In this article we will discuss about the structure and life history of flagellate or Giardia intestinalis (explained with diagram).
Structure of Flagellate:
Flagellate occurs in two different forms:
1. Trophozoite or feeding stage and
2. Cystic stage.
1. Trophozoite or Feeding stage:
It possesses a bilateral symmetrical body with organelles occurring in pairs and measures 10-18 μm length. The body of trophozoite is a ‘tear-drop shaped’ with a convex dorsal surface and a concave ventral one.
This concave ventral surface is provided with two depressions, sometimes called adhesive discs which make contact with the intestinal cells of the host. A single or double median body is found just below the adhesive discs. Flagella occur in four pairs (anterior pair, posterior pair, ventral pair and caudal pair). Two nuclei occur at the broader end of the body.
There are double sets of nuclei and kinetosomes (axostyles). Each nucleus has been shown to contain a haploid number of chromosomes. No structures identifiable as mitochondria, smooth endoplasmic reticulum or Golgi-complex have been identified in this stage (Fig. 2.1).
Note:
Trophozoites of Entamoeba histolytica or Giardia intestinalis may also be found in the stool specimens for which fresh stool specimens are required to be examined to study the mortality of trophozoites. Trophozoites should always be studied in normal saline preparation.
2. Cystic stage:
The fully formed cyst is oval in shape and measures 12 μm long by 7 μm broad. The cyst wall is thin and the organism does not fill the entire cyst. There are four nuclei which may remain clustered at one end or lie in pairs at opposite poles. The remains of the disintegrated flagella forming a central ‘streak’ visible in iodine and the margins of the sucking disc may be seen inside the cytoplasm.
Life History of Flagellate:
In the trophozoite stage Giardia undergoes multiplication in the upper part of the intestine by binary fission. When conditions in duodenum are unfavourable, encystment takes place, usually in the large intestine. An acid environment often causes the parasite to encyst. During encystment, a tough resistant wall is formed by the parasite and the cell undergoes division to produce two cells within the cyst.
Infection in man is occurred by ingestion of cysts within about 30 minutes of ingestion, the cyst hatches out two trophozoites in the host’s intestine which then multiply in enormous number and are colonised or confined in the small intestine, particularly the duodenum. Giardia often invades the bile ducts to avoid the high acidity of duodenum.
