The infective stage of Ascaris , is the egg containing an L2 larva, which is ingested by the host and passes through the stomach and hatches in the duodenum. The larva molts to the L3 stage and penetrates the intestinal mucosa, enters the blood system and is carried via the heart to the lungs, where they enter the alveolar spaces. During a period of 10 days they molt to the L4 stage and migrate up the trachea and are swallowed. In some cases L3 larvae also undergo this migration, but only the L4 stage can tolerate the extreme acidity and gastric enzymes of the stomach. In the small intestine they molt and mature.
Eggs pass out with the faecal material unembryonated.
This rather bizarre migration undergone by the parasite to effectively get back to where it started, has led to the suggestion that, perhaps from an evolutionary perspective, Ascaris originally had two host and that the migratory path originally took place in an intermediate host and it has secondarily lost its intermediate host. Pathology Secondary complications can arise with Ascaris infections because sometimes, when the worms are undergoing the tissue migration phase they appear to get lost and start wandering through other organs such as the brain, liver, lymph nodes, spleen and even the eye, where they often cause an inflammatory response. Symptoms are vague and frequently difficult to diagnose. The worms which penetrate the intestine cause little symptoms but depending on the infection dose, the act of bursting out of the blood capillaries into the alveolar spaces of the lungs causes haemorrhaging at the site. With heavy infections this can lead to edema resulting in clogged airspaces.