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Term Paper on Parasites
Term Paper Contents:
- Term Paper on the Definition of Parasites
- Term Paper on the Type of Parasites
- Term Paper on the Parasitism and Host Specificity
- Term Paper on the Parasitic Effect on Parasites
- Term Paper on the Parasitic Adaptations in Parasites
Term Paper # 1. Definition of Parasites:
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The word parasite has been derived from two Greek words, para = beside and sitos = food, which means eating beside one another. Parasites may be defined as “the species which exist at the expense of certain other species called hosts, and are biologically and economically closely connected with them throughout their life-span”.
Parasitism is an association in which one organism called the parasite which lives on or within the body at the expense of another organism known as host. The parasitic mode of life is a secondary state, the parasite having arisen frequently and independently from free-living ancestors.
The parasitic adaptations in the organization and life history of a parasite can be defined as the profound changes and modifications occurring in pursuit of successful parasitism so that the parasite is fully adapted inside the body of the host. Parasitism is an association between two organisms when one the parasite lives on another (host) receiving nourishment and shelter without any compensation for the host. The parasitic lives at the cost of the host, causing it harm and sometimes prove so harmful that it ultimately results in its death.
Term Paper # 2.
Type of Parasites:
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The animals inhabiting the outer surface of the host are called ectoparasites. The protozoa living inside the body of the host are called endoparasites. The endoparasites may further be coeloznic, i.e. living in the cavities of the body such as Entomoeba, Trichomonas, Balantidium, etc. or intercellular (bistozoic) living between. Finally, the form living inside the cell such as Plasmodium is called intracellular or cytozoic.
The endoparasites may of following types:
(i) Parasites of Digestive Tract:
Those endoparasites which dwell inside the lumen of alimentary canal of host. For example giardia lamblia, a parasitic flagellate, Entamoeba histolytica, a parasitic amoeba, Isospora hominis, a parasitic coccidian, Balatidium coli, a parasitic ciliate.
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(ii) Parasites of Mouth:
Those endoparasites which reside in mouth cavity of hosts. In man Entamoeba gingivalis and Trichomonas tenax are found in between the gums and teeth.
(iii) Parasites of Genital Tract:
Those endoparasites which inhabit the genital tract of hosts. For example Trichomonas vaginalis lives in vagina.
(iv) Parasites of Body Issues:
Those parasites which live within tissues of hosts and may enter through skin or from digestive tract. For example, species of Trypanosoma, Leishmania, Plasmodium and Babesia are common blood parasites of vertebrates.
Symbiosis:
In such type of relationship, there is reciprocal benefit between one animal called a symbiont and the other called a host. The symbiont lives in the body of the host e.g. Trichonympha lives in the gut of termites in symbiotic relationship. Trichonympha obtains food and lodging and in return it digests the wood eaten by termites. The termites are incapable of digesting wood.
Commensalism:
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In such type of relationship, one organism called a commensal is benefited and the other organism known as the host is neither benefited nor harmed e.g. Nyctotherus in cockroach gets food from the host, but the host is not injured in any way. The distinction between symbiosis and commensalism is not very sharp. E. coli in man is a commensal, but it may become symbiotic when it eats up bacteria which may be harmful to man.
Term Paper # 3.
Parasitism and Host Specificity:
In such type of relationship, one organism, the parasite lives on the body or inside the body and at the expense of another organism known as the host. Parasitic mode of life is a secondary state, the parasites having arisen frequently and independently from free living ancestors.
The relationship of a parasite to its host is of varying degrees of intimacy, the parasite may be epizoic or ectoparasitic which lives on the external surface of the host though it may wander into the buccal cavity or rectum, e.g. kerona. The parasite may be entozoic or endoparasitic which lives inside the body of the host.
Host Specificity:
Two general trends are exhibited regarding the development of host specificity in parasitic Protozoa. Firstly, some parasites can successfully parasitize a wide variety of hosts. Trypanosoma, Entamoeba and Elimeria belong to this group. Secondly, some parasites have become restricted to only a few specific host coccidia of mammals. Thus are parasite can complete its life cycle only is their specific host only.
Term Paper # 4.
Parasitic Effect on Parasites:
Although a given host may be immune to a parasite by virtue of being constitutionally unstable as a habitat, it is important to recognize that the establishment of a parasite in a host also involves the capacity of the parasite to resist reactions of the host that might be deleterious.
These host reactions would include inflammatory tissue response and immune responses of a humoral character. Both types of host reaction occur in most parasitism involving characters. The immune system in higher vertebrates is composed of several tissue components.
Due to this specialized mode of life, the parasites undergo great morphological and physiological changes. In the early stages of parasitism, the adaptations are only physiological but in extreme cases both morphological and physiological modifications are brought about.
These structural modifications include:
(a) Reduction or simplification of the organs of locomotion as the parasite is transported by the host thus in intracellular parasite like Plasmodium, the locomotor organs are completely lost;
(b) The form and shape of the body becomes much simplified;
(c) In some parasites like Gregarina, some organs appear which provide holdfast for attachment on the body of the host;
(d) The organs of nutrition such as undulating membrane, etc. are simplified or lost as the food is absorbed through the body surface;
(e) Anoxybiotic respiration in some cases;
(f) Cyst formation in order to avoid desolation by the digestive enzymes of the host;
(g) Ability for rapid multiplication;
(h) Many parasites have two hosts in their life cycle and one of the hosts also acts as a vector to disseminate the parasite.
