Introduction to

 MEDICAL PROTOZOOLOGY

 

This is the kingdom comprising some of the morphologically simplest organisms of the animal kingdom.

The parasitic protozoa, unlike helminthes, can replicate within the host body. This explains their survival and overwhelming infections that develop from single exposure.

 

MORPHOLOGY
  Protozoa are unicellular organisms, sometimes called noncellular or acellular, being not divided into cells, but formed of one and only cell. Each protozoon performs all the functions of life.

The protozoon is formed of a mass of protoplasm differentiated into:

  • Cytoplasm

 

The cytoplasmic matrix consists of very small granules and filaments suspended in a low density medium with properties of a colloid, and it is divided into:
a) Central zone called endoplasm, which bears the nucleus and other membranous organelles as mitochondria, Golgi apparatus, micromeres, food vacuoles and volution granules.

  1. b) Outer zone called ectoplasm, which is more transparent and in the gel state which gives rigidity to the protozoon’s body. The bases of cilia and flagella are embedded in it.
  • NucleusThe protozoan nuclei are typically oval, discoid or round, usually vesicular, but in others as ciliates the nucleus may be dense, elongated, chain like or branched. 

 

  1. LOCOMOTOR ORGANELLES
  2. Like all cells, the body of protozoa is covered by a trilaminar plasma membrane. In some organisms it has a glycoprotein surface coat which has immunological importance.
  3. All protozoa are eukaryotes i.e. true cells. They have nucleus, nuclear membrane and cell wall. Their D.N.A. is carried on well-defined chromosomes which are contained within the nucleus.
  1. Pseudopodia: which are temporary extensions of the cell membrane and ectoplasm, which is followed by the rest of the cell in its direction. Pseudopodia are used also in food vacuole formation.
  2. Flagella: a flagellum is a slender, whip-like structure composed of a central axoneme originating from kinitosome, and an outer sheath which is continuation of cell membrane. The wavy movement of the flagellum results in directional movement of the cell.
  3. Apical complex and microtubules: gliding movement of some protozoa is caused by epicytic folds which are longitudinal surface ridges under which there are microtubules which are connected to an apical complex. The apical complex and the microtubules act as a brain and nerves thus the folds give gliding movement.
  4. Cilia: These are small, numerous and structurally similar to flagella. The cilia cover the whole cellular surface in longitudinal rows. Their kinitosomes are connected to each other such as a cable network thus all cilia move in one and the same direction.REEPRODUCTION AND LIFE CYCLE
  5.  
  • Asexual reproduction:
    1. Binary fission: It means that one parasite becomes two. The nucleus and the cytoplasm divide resulting in two daughter organisms. The plane of fission is random in amoeba, longitudinal in flagellates and transverse in ciliates.
    2. Multiple fission (schizogony): The nucleus is divided repeatedly giving daughter nuclei which arrange peripherally, surround themselves with portions of cytoplasm and cell membrane within the mother cell. The mother cell then bursts giving merozoites and residual cytoplasmic mass. During this process the mother cell is called schizont or segmenter.
    3. Endodiogeny or internal budding: This is the formation of daughter cells within the mother cell similar to it in all parameters. This process may either result in that the mother cell membrane remains thin and contains many daughter cells and so forms a psuedocyst ass in Toxoplasma, or in transformation of the mother cell wall in a true cyst wall as in Sarcocystis.
    4. Sexual reproduction:
      1. This is called also gametogony or sporogony when the cells divide by miotic division into male and female gametes. Gametes of different sexes unite to give zygote, this transforms into oocyst ------ sporocyst ---------- sporozoites which start endodiogeny in the new host.
      2. Conjugation:      Many protozoa can secrete a resistant covering cyst wall and change into a resting stage. Cyst wall may contain chitin or sometimes cellulose. During encystment, the cyst wall is secreted and some food reserves are formed as starch or glycogen and protein material called chromatiod bodies in some species. In old cysts the food reserve is consumed and no longer present. Most protozoa are particle feeders. Their mouths are either:
      3. FEEDING AND METABOLISM
      4. Excystation is stimulated by return of favorable conditions. Mechanisms for excystation may include water absorption leading to swelling of the cyst, secretion of lytic enzymes by the protozoon- and action of host’s digestive secretions on the cyst wall. Excystation is followed by rapid division of the nuclei and then cytoplasm leading to formation of double number of trophozoites.
      5. The conditions favoring encystment may be food deficiency, desiccation, decreased oxygen concentration or pH and temperature changes.
      6. ENCYSTMENT AND EXCYSTATION
      7. This occurs in ciliates where the nuclei of two trophozoites fuse together, then divide into two nuclei followed by cytoplasmic division. The resulting pair divides by binary fission resulting in four daughter cells. This type of reproduction thus includes fusion and interchange of nuclear chromatin between trophozoites before binary fission.

 

  • Temporary pits that form in the plasma membrane and proceed to form food vacuole as in amebae.
  • Cytostome as in some flagellates and ciliates.
  • Micropore as in Apicomplexa. Most protozoa excrete nitrogen as ammonia. Other waste products are CO2, lactate, pyruvate and short chain fatty acids. These are extruded outside the cell either through food vacuoles or the plasma membrane itself. 
  • Contractile vacuole is involved in osmoregulation. This is present in protozoa which are hyper tonic to their environment as the excess water is pumped into the vacuole, e.g. Balantidium coli.
  • EXCRETION AND OSMOREGULATION
  • Endocytosis is either by pinocytosis for fluids and phagocytosis for particulate matter. Metabolism of protozoa is by glycolysis.

 

CLASSIFICATION OF PROTOZOA

 

SUBKINGDOM: PROTOZOA

PHYLUM:

 

 

SARCOMASTIGOPHORA

I.  Subphylum Sarcodina

Class Rhizopoda

-Order: Amoebidae

*Family: Endamaebidae

Genus : Entamaeba

             Endolimax

             Iodamaeba

 

-Order: Schizopyredina

*Family: Valkamphidae

Genus: Naegleria

*Family Hartmanellidae

Genus: Acanthamaeba

 

II.Subph. Mastigophora

Class: Zoomasigophora

-Order Retromonadida

*Family:Retromonadidae

Genus:Chilomastix

Genus: Retromonas

 

-Order Diplomonadida

*Family Hexamitidae

Genus:Giardia

 

-Order Trihomonadia

*Family:Trichomonadidae

Genus: Trichomonas

 

-Order: Kinetoplastida

Family Trypanosomatidae

Genus: Tpryanosoma

            Lieshmania

     

APICOMPLEXA

Class Sporozoea

I.Sublass Coccidia

Order Eucoccidia

-Suborder:-            Haemosporina:

+Family: Plasmodiidae

Genus Plasmodium

 

-Suborder:- Eimeriina

+Family: Eimeriidae

Genus Eimeria

           Isospora

+Family: Sarcocystidae

Genus  Toxoplama

            Sarcocystis

+Family:

     Cryptosporidiidae

Genus: Cryptosporidium

 

II.Subclass:

       Piroplasmea

Order Piroplasmida

Genus Babesia

 

 

CILIOPHORA

Order: Trichostomatida

Family: Balantidiidae

Genus: Balantidium

MICROSPORA

Genus: Enterocytozoon

Genus: Encephalitozoon