An estimated 807 million–1.2 billion people in the world are infected with Ascaris lumbricoides (sometimes called just Ascaris or ascariasis). Ascaris, hookworm, and whipworm are parasitic worms known as soil-transmitted helminths (STH). Together, they account for a major burden of parasitic disease worldwide. Ascariasis is now uncommon in the United States.

Ascaris parasites live in the intestine and Ascaris eggs are passed in the feces (poop) of infected people. If an infected person defecates outside (for example, near bushes, in a garden, or in a field), or if the feces of an infected person are used as fertilizer, eggs are deposited on soil. The eggs can then mature into a form of the parasite that is infective. Ascariasis is caused by ingesting eggs. This can happen when hands or fingers that have contaminated dirt on them are put in the mouth, or by consuming vegetables or fruits that have not been carefully cooked, washed, or peeled.

People infected with Ascaris often show no symptoms. If symptoms do occur they can be light and include abdominal discomfort. Heavy infections can cause intestinal blockage and impair growth in children. Other symptoms such as cough are due to migration of the worms through the body. Ascariasis is treatable with medication prescribed by your health care provider.

Humans can also be infected by pig roundworm (Ascaris suum). Ascaris lumbricoides (human roundworm) and Ascaris suum (pig roundworm) are indistinguishable. It is unknown how many people worldwide are infected with Ascaris suum.

Image: Left/Right: Fertilized eggs of A. lumbricoides in unstained wet mounts of stool. Center: Adult female A. lumbricoides. Credit: DPDx, Orange County Public Health Laboratory, Santa Ana, CA.