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Lecture - 4 (1st DDS program)

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Morphological adaptations of parasites to parasitism

Parasites (nematodes, trematodes, cestodes) have become adapted in different ways to the microenvironment of the host’s body (especially intestine). Life in this specialized habitat affords parasites a reliable source of nutrients, a relatively homeostatic environment, and protection from predators but, in exchange for these advantages, presents the special challenges of exposure to digestive enzymes, normal peristalsis, and host immune response to infection. Morphological adaptations of helminths to their microenvironment include modification of the tegumental surface that affords protection and increases absorptive surface area, development of specialized attachment organs, and, in some cases, complete loss of their own internal digestive system. This way the adaptations occur either in the form of degeneration of certain organs or development of new organs.†
Degeneration of organs:
1. Organs of locomotion:
Since the parasite reside in the host body where they live well protected and nourishment readily available, there is no need to move. Hence, the locomotary organelles are completely lost. However, in cases where the larval forms are free living, the locomotary organs in the form reappears:†
- flukes and miracidium larva (ciliated),
- Schistosoma and furcocercaria (tail),
- In Pediculus and Pthirus legs are short and terminate with a single claw and opposing "thumb". Between its claw and thumb, the louse grasps the hair of its host. With their short legs and large claws, lice are well adapted to clinging to the hair of their host. These adaptations leave them incapable of jumping, or even walking efficiently on flat surfaces. Lice can climb up strands of hair very quickly, allowing them to move quickly and reach another host.
2. Trophic organs:
The organs which are concerned with nutrition are called trophic organs. As the parasite derives fully digested or partially digested nutrition from the host’s body, the alimentary canal has either totally disappeared (tapeworms) or exhibit fair degree of degeneration (flukes).
3. Nervous system and sense organs:
The endoparasite live in a well-protected and more or less stable environment inside host’s body in perpetual darkness, there is no need of complex form of nervous system, consequently the photoreceptor organs (eyes) and other sense organs have completely lost. The central and peripheral nervous system have also reduced considerably as compared to the other free living species.
Development of new organs
1. Size
Many parasites are large compared with their free-living relatives. It depends on the microenvironment where they live. Usually parasites in host digestive system reach long size whereas the extra-intestinal parasites (muscles, liver, subcutaneous tissue) are relatively small.
2. Shape of the body:
In most internal parasites the shape of the body have become round, dorso-ventrally flattened or ribbon like which enable them to fit in the space of host’s body where they reside.†
In ectoparasites (fleas) the body is laterally flattened because of the movement through the hair. The legs are adapted for jumping.
2. Development of protective covering:
- tegument of tapeworms and flukes;
- cuticle of nematodes.
In external layer helminths have lost epidermis and have developed several layered thick protective covering as cuticle. The cuticle is resistant to host’s digestive enzymes, antitoxin and abrasive action of the food and roughage passing through the digestive tract. The cuticle is permeable to water and also help in the absorption of food. Protective spines have also developed in the cuticle of several trematodes.
3. Development of adhesive organs:
The endoparasites live in an environment where there is always a danger of being dislodged or swept away along with the hosts body fluid or peristalsis of the alimentary canal. Hence, there is always a demand for certain organs of attachment to keep the parasite in their respective position.†
- Suckers: are strong organs or attachment found in both trematodes as well as in cestodes. In Fasciola hepatica, there are two suckers, an anterior sucker surrounding mouth and a large ventral sucker. In Taenia solium, there are four suckers on the scolex.
- Hooks: the anterior end of the body of cestodes and trematodes bear hooks and spines as organ of attachment.
- Jaws: in nematodes (Ascaris) chitinous jaws are present inside mouth which helps them to anchor with the wall of gut.
- Glands: the secretory glands present near the mouth in certain helminths help them to tissues by secreting histolytic juices.
4. Development of protective elements in dispersive stages
- Cyst, oocyst: infective stages of some parasites like Entamoeba histolytica, Giardia, Cryptosporidium need to survive outside the body of host in the open environment. Hence, they are exposed to harsh environmental conditions, they have to survive over the unfavourable conditions , they are enclosed in a protective envelope forming cyst.
- Eggs of Ascaris, Taenia, Trichuris.

 Author: Piotr Nowosad date: 2020-03-18  print    back  
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