The Nematodes

Ascaris suum- life cycle

The predilection site for Ascaris suum is the small intestine of swine.

Pre-parasitic phase

Eggs laid by female worms pass to the external environment in host feces (A). These eggs are oval and thick-shelled with a rough mammillated, sticky external coat and are laid in the one-celled stage. Development to the infective stage takes place inside the egg and consiss of one molt only. Therefore the infective stage for Ascaris suum is an egg containing a second stage (L2) larvae (C).

Earthworms (D) and dung beetles may ingest ascarid eggs while feeding on soil and feces. When that happens eggs will hatch and L2s will migrate to tissues and encyst giving rise to the possibility that these invertebrates may serve as paratenic hosts for Ascaris suum.

Parasitic phase

Pigs are infected by ingesting (E) eggs or paratenic hosts containing second stage larvae.  Following hatching in the small intestine (F), L2s burrow into the intestinal wall, enter the hepatic portal system and are carried to the liver (G) within 24 hours of infection. Here the first parasitic molt (L2 to L3) takes place.

(If infected paratenic hosts (D) are ingested by pigs, L2s are released during digestion in the stomach and small intestine. Their migratory route and parasitic development are the same as infective larvae from eggs).

Third stage larvae (L3s) continue their migration from the liver to the lungs via the venous system, right heart and pulmonary arteries, reaching the lungs (H) by 4 to 6 days after infection. They break out of the alveolar capillaries and migrate up the bronchial tree to the pharynx (I) where they are swallowed.

The final two parasitic molts (L3 to L4 to immature adults) are completed in the small intestine (F) by 3-4 weeks after infection. Mature, gravid females begin to lay eggs approximately 6 to 8 weeks after infection.

Rbtn_pre.gif (1751 bytes) Rbtn_nex.gif (1502 bytes)
Rbtn_cont.gif (1897 bytes) Rbtn_top.gif (1435 bytes)

Parasites & Parasitic Diseases of domestic animals
Dr. Colin Johnstone (principal author)

Copyright University of Pennsylvania
This page was last modified on February 21, 2000