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Parasitism in Protozoa |Types of Parasites |Host specificity

Parasitism in Protozoa |Types of Parasites |Host specificity

Parasite and parasitism

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 between the parasites and their hosts. It may be defined as “an association between two organisms of such kinds that one (parasite) lives and feeds, temporarily or permanently, either in or on the body of the other (host)”. Parasitic species occur in all groups of Protozoa, and one of them, Sporozoa, is exclusively parasitic. A brief account of parasitism in Protozoa may be given as follows

Types of parasites

  1. Ectoparasites– Those Protozoa which inhabit the external surface of their hosts. Hydramoeba hydroxena, feeding on the ectodermal cells of Hydra and Ichthyophthirius multifilis, burying in the epidermis of freshwater fishes, are examples of ectoparasitic Protozoa.
  2. Endoparasites Those parasites which live inside the body of their hosts. These are divided into four categories :

(a) Parasites of digestive tract- Those endoparasites which dwell inside the lumen of alimentary canal of hosts. Giardia lamblia, a parasitic flagellate, Entamoeba histolytica, a parasitic amoeba, Isospora hominis, a parasitic coccidian, Balantidium coli, a parasitic ciliate, are all intestinal endoparasites of man.

(b) Parasites of mouth- Those endoparasites which reside in mouth cavity of hosts. In man Entamoeba gingivalis and Trichomonas tenax are found in pockets between the gums and teeth.

(c) Parasites of genital tract- Those endoparasites which inhabit the genital tract of hosts. In human female, Trichomonas vaginalis lives in vagina.

(d) Parasites of body tissues- Those parasites which live within tissues of hosts and may enter through skin or from digestive tract. Species of Trypanosoma, Leishmania, Plasmodium and Babesia are common blood parasites of vertebrates. Species of Eimeria and Isospora occur in the epithelial lining of gut of their respective hosts.

  1. Hyperparasites– These Protozoa parasitizing other species of parasitic Protozoa. the opalinid (Zelleriella) which lives in the frog’s intestine, is parasitized by a certain amoeba. Nosema notabilis parasitizes Sphaerospora polymorpha which is a parasite of urinary bladder of toad fish.
  2. Pathogenic parasites- Most of the parasitic protozoans do not cause disease conditions in their hosts except producing minor symptoms. On the other hand, certain parasites act as disease-causing organisms in man and other animals. Such parasites are referred to as pathogenic parasites. Important pathogenic parasites of man Leishmania donovani, Trypanosoma gambiense, Plasmodium vivax, Entamoeba histolytica, etc.

Host specificity

Two general trends are exhibited regarding they are development of host specificity in parasitic Protozoa. Firstly, some parasites can successfully parasitize a wide variety of hosts. Typanosoma, Entamoeba and Eimeria belong to this group. Secondly, some parasites have become restricted to only a few specific host. Coccidia of mammals (eg. Plasmodium) are such parasites.


Protozoan parasites have different ways of infecting their hosts. Entamoeba gingivalis is transferred directly from one man to another through mechanical contact, like kissing (direct transfer). Some, such as Entamoeba histolytica and Eimeria tenella, are transferred by cysts in water or food (contaminative transfer). Species of Trypanosoma, Plasmodium, etc. are transmitted by certain invertebrate vectors (inoculative transfer). Transmission by invasion of ovary or egg takes place for species of Babesia. Placental transference has been reported for Plasmodium in man (congenital transfer).

Life cycle

Many parasites, such as Eimeria and Monocystis, have only a single host during their life cycle, only a part spent outside the host. These are called monogenetic parasites. Other protozoan parasites (e.g. Plasmodium, Trypanosoma) have two or more hosts, belonging to widely separated animal groups. The two hosts are usually designated as the primary host, in which the parasite’s ancestors evolved and the secondary host or vector, which acts as a transmitting agent for the parasite to the other host. These are called digenetic parasites. If the parasite undergoes part of its life cycle in vector, its transmission is called cyclical, if not, it is referred to as mechanical transmission. Besides the twohosts, some other animals may be infected by the parasites and serve as a source for infecting other animals. These animals constitute the reservoir hosts.

Effects of parasites on their hosts

Parasitic Protozoa bring about change within their hosts to a lesser or greater degree. Some prove to be injurious for their hosts, while others produce almost no effect. Entamoeba histolytica destroys host’s large intestine causing large ulcerations. Eimeria stiedae is known to cause hyperplasia of the hepatic cells of rabbits. The coccidian parasite of earthworm, Polymnia nebulosa, brings about hypertrophy of the sperm mother cells. The malarial parasite of birds, Plasmodium gallinaceum, clogs the fine blood capillaries. Similar examples are not few but numerous in the world of protozoan parasites.

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