Q.Describe the life cycle of Ascaris.

Ascaris lumbricoides, the common human roundworm, completes its life cycle directly in a single host, involving egg passage in faeces, soil development, ingestion, and a striking larval migration through the body before maturing back in the small intestine.

1. Adult worm and egg-laying

• Adult Ascaris worms (large, cylindrical, unsegmented roundworms) live free in the lumen of the small intestine of the human host (the only definitive host; no intermediate host is required).
• After mating, the female produces enormous numbers of fertilised eggs -- roughly up to 200,000 per day -- which pass out of the body along with the host's faeces.

2. Development of eggs in soil

• The eggs, protected by a thick, resistant shell, require warm, moist, shaded soil conditions to develop.
• Over about 2-4 weeks, the fertilised egg develops internally into the infective stage, containing a fully formed first-stage (rhabditiform) larva within the egg shell.

3. Infection of a new host

• A new host becomes infected by ingesting these infective (embryonated) eggs, typically through contaminated vegetables, water, or soil-contaminated hands (faecal-oral transmission), especially where sanitation is poor.

4. Hatching and larval migration ('lung migration phase')

• Once swallowed, the eggs hatch in the duodenum, releasing motile larvae.
• The larvae bore through the intestinal wall and enter the blood or lymphatic vessels.
• They are carried via the portal circulation to the liver, then to the heart, and from there to the lungs via the pulmonary circulation.
• In the lungs, the larvae break out of the pulmonary capillaries into the alveoli, undergo two moults, and grow.
• They then migrate up the bronchioles, bronchi, and trachea to the pharynx (throat).

5. Return to the intestine and maturation

• On reaching the pharynx, the larvae are swallowed again and pass down the oesophagus back into the small intestine. …