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(Threadworm Infection)


Chelsea Marie

, PhD, University of Virginia;

William A. Petri, Jr

, MD, PhD, University of Virginia School of Medicine

Reviewed/Revised Oct 2022
Topic Resources

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. The larvae migrate by various routes to 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 and disseminated strongyloidiasis

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, other corticosteroids, or drugs used to prevent rejection of an organ or bone marrow transplant. This syndrome also occurs in people with AIDS, but much less often than might be expected.

Hyperinfection syndrome causes widespread disease that affects the intestines, lungs, and skin, which are organs that are involved in the normal strongyloides life cycle, and then progresses to disseminated strongyloidiasis, which can affect other tissues not typically affected by strongyloidiasis, such as tissue covering of the brain and spinal cord (meninges), brain, liver, or other organs. 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.

Symptoms of Strongyloidiasis

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 and disseminated strongyloidiasis

People with hyperinfection syndrome often have severe symptoms involving the lungs and/or digestive tract, organs involved in life cycle of the parasite. Lung symptoms include severe shortness of breath, coughing up blood, and respiratory failure Respiratory Failure Respiratory failure is a condition in which the level of oxygen in the blood becomes dangerously low or the level of carbon dioxide in the blood becomes dangerously high. Conditions that block... read more . Digestive tract symptoms include intestinal blockage, bleeding, and severe problems absorbing nutrients (malabsorption Overview of Malabsorption Malabsorption syndrome refers to a number of disorders in which nutrients from food are not absorbed properly in the small intestine. Certain disorders, infections, and surgical procedures can... read more ).

Hyperinfection and disseminated disease are often fatal in people with a weakened immune system, even when they are treated.

Diagnosis of Strongyloidiasis

  • Examination of a stool sample

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

  • Blood tests to detect eosinophilia and 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.

Eosinophilia High number of eosinophils Eosinophils are a type of white blood cell that play an important role in the body's response to allergic reactions, asthma, and infection with parasites. These cells have a role in the protective... read more High number of eosinophils is common on blood tests. Eosinophilia is a higher than normal number of eosinophils, which are a type of disease-fighting white blood cell that play an important role in the body's response to allergic reactions, asthma, and infection with parasitic worms (helminths).

Blood tests to check for antibodies to Strongyloides are also done. (Antibodies Antibodies One of the body's lines of defense ( immune system) involves white blood cells (leukocytes) that travel through the bloodstream and into tissues, searching for and attacking microorganisms and... read more 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

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)

Treatment of Strongyloidiasis

  • 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 1-2 days

  • Albendazole, given twice a day for 7 days

Ivermectin is more likely to cure the infection than albendazole. If people with strongyloidiasis have lived in or traveled to areas of Africa where Loa loa is transmitted, doctors check them for loiasis Loiasis 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 Loiasis before giving them ivermectin because ivermectin can cause serious brain inflammation (encephalitis) in people with loiasis.

For hyperinfection syndrome and disseminated strongyloidiasis, 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.

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