Q.Describe, with reference to each stage, the complete life cycle of Ascaris lumbricoides.

[!TLDR] Ascaris lumbricoides has a single-host life cycle in which a swallowed egg hatches into a larva that migrates through the liver, heart and lungs before returning to the small intestine to mature into an egg-laying adult.

A swallowed embryonated egg hatches in the small intestine; the larva bores into gut blood vessels, is carried via the portal circulation to the liver, then via the heart to the lung capillaries, breaks into the alveoli, moults twice, migrates up the airway to the pharynx, is swallowed again, and matures into an adult worm in the small intestine, where the female lays mammillated eggs passed out in fa …

Ascaris lumbricoides has a direct life cycle involving only a single host, man, but is unusual among intestinal parasites in that its larva undertakes an elaborate migratory journey through several internal organs before finally settling down as an adult in the intestine.

Stage 1: Ingestion of the embryonated egg

Infection begins when a person swallows an embryonated (larvated) egg present in food, water, or on unwashed hands or vegetables contaminated with soil carrying the parasite's eggs. The egg's thick, mammillated (knobbly) outer coat protects it during this journey through the environment and through the acid stomach.

Stage 2: Hatching in the small intestine

On reaching the small intestine, the egg hatches, releasing a first-stage (rhabditiform) larva.

Stage 3: Penetration into the gut blood vessels

Rather than developing directly into an adult worm within the gut, the newly hatched larva bores actively through the wall of the small intestine and enters the small blood vessels of the gut wall — the beginning of its extra-intestinal migration.

Stage 4: Transport to the liver via the portal circulation

The larva is carried passively by the portal blood circulation to the liver, where it spends some time within the liver tissue before continuing its journey.

Stage 5: Transport through the heart to the lungs

From the liver, the larva travels via the hepatic vein into the inferior vena cava, then through the right side of the heart, and out into the pulmonary circulation, eventually arriving at the capillaries of the lungs.

Stage 6: Breaking into the alveoli and moulting

In the lungs, the larva breaks out of the pulmonary capillaries into the air-filled alveoli. Here it undergoes two successive moults (shedding its outer cuticle each time) as it continues to grow in size.

Stage 7: Migration up the respiratory tract to the pharynx

The growing larva then migrates upward through the bronchioles, the bronchi, and the trachea, finally reaching the pharynx (throat).

Stage 8: Second swallowing and return to the small intestine

On reaching the pharynx, the larva is swallowed a second time by the host, carrying it back down through the oesophagus and stomach into the small intestine, the same organ where the cycle began.

Stage 9: Final moulting and maturation into the adult worm

Back in the small intestine, the larva undergoes further moulting and finally matures into an adult male or female worm. The two sexes can be distinguished: the female is longer and thicker with a straight posterior end, while the male is shorter and thinner with a ventrally curved posterior end bearing copulatory spicules used in mating.

Stage 10: Mating and egg production

The mature male and female worms mate within the small intestine, and the fertilised female begins producing enormous numbers of mammillated eggs — eggs with a thick, knobbly, resistant outer covering.

Stage 11: Egg release and transmission to a new host …

• Assuming the Ascaris larva matures directly into an adult in the intestine without any extra-intestinal migration — the liver-heart-lung-pharynx journey is an essential, examinable part of the life cycle. …