MSD Manual

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(Threadworm Infection)


Richard D. Pearson

, MD, University of Virginia School of Medicine

Last full review/revision May 2019| Content last modified May 2019
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Strongyloidiasis is infection caused by the roundworm Strongyloides stercoralis, which enters the body when bare skin comes in contact with soil contaminated with the worm.

  • Usually, people are infected when they walk barefoot on contaminated soil.

  • Most people with this infection do not have any symptoms, but some have a rash, cough, wheezing, abdominal pain, diarrhea, and weight loss.

  • Rarely, a severe, life-threatening infection develops in people who have a weakened immune system because of a disorder (such as cancer) or drugs that suppress the immune system.

  • Doctors diagnose the infection by finding larvae in a stool sample or by detecting antibodies to Strongyloides in blood.

  • Ivermectin or albendazole is used to treat strongyloidiasis.

Strongyloidiasis occurs in warm, moist areas such as the subtropics and tropics, including rural areas of the southern United States.

Strongyloides worms are sometimes called threadworms.

Transmission of Strongyloides

Adult Strongyloides worms live in the small intestine. The females produce eggs, which hatch and release larvae. Most of the larvae are excreted in stool. After a few days in soil, larvae develop into a form that can cause infection. If Strongyloides larvae come in contact with a person's bare skin, they penetrate it. Then they travel through the bloodstream to the lungs and up the respiratory tract to the throat, are swallowed, and end up in the small intestine, where they mature into adults in about 2 weeks.

The larvae that do not come in contact with people may develop into adult worms that can reproduce in soil for several generations before their larvae come in contact with a person.

Some of the larvae in the small intestine can re-infect the person by either

  • Penetrating the wall of the intestine and re-entering the person's bloodstream directly

  • Being excreted in stool and penetrating the skin around the anus or the skin of the buttocks or thighs

In both cases, the larvae travel through the bloodstream to the lungs and then to the throat and back to the intestine to cause another infection—called autoinfection (infection of self).

Hyperinfection syndrome

Rarely, a severe infection (called hyperinfection syndrome) develops in people who have a weakened immune system because of a disorder, such as cancer, or drugs that suppress the immune system (immunosuppressants), such as prednisone or drugs used to prevent rejection of an organ transplant. This syndrome also occurs in people with AIDS, but less often than might be expected.

Hyperinfection syndrome causes widespread disease that can affect the lungs, brain, liver, or heart in addition to the intestine. Strongyloides larvae may carry bacteria from the digestive tract with them. When the larvae travel through the body, these bacteria can cause infections in the bloodstream, brain and spinal fluid, lungs, or other parts of the body.


Most people with a strongyloidiasis do not have symptoms. When symptoms occur, they usually involve the skin, lungs, and/or digestive tract.

People who have an autoinfection develop a rash caused by the larvae as they travel through the skin. The rash often occurs around the anus. As the larvae travel, the rash can rapidly spread to the thighs and buttocks, causing intense itching.

Severe infections may cause lung and/or digestive symptoms. Some people cough and wheeze. Some have abdominal pain and tenderness, diarrhea, nausea, and vomiting. They may lose their appetite. They may not absorb nutrients normally, resulting in weight loss.

Hyperinfection syndrome

People with hyperinfection syndrome often have severe symptoms involving the lungs and/or digestive tract. Lung symptoms include severe shortness of breath, coughing up blood, and respiratory failure. Digestive tract symptoms include intestinal blockage, bleeding, and severe problems absorbing nutrients (malabsorption).

If other organs are infected, people may have other problems, such as inflammation of the tissues covering the brain and spinal cord (meningitis), an abscess in the brain, or hepatitis.

Bacterial infections, such as a serious infection of the blood (sepsis) or an infection of the abdominal cavity (peritonitis), may occur as complications of Strongyloides hyperinfection.

Infection may be fatal in people with a weakened immune system, even when they are treated.


  • Examination of a stool sample

  • For hyperinfection syndrome, examination of a sputum (phlegm) sample and chest x-rays

  • Blood tests to detect antibodies to Strongyloides

Doctors can sometimes see Strongyloides larvae when they examine a stool sample under a microscope. Often, they must examine many samples.

Doctors may use a flexible viewing tube (endoscope) inserted through the mouth into the small intestine to take a sample of tissue there. A thin tube is threaded through the endoscope and used to suction up a sample of tissue. Doctors use the endoscope to determine where to take samples.

If doctors suspect hyperinfection syndrome, they also examine a sputum sample for larvae and take chest x-rays to look for evidence of lung infection.

Blood tests to check for antibodies to Strongyloides are usually done. (Antibodies are proteins produced by the immune system to help defend the body against attack, including that by parasites.) However, these tests cannot distinguish between new and old infections or sometimes between Strongyloides and other roundworm infections.


Prevention of strongyloidiasis involves the following:

  • Using latrines or toilets

  • Preventing the skin from directly contacting the soil in areas with strongyloidiasis (for example, by wearing shoes and using a tarp or other barrier when seated on the ground)


  • Ivermectin or albendazole

All people who have strongyloidiasis are treated.

For most cases of strongyloidiasis, one of the following is used:

  • Ivermectin, given once a day for 2 days

  • Albendazole, given twice a day for 7 days

Ivermectin is more likely to cure the infection than albendazole. If people with strongyloidiasis 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.

For hyperinfection syndrome, ivermectin is given until sputum and stool have been free of larvae for 2 weeks.

If people have a weakened immune system, they may need to take drugs for a long time.

If people are severely ill and unable to take drugs by mouth, a rectal preparation or sometimes an under-the-skin (subcutaneous) preparation (designed to treat animals) is used.

Antibiotics are used to treat bacterial infections, which may be complications of strongyloidiasis.

To determine whether the infection is eliminated, doctors examine stool samples for larvae or do blood tests to determine whether levels of antibodies to the worm have decreased. If Strongyloides larvae are still present in stool after treatment or if antibody levels do not decrease, people are treated again.

NOTE: This is the Consumer Version. DOCTORS: Click here for the Professional Version
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