Term Paper # 5. Parasitic Adaptations in Parasites:
Changes from a free-living condition to a parasitic life involve changes in the body of the animal. Due to parasitic mode of life certain new structures are developed due to new environment requirements similarly certain characters, which are of no use of new environment, and lost. All such changes are included with effect on parasitism on the parasite. The helminthes are modified morphologically as well as physiologically to live in their particular environment.
I. Morphological Adaptations:
Various morphological adaptations have developed along two lines in helminthes.
These are:
A. Degeneration or loss of organs and systems,
B. Development of new organs and devices.
A. Degeneration of Organs and Systems:
In general the parasitic helminthes have lost locomotory digestive and sensory organs.
(i) Locomotory Organs:
Locomotion is actually an effort for procuring food. But parasites habitually inhabit such places in the host’s body, where sufficient food is available without effort. Thus, the organs of locomotion, such as the cilia of free-living turbellarians, are absent in the parasitic forms. It is interesting to note that the locomotory organs are duly present in free-living larvae of parasitic forms; the miracidium possesses cilia and the cercaria possesses a tail for locomotion.
(ii) Digestive Organs:
The parasites feed on digested and semi-digested food of the host. It does not require digestive glands and well developed digestive system. In the trematodes the alimentary canal consists of suctorial pharynx (to suck the food) and branched intestine which does not open to outside. In the tapeworms, the digestive system is completely absent. It absorbs its food through its tegument (body wall). The parasitic larval stages of helminthes also lack well developed digestive system.
(iii) Nervous System and Sensory Organs:
Parasitic helminthes lead a safe, luxurious and protected life inside the host’s tissue and, therefore, do not need highly developed nervous system and sense organs. As a result, these parasites do not contain sense organs, and the nervous system too is not well developed.
B. Development of New Organs and Devices:
Several new structures like adhesive organs, organs for absorbing food and protection have developed in the helminth parasites.
(i) Tegument:
The body wall or the tegument of parasitic helminthes is highly specialized because it protects the parasite from the digestive enzymes of the host, and also serves as organ of food absorption (ex. Taenia), respiration and excretion. Tegument is non-cellular. It is formed by the cytoplasmic extensions of the cells located in the parenchyma.
If often contains spines or scales which firmly anchor the parasite in the host’s tissue. The tegument contains numerous microvilli which tremendously increase the absorptive surface. The parasite absorbs its food through these microvilli. The tegument is continuously renewed by the cells forming it.
(ii) Adhesive Organs:
For a firm grip on or in the host’s body some special organs of adhesion are necessary. The flatworms, for this purpose, are variously armed with suckers, hooks and spines. The suckers themselves may be with or without hooks and spines.
(iii) Organs of Penetration:
The parasites must penetrate the host’s body for entering it. So the parasites develop certain structures for penetration. Miracidium larva has a conical process at the anterior end called apical papilla. These are a pair of penetration glands inside the body near the anterior end.
(iv) Digestive System:
All the endoparasites except cestodes have a suctorial pharynx by means of which liquid food is sucked into the intestine. Intestine is always branched.
(v) Osmoregulation:
Flame cell system is well developed and well adapted for osmoregulatory needs of the parasite.
(vi) Reproductive System and Devices:
It is highly developed and the production of eggs is prolific. The parasitic flatworms are hermaphrodite. In cestodes the reproductive organs are much more elaborate and are repeated in each proglottid. Each mature proglottid possesses one (Taenia solium) or two sets (Diphylidium) of male and female reproductive organs. In each gravid proglottid all other organs of the system degenerate to make room for the uterus which is greatly enlarged and branched to accommodate a large number of eggs.
II. Physiological Adaptations:
(i) Protective Mechanism:
The intestinal parasites protect themselves from the digestive action of intestinal enzymes of the host by:
(a) Stimulating the intestine to secrete mucus which forms protective cover around the parasite.
(b) By secreting antienzymes to neutralize the digestive enzymes of the host.
(c) By continuously renewing their tegument.
(ii) Anaerobic Respiration:
The pH tolerance of the parasite is high from 4-11. The intestinal parasites live in an environment completely devoid of oxygen. The respiration is of the anaerobic type consisting of extracting oxygen from the food stuffs. In the absence of oxygen energy is obtained by the fermentation of glycogen in which glucose in broken into lactic acid.
(iii) Osmoregulation:
Usually the osmotic pressure of the body fluids of the parasite (especially trematodes) is almost equal to that of the host, and thus the parasite has no osmoregulatory problem. In intestinal tapeworms the osmotic pressure is a little higher. It allows absorption of digested food from the intestine of the host. Osmoregulation is done by the flame cells.
(iv) Periodicity:
There is some parasite which appears at definite period. The larval of Wuchereria bancrofti circulate into the peripheral blood circulation in the night between 10 P.M. to 4 A.M. for further development they require culex mosquitoes. The culex bites a man at night i.e. nocturnal in habit. Their migration is correlated with the nocturnal habit of culex.
(v) Over Production:
Parasites develop enormous fertility, producing millions of fertile eggs. This is correlated with the passive transference of the infective stages of the parasites from one host into another. While passing through the complex life-cycle, these potential off-springs face several hazards as a result of which a very small percentage of the total eggs produced reach adulthood.
For example in Fasciola, if the eggs fall in water, if the first larva (miracidium) finds a suitable snail within a limited period, if the cercaria encyst, on vegetation within the reach of final host and if the metacercaria happen to be ingested by the proper final host, only then the life-cycle is completed.
Not only this, the larval stages themselves multiply say for example for sporocyst produces a number of rediae. A single redia produces a number of cercaria larvae. Similarly a single hexacanth may multiply by generating buds which produce daughter and grand-daughter scolices (hydratids of Echinococcus).