(See also Overview of Parasitic Infections.)
Ascariasis is infection caused by Ascaris lumbricoides, an intestinal roundworm, or occasionally by Ascaris suum (which causes ascariasis in pigs).
People acquire the infection by swallowing the roundworm eggs, usually in food.
When first infected, people may have no symptoms or may develop fever, coughing, wheezing, abdominal cramps, nausea, and vomiting.
Children with a heavy, chronic infection may not grow normally, or worms can block the intestine or bile duct, resulting in severe pain and vomiting.
Doctors usually diagnose the infection by identifying the eggs or worms in a stool sample.
People are treated with antiparasitic drugs such as albendazole.
Ascariasis is the most common roundworm infection in people, occurring in about 800 million people worldwide. About 2,000 infected people (mostly children) may die each year because the worms block the intestine or bile ducts (tubes that connect the liver and gallbladder to the small intestine).
The infection is common in tropical or subtropical areas with poor sanitation. In the United States, ascariasis occurs most often in refugees, immigrants, and people who have traveled to areas where ascariasis is common.
Infection begins when a person swallows fertilized Ascaris eggs. Only fertilized eggs can cause infection. People may swallow the eggs in food that came in contact with soil contaminated by human stool (feces) containing the eggs. Ascaris eggs are hardy and can survive in the soil for years.
Once swallowed, Ascaris eggs hatch and release larvae in the intestine. Each larva migrates through the wall of the small intestine and is carried through the lymphatic vessels and bloodstream to the lungs. Once inside the lungs, the larva passes into the air sacs (alveoli), moves up the respiratory tract and into the throat, and is swallowed. The larva matures in the small intestine, where it remains as an adult worm. This process takes 2 to 3 months. Adult worms range from 6 to 20 inches in length and from 1/10 to 2/10 inch in diameter. They live 1 to 2 years. Eggs laid by the adult worms are excreted in stool, develop in the soil, and begin the cycle of infection again when they are ingested.
Also, people can be infected with Ascaris suumif they swallow fertilized eggs in food contaminated with feces from pigs or the larvae in raw or undercooked pork. Ascaris suum usually causes ascariasis in pigs.
If people have not been previously exposed to Ascaris, the migration of larvae through the lungs can cause fever, coughing, wheezing, and sometimes blood in phlegm (sputum).
A small number of worms in the intestine does not usually cause digestive symptoms. A large number of worms can cause abdominal cramps and, occasionally, a blockage of the intestine, most commonly in children living in areas with poor sanitation. A blockage can cause nausea, vomiting, abdominal swelling (distention), and abdominal pain.
Sometimes adult worms migrate to the mouth or nose, are vomited up, or passed in the stool—situations that can be psychologically distressing. Adult worms occasionally block the opening into the appendix, biliary ducts, or pancreatic duct, producing severe abdominal pain.
Undernutrition may develop in infected children. Children with a heavy infection may not grow or gain weight normally.
Ascariasis is diagnosed by identifying eggs or adult worms in a stool sample or, rarely, by seeing adult worms in stool or coming out of the mouth or nose.
If computed tomography (CT) or ultrasonography is done for other reasons, adult worms may be seen. Rarely, the effects of larvae migrating through the lungs can be seen on a chest x-ray.
The best strategies for preventing ascariasis include
Effective sewage disposal systems can help prevent this infection from spreading.
To treat a person with ascariasis, a doctor prescribes albendazole, mebendazole, or ivermectin, which are taken by mouth. However, because these drugs may harm the fetus, doctors must balance the risk of treating an infected pregnant woman with the risk of not treating the infection.
If people have been to areas of Africa where Loa loa is transmitted, doctors check them for loiasis before giving them ivermectin because ivermectin can cause serious brain inflammation (encephalitis) in people with loiasis.
If the worms cause a blockage in the intestines, people may be treated with one of the above drugs, or the worms may be removed surgically or through an endoscope (a flexible viewing tube) inserted through the mouth into the intestine.
When the lungs are affected, treatment focuses on relieving symptoms. It includes bronchodilators and corticosteroids. Albendazole or other anthelmintic drugs are typically not used to treat lung infection.