What is strongyloidiasis?

Strongyloidiasis is infection by a roundworm, or nematode, called Strongyloides stercoralis. The S. stercoralis roundworm is a type of parasite. A parasite is an organism that lives in the body of a different species from which it obtains nutrients. The infected organism is called the host.

S. stercoralis infection is uncommon in the United States. The roundworm is usually found in warm climates, like those of tropical and subtropical countries. It’s more common in rural areas and institutional settings, such as nursing homes.

Usually, strongyloidiasis causes no symptoms. S. stercoralis infection can generally be prevented through good personal hygiene.

In around 50 percent of cases, strongyloidiasis causes no symptoms. If symptoms are present, they may include:

Rashes may occur immediately after contact with the S. stercoralis roundworm. Gastrointestinal symptoms typically appear two weeks after a person is first infected.

Strongyloidiasis is caused by the parasitic roundworm S. stercoralis. This worm infects mainly humans. Most humans get the infection by coming into contact with contaminated soil.

It’s most often found in tropical and subtropical climates, but it can occasionally be found in more temperate climates. This may include parts of the southern United States and Appalachia.

Once a person comes into contact with S. stercoralis, the infection follows the lifecycle of the worm. The worm’s lifecycle includes the following stages:

  1. The tiny worms penetrate your skin and enter your bloodstream.
  2. The worms then move through your bloodstream and pass through the right side of your heart and into the lungs.
  3. The parasites travel from the lungs up the windpipe and into your mouth.
  4. You unknowingly swallow the worms, and they travel into your stomach.
  5. The worms move into your small intestine.
  6. The worms lay eggs that hatch and become larvae.
  7. The larvae are expelled from your body in your feces.
  8. The larvae can infect your body by penetrating the skin around your anus, or they can develop into mature worms and infect someone else.

The worms can also live and reproduce in soil, without a host.

Rarely, the worms can penetrate the intestine of the host as larvae rather than pass out of the body through the feces.

You’re at an increased risk for infection if:

  • you travel to or live in South America, Africa, or other tropical regions
  • you live in or travel to rural areas, areas with unsanitary living conditions, or areas without adequate public health services
  • your job involves regular contact with soil
  • you don’t practice good personal hygiene
  • you have a weakened immune system, such as can occur from HIV or AIDS

According to the Centers for Disease Control and Prevention (CDC), most of the infections in the United States are spread by people who have lived in endemic areas for an extended period of time. This would include immigrants, refugees, and military veterans.

The following tests may be performed to diagnose an infection with S. stercoralis:

  • Duodenal aspiration. During this test, your doctor