. Trematodes, eg. the group Dignea, are digenetic. This means that there is a two host minimum for maturation and growth. Each at different stages of maturity. Trematoda are found in lakes attached to internal organs of fish or in the marshes in snails in a lifecycle which will be discussed later. Cestoda are the parasitic class of flatworms which are best known as Tapeworms. Living comfortably in the G.I tracts of vertebrates when mature and other organsims when not, the tapeworm is an endoparasitic member of platyhelminths and it's alternating life cycle has been crucial in ensuring it's parasitic survival.
Parasitism involves harming but not killing the host meaning that platyhelminths has evolved to become not selfish.
The class Cestoda of platyhelminths are often found in the digestive tract in humans however larvae may migrate and form cysts on other vital organs causing potentially fatal conditions eg Cysticercosis . Symptoms of an infection vary widely with many reporting no symptoms and others complaining of Nausea, weakness, abdominal pains , weight loss and poor absorption of nutrients. Tapeworms function pathogenically using a few mechanisms. Their anterior structure is covered in suckers and hooks for adhering to the intestinal lining. At this point the worm (which unlike other classes has no mouth or GI cavity) absorbs nutrients that are being released by enzyme activity in the gut. This is relatively harmless aside from possible malnourishment. The head or Scolex of the worm is anterior to the reproductive proglottids which creates thousands of eggs. Through defaecation and horizontal transmission the eggs are transfered into a secondary host e.g. pig or cow. Proper cooking should kill any potential invaders but meat is often undercooked. Once in the body again, the lifecycle is complete and the larvae encyst in various tissues of the host. A good example of this is the food-borne disease Cysticercosis. Often caused by Taenia Solium from undercooked pork products, a build up of cysts can lead to an inflammatory response in the body. In one case report (Chung et al,1998) a 47 year old man presented a large intrahippocampal solid mass which was caused by T.Solium. The neurocystcircosis lead to temporal lobe epilepsy which remained until surgically removed. Developing countries experience increased counts of late onset epilepsy due to poor food preparation and subsequent contamination and proliferation.
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Living in the gut also, the worm must be well equipped to deal with digestive enzymes. Cestoda flat worms eg Hymenolepsis diminuta secretes a proteolytic inhibitor for protection. Treatments for Cestoda usually come in oral drugs such as niclosamide which halts the oxidative phosphorylatation in the Cestodas mitochondria robbing them of energy.
Monogenea and Trematodes are ectoparasitic flat worms (although Trematodes are more endoparasitic). They have a mouth and partial GI cavity and so are not reliant on the intestines breaking down the food like Cestoda are. This allows them to be more versatile and leech on the outside and inside of a host. The basic structure of Monogenea and Trematoda is the same and both parasites are given the term "flukes". A key difference between the two is that monogenea as the name might suggest has orders below it which are monogenetic and develop in one stage. However Trematoda are digenetic and require two(although more than two is common). Trematoda is also far more dangerous. Common diseases causes by trematoda are done by Schistosomas (blood flukes). A blood fluke can lead to "swimmers' itch" and various types of schostasomiasis.
Symptoms of infection or contact with a schostosoma include anemia, dysentary and even eosinophillia, a noticable drop in a type of white blood cell resulting in lower immune activity. Approximately 200 million people in the world are infected with blood flukes. The Trematoda river flukes along side the other parasitic classes differ from Turbellaria in that a host is used for sexual reproduction. This is carried out by mature flukes, living unnoticed, in intestinal blood vessels. Eggs are released via faeces possibly into a lake area. Once in a wet area with exposure to other animals the leggs develop into larvae with cillia. This is the miracidium stage. Now the larvae infects a snail.The snail may not be the desired host yet it is infected to use as a reproduction ground intermediate. Inside the Snail the larvae reproduce assexually which releases a sprorocyst into the environment. The motile larvae bury in any blood vessel or opening on a human they can get. When in the human further cyst formation occurs and develops. As it travels through a minimum of two hosts there is a minimum of two immune responses to encounter and avoid. A strong and effecient mechanism had to have developed to make such a resilient organsism. The first mode of defence comes from the multi component cover, the teguement of the parasite. This acts as a preventitive physical barrier against enzymes and other immune responses. An array of antioxidant proteins used by the worm can prevent toxic free radicals from forming caused by the hosts immune system. Lastly, the worm may wish to avoid the immune system entirely. T.Solium alters its surface proteins such as GTPases, to evade detection. However when parasitic cysts grow too large they may rupture and present antigens to the host.
Schistomiasis is endemic in 74 countries with intestinal schistomiasis found prevalent in 54 (4)
Potential targets for drugs have been worked on with the S.mansoni gene project and the redox enzyme thoiredoxin glutathione reductase looks promising (5).
The second fluke class is not of grave concern to humans except when it comes to eating freshwater fish. . Most fish harbour one or more species of parasite without showing any signs (Feist SW 2008). This fish is the only host for the parasite in it's lifecycle as it is monogenetic. It has cilia in larvae form and may swim to find a host and latch on with hooks on its opisthaptor (posterior bulbous structure). Although they are ectoparasites they may live in the bladders of frogs and turtles and one species Oculotrema hippopotami lives around the eye of a hippopotamus. At no longer than 2 cm long Monogenean flukes are very versatile.(6)
The impact of a parasite on a host can affect more than it's nutrient intake. Primal behaviour was found to altered in snails infected with the blood fluke Trematoda. The flukes infecting the snails were able to alter a hormonal balance in the snail and cause it to seek out the light and at the same time ignore predators. This gave the blood fluke an opportunity to change its host from the snail to it's next host, a bird where it could undergo further development. This is a perfect example of the flatworm Platyhelminths adapting to it's niche as a parasite. Tapeworms often go undetected for years then are surgically removed spanning several metres. The evasion of the immune system in that example shows how truly effecient this phylum can be at surviving and thriving.
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