Ascaris lumbricoides and Ascariasis

Ascaris lumbricoides and Ascariasis

Ascaris lumbricoides, commonly called the giant intestinal roundworm, is one of the largest and most common nematode parasites of the human small intestine, and the causative agent of ascariasis.

Structure

The adult worm is elongated, cylindrical, unsegmented and tapers at both ends, covered by a tough, non-living, resistant cuticle. The two sexes can be told apart by size and by the shape of the posterior end: the female is longer and thicker, with a straight posterior end, while the male is comparatively shorter and thinner, with a posterior end that is curved ventrally and bears a pair of copulatory spicules used during mating. A mature female is an extraordinarily prolific egg-layer. The fertilised eggs that pass out in the host's faeces have a thick, mammillated (bumpy, knobbly-surfaced) outer covering — the mammillated egg — which gives it strong resistance to adverse environmental conditions and allows it to remain viable in soil for long periods.

Life cycle

Ascaris lumbricoides has a direct life cycle with a single host (humans) but, unusually, the larva must undertake an elaborate migratory journey through host tissues before it can return to the intestine as an adult.

1. A person swallows embryonated (larvated) eggs present in soil-contaminated food, water, or on unwashed hands/vegetables.
2. The egg hatches in the small intestine, releasing a first-stage rhabditiform larva.
3. Extra-intestinal migration — rather than developing directly into an adult in the gut, the larva bores through the intestinal wall into the blood vessels of the gut, and is carried by the portal blood circulation to the liver.
4. From the liver, the larva is carried via the hepatic vein, the inferior vena cava and the right side of the heart into the pulmonary circulation, arriving in the capillaries of the lungs.
5. In the lungs the larva breaks out of the pulmonary capillaries into the alveoli, moults twice as it grows, then migrates up the bronchioles, bronchi and trachea into the pharynx.
6. On reaching the pharynx the larva is swallowed a second time, returning to the small intestine.
7. Back in the small intestine, the larva moults further and matures into an adult male or female worm, where mating occurs and the female begins laying enormous numbers of fertilised, mammillated eggs, which are passed out in the faeces to begin the cycle again in a new host via soil contamination.

This extra-intestinal (liver-heart-lung-throat-gut) migratory route means that a heavy Ascaris infection can produce not just intestinal symptoms but also transient pulmonary (lung) symptoms while the larvae are migrating through the lungs.

Ascariasis: symptoms and prevention …