Merck Manual

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Chelsea Marie

, PhD, University of Virginia;

William A. Petri, Jr

, MD, PhD, University of Virginia School of Medicine

Reviewed/Revised Oct 2022
Topic Resources

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 500 million people worldwide, and ascariasis contributes to malnutrition in areas with poor sanitation. About 2,000 to10,000 people die each year from ascariasis. Most deaths occur in children as a result of worms blocking 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 or lived in areas where sanitation is poor.

Did You Know...

  • About 500 million people worldwide have a roundworm infection called ascariasis.

Transmission of ascariasis

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. Infection can also occur when people put their hands or fingers with contaminated dirt on them in their mouth. 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 (15 to 51 centimeters) in length and from 1/10 to 2/10 inch (¼ to ½ centimeter) 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.

People can also be infected with Ascaris suum from pigs. Infection begins when a person swallows fertilized eggs because their hands were contaminated from touching infected pigs or from consuming undercooked vegetables or fruits contaminated with pig feces. Whether Ascaris suum is a distinct species from Ascaris lumbricoides is debated.

Symptoms of Ascariasis

The migration of Ascaris 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.

Diagnosis of Ascariasis

  • Examination of a stool sample

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.

Prevention of Ascariasis

The best strategies for preventing ascariasis include

  • Washing hands thoroughly with soap and water before handling food

  • Washing, peeling, and/or cooking all raw vegetables and fruits before eating, particularly those that have been grown in areas where human or pig feces is used as fertilizer

  • Not defecating outdoors except in latrines with proper sewage disposal

Effective sewage disposal systems can help prevent this infection from spreading.

Treatment of Ascariasis

  • A drug used to treat worm infections (anthelmintic drug)

To treat a person with ascariasis, a doctor usually prescribes albendazole or mebendazole. Ivermectin is an alternative. These drugs 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 lived in areas of Africa where Loa loa is transmitted, doctors check them for loiasis Treatment Loiasis is infection of tissues under the skin or under the clear outer membrane that covers the eye (conjunctiva) with the roundworm Loa loa. Itchy swellings may appear, mainly on the... read more Treatment before giving them ivermectin because ivermectin can cause serious brain inflammation (encephalitis) in people with heavy Loa loa infection.

When Ascaris 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.

Drugs Mentioned In This Article

Generic Name Select Brand Names
Emverm, Vermox
Sklice, Soolantra, Stromectol
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