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In this article we will discuss about the effects of parasitic life on parasites and host.
Effects of Parasitic Life on Parasites:
In early parasitism there are no morphological changes in the parasite though physiological adaptations take place, later the following changes occur:
(a) There is a reduction in the organelles of locomotion’s, since the parasites are transported by the host, so that the locomotor organelles are simplified and finally lost. In some intestinal Sporozoa (Gregarina) only metaboly occurs, but in intracellular parasites (Plasmodium) there is no locomotion.
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(b) The form and shape of the body become very simple with no complex organelles (Plasmodium).
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(c) Organelles of fixation appear in some intestinal parasites (Gregarina).
(d) Organelles of nutrition are simplified (Balantidium) or even lost (Plasmodium) since food is absorbed by the body surface.
(e) Parasites acquire an ability for rapid multiplication to form numerous young ones, this ensures that at least some of the offspring will find a suitable host and continue the species (Plasmodium).
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(f) Many have two hosts in their life cycle, and one of the hosts also acts as a vector to disseminate the parasite (Trypanosoma in man and tsetse fly).
Host Specificity:
Some parasites are restricted to a few hosts only, e.g., Gregarina in a few insects, or Opalina in Anura only, but some parasites have become adapted to a large variety of hosts, e.g., Trypanosoma is found in all classes of Vertebrates in which it parasitises some five hundred species.
Thus, in the development of host relationship the above two general trends are seen, this is due partly to the infective powers of the parasite, and partly to the degree of susceptibility of the host.
Effects of Parasitism on the Host:
The following pathological conditions may be caused by parasites in their hosts:
(i) Destruction of cells and tissues of the host may take place by movement or feeding activities of the parasite, e.g., Entamoeba histolytica eats the tissue cells of the colon and red blood corpuscles of the host; Plasmodium feeds on liver cells and erythrocytes.
(ii) Parasites may cause enlargement and disorders of lymph glands, spleen and liver, e.g., Leishmania or parasites may cause ulcers in the intestine, liver and brain, e.g., Entamoeba.
(iii) Parasites may secrete poisonous toxins which cause some disease in the host, e.g., Plasmodium causes malaria.
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But in most cases of parasitism there is a mutual adaptation between the host and the parasite, the parasite is able to live and reproduce without any apparent injury, and the host offers a resistance or acquires an immunity against the parasite by producing antibodies which neutralize the effects of the parasite, or by becoming immune due to previous infection, or by increasing its powers of repairing and regenerating the injured tissue cells.
At times the host destroys the parasite by phagocytosis with the aid of leucocytes or cells of the spleen, bone marrow and liver.
The host may succeed in destroying the parasite or it may remain infected but become immune, so that it becomes a carrier of the parasite. Generally there is a delicate adjustment between the parasite and the host and they come to an elaborate compromise, if this mutual adjustment is lacking, then either the parasite is killed or the host is destroyed.
