Question

Describe the life cycle of Ascari lumbricoides with the help of a diagram​

Answer 1

Answer:

Ascaris is monogenetic i.e., it requires only one host to complete its life cycle and no intermediate host is required. Comprehensive reviews of the life cycle have been given by Crompton and Pawlowski (1985) and Crompton (1989). Man is the only known definitive host of Ascaris lumbricoides.   he various stages in the life cycle are described below:

Copu­lation occurs in the small intestine of host (man) where the adult worm lives. During copulation the male orients its body at right angle to that of the female in such a way that its cloacal aper­ture apposes the vulva of the female and the sperms are easily transferred into the vagina from where they ascend up in the uterus and fertilizes the eggs in the oviduct.

The eggs are laid in the host’s intestine which are deposited outside along with faeces of host. A female Ascaris produces roughly about 2,00,000 eggs daily. The egg production is astounding. Cram (1925) estimated the number of eggs con­tained in a mature female worm to be as high as 2,70,00,000 and the eggs per gram of faeces for each female worm may be in excess of 2000.

When the eggs are passed in faeces, their fur­ther development is largely dependent on oxygen tension, moisture content and temperature of their environment. Due to high temperature inadequate moisture and oxygen supply in the host’s intes­tine, the fertilized eggs do not start their further development.

The fertilized eggs are round or oval in shape. They usually measure about 52-84 μm by 45-67 μm. The zygote has a thick, clear inner shell covered over by a warty, albuminous coat which is always bile-stained and brownish (golden-brown) in colour. The egg contains a very large conspicu­ous, unsegmented ovum (the nucleus is concealed by a large amount of coarse yolk granules). There is a clear crescentic area at each pole of the zygote Stage 3. Cleavage (Segmentation of ferti­lised egg) and early development:

Cleavage of fertilised egg is of spiral and determinate type. The first division is transverse which results in a dorsal cell and a ventral cell. The dorsal cell divides vertically into an anterior and a posterior cell, while the ventral cell divides horizontally into an upper and a lower cell. The four celled embryo, thus formed, is first T-shaped in appea­rance.

In the next cleavages, the 4-celled embryo becomes the 16-celled embryo and attains the form of a hollow ball. It is termed as blastula con­taining the blastocoel. The blastula undergoes the process of invagination and becomes the gastrula. The juvenile is formed within 10-14 days from the onset of cleavage. It has an alimentary canal, a nerve-ring and a larval excretory system.

For its close resemblance with Rhabditis (a nematode found in the soil and human faeces), the juvenile is also termed as Rhabditiform larva. This larva of the first stage is not infective. In another week’s time it undergoes moulting within the egg-shell and becomes the second stage of Rhabditoid which is capable of infecting the host. Under suit­able conditions of moisture, oxygen and tempera­ture, the infective eggs are known to remain viable for about six years.

In the small intestine by the action of host’s digestive juice the egg-shells dissolve and the juveniles hatch out. It performs active thrashing move­ments and bores through the intestinal epithelium to enter in the hepatic circulation which carries it to the liver.

According to Douvres et al (1969), on hatching the larvae burrow into the intestinal mucosa, pene­trate blood vessels and appear as second stage larvae in the liver within six hours of post-infection. Some larvae penetrate lymphatics but apparently become inhibited and it is doubtful if these larvae develop further. They remain in the liver for a few days and develop to the early third stage larva.

From the liver it finally reaches the heart through the post caval vein. Larvae are then car­ried to the lungs via pulmonary arteries. The larva generally remains in the lung for few days and gradually increases in size. Then it ruptures out of blood capillary and finally bores its way into the lung alveolus.

After about six days stay there, the larva moults there for the second time. Then they pass through the trachea with cough and when the cough is swallowed, pass to the oeso­phagus, stomach and finally to the intestine. The larva here undergoes moulting for two times and becomes adult. The period of migration from the time of infection to that of reaching the intestine is said to be about 10 days.

Answer 2

Answer:

In Adult Warms,

Explanation;

1. live in the lumen of the small intestine. A female may produce approximately 200,000 eggs per day, which are passed with the feces.

2.  Unfertilized eggs may be ingested but are not infective. Larvae develop to infectivity within fertile eggs after 18 days to several weeks

3. depending on the environmental conditions (optimum: moist, warm, shaded soil). After infective eggs are swallowed

4. the larvae hatches

5. invade the intestinal mucosa, and are carried via the portal, then systemic circulation to the lungs

6. The larvae mature further in the lungs (10 to 14 days), penetrate the alveolar walls, ascend the bronchial tree to the throat, and are swallowed

7.Upon reaching the small intestine, they develop into adult worms. Between 2 and 3 months are required from ingestion of the infective eggs to oviposition by the adult female. Adult worms can live 1 to 2 years.