Ascaris lumbricoides

Ascaris lumbricoides is the giant roundworm of humans, belonging to the phylum Nematoda. An ascarid nematode, it is responsible for the disease ascariasis in humans, and it is the largest and most common parasitic worm in humans. A quarter of the human population is estimated to be infected by this parasite. Ascariasis is prevalent worldwide and more so in tropical and subtropical countries.Ascaris lumbricoides, or "roundworm", infections in humans occur when an ingested infective egg releases a larval worm that penetrates the wall of the duodenum and enters the blood stream. From here, it is carried to the liver and heart, and enters pulmonary circulation to break free in the alveoli, where it grows and molts. In 3 weeks, the larvae pass from the respiratory system to be coughed up, swallowed, and thus returned to the small intestine, where they mature to adult male and female worms. Fertilization can now occur and the female produces as many as 200,000 eggs per day for a year. These fertilized eggs become infectious after 2 weeks in soil; they can persist in soil for 10 years or more.

The eggs have a lipid layer, containing ascarocides and it makes them resistant to the effects of acids and alkalis as well as other unpleasant chemicals. This resilience helps to explain why this nematode is such a ubiquitous parasite.

Most diagnoses are made by identifying the appearance of the worm or eggs in feces. Due to the large quantity of eggs laid physicians can diagnose using only one or two fecal smears.

Infections can be treated with drugs called ascaricides. The treatment of choice is Mebendazole. The drug functions by binding to tubulin in the worms' intestinal cells and body wall muscles. Nitazoxanide and ivermectin can also be used.