The survival of a parasite in the body of the host depends upon its abiity to adapt to the surrounding environment at the site of its infection,this is called microenvironment. To adapt to this microenvironment, certain morphological, anatomical and physiological changes occur and because of which the parasite survives in the host. Such changes which facilitate a parasite to adapt to parasitic mode of life in the host itself are
called parasitic adaptations.
Differrent types of parasitic adaptations are as follows:
1.Body shape and size of he parasite depends upon the space available at the site of infection.
e.g., Intracellular parasites are very small in size – Plasmodium.
Enerozoic parasites are usually large in size – Taenia solium, Ascaris
Hepatophagic parasites are moderately large and flat – Fasciola.
Parasites dwelling in the blood vessels are slender and elongated – Schistosomes.
2. Locomotory struturres almost absent in the parasitic forms , as they need not move in search of food.
3.A parasite in the body, if occurs, without attaching to the surrounding
tissues , it gets carried away to a nonspecific site or to the exterior of the body , in which case, is survival is not possible. Thererfore , to remain at the specific site of infection , they attach to the surrounding tissues. For this purpose , all the parasites are provided with adhesive structures like hooks.
e.g., Echinostomes , Monogenetic trematodes are equipped with suckers.
Trematodes and Cestodes are equipped with rostellum.
Taenia solium (a cestode) is equipped with rostellum , hooks and suckers.
4.In the case of intestinal parasites , there is every possibility, that they are either harmed , injured or digested by the gastrointestinal secetions of the host.
To protect from the harmful effects of digestive enzymes , the parasite’s body surface is covered with
cuticle( Nematodes – Ascaris lumbricoides) or
Tegument ( Flatworms – Taenia soium).
Intestinal worms may also secrete some antienzymes that neutralize the digestive enzymes of the host.
e.g., Taenia soium.
5. In the bodyof a host where parasites live , the amount of oxygen may very high( lungs) or relatively less (intestine and liver ).
The catabolic activity of parasites living in oxygen rich environment are predominantly of aerobic type.
e.g., Human lung fluke – Paragonimus vestermoni.
The parasites live in less or no oxygen content are facultative anaerobes. They can carry on aerobic respiration if oxygen is available and anaerobic respiration in the absence of oxygen.
e.g., Ascaris lumbricoides.
Some parasites live as obligatory anaerobes as the availability of oxygen is very rare for them.
e.g ., Taenia solium , Entamoeba histolytica.
6.The reproductive potential of parasites is veery high.
e.g., Taenia soium lays about 35,000 eggs/day.
Fasciola hepatica lays about 10,000 eggs/day,
Ascaris lumbricoides lays about 2,00,000 of eggs/day.
Inspite of numerous risks during development, such high reproductive potential ensures the continuation of parasitic race.
7.In addition to the production of enormous number of eggs , there will be multiplication of larval stages resulting in the production of several infecting agents. This process called as polyembryony occurs in trematodes.
e.g., The life cycle of trematode includes five larvalstages viz.,
miracidium , sporocyst, redia ,cercaria and metacercaria. Of these , sporocyst exhibits polyembryony producing a number of rediae and redia exhibits polyembryony producing a number of
cercariae. Thus the life cycles of endoparasites are very complex as they exhibit high host-paarasite specificity.
8. Infective stages of the monogenetic parasites like Entamoeba histolytica need to survive outside the body of host in the open environment. Hence, they are exposed to harsh environmental conditions , they are to tide over the unfavorable conditions , they are enclosed in a protective envelope forming cyst.
9. Due to the presence of the parasites , host’s immunological system gets activated producing specific antibodies to eliminate parasites. However , parasites elude the action of antibodies generally by a process of molecular mimicry , in which the parasites covered by molecules that are similar to that of the host. These are considered by host’s antibodies as self molecules and fail to counteract.
9. Some parasites e.g., Plasmodium and HIV constantly change the antigenic polypeptides. The parasitic antigens change in different stages of development. Hence , it is not easy for the host to switch on the required defense mechanisms or to produce suitable vaccines against these diseases.