Seven important stages involved in the development of ascaris inside the host

Stages involved in the development of ascaris inside the host are given below:

1. Copulation and Fertilization:

Copulation occurs in host’s intestine, where the adult worms live. The male and female lie in such a position that the cloacal aperture of male and vulva of female come in close contact.

Penial setae of male help to open the vulva and sperms are transferred into vagina. Then the sperms pass on to the proximal end of uteri to fertilize the eggs.

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2. Zygote:

After fertilization the glycogen globule of the egg comes on surface to form a fertilization membrane. It becomes harder and forms a thick, clear and chitinous shell. The fat globule of the egg forms a thin lipoid layer below shell. As the zygote moves into the uterus, its wall secretes a brown albuminous coat.

A mature female ascaris may contain 21 million eggs in its uterus. Zygotes then pass out through gonopore into the host’s intestine and finally in the soil with host’s faeces.

Under suitable conditions of temperature, moisture, and oxygen it undergoes cleavage and develops into the infective stage.

3 .Cleavage and early development:

Cleavage is of spiral and determinate type. Fate of various cells of the zygote at 16 cell stage is fixed and it contains a cavity to become blastula.

Blastula undergoes a process of invagination and becomes gastrula. It grows in length to become an active juvenile in 10-14 days from the start of cleavage.

This juvenile closely resembles with another nematod, rhabditis and therefore is termed as rhabditi form larva of first stage.

At this stage it is not infective. In another week’s time it moults within the egg shell and becomes the second stage of rhabditoid, which is capable of infecting the host.

Under suitable conditions of moisture, 0₂ and temperature, these infective eggs are known to remain viable for about six years.

4. Later development and migration into new host:

Being a sole host, man acquires infection by consuming egg contaminated food and water. In the small intestine, by the action of host’s digestive juices egg shells disolve and the juveniles (second stage larva) hatch out.

It performs active thrashing movements and bores through the epithelium of host’s intestine and starts its migration into the different organs.

5. Primary migration:

Firstly larva enters the hepatic portal circulation from the wall of the intestine and reaches into the liver.

Then it migrates to the heart and finally goes to the lungs via pulmonary artery. It grows in size and ruptures out of blood capillary into the alveoli of the lung.

After about 6 days of stay it moults to become third stage larva. After 4 days it again moults to become fourth stage larva and grows upto a length of 2 to 3 mm.

6. Secondary migration:

Fourth stage larva then migrate from lung alveoli to the pharynx through trachea. From trachea it is couged up and then swallowed for the second time into gut. In the intestine it moults for the fourths, the last time to become an adult.

Adult attains sexual maturity within 8-10 weeks. The average life span of ascaris in the host is 9-12 months.

7. Aberrant migration:

Sometimes larva does not follow its usual path of migraton, but reaches the brain or spinal cord or any other such vital organ, which is called aberrant migration.

Larva is not able to survive in these organs and a calcareous cyst is formed around it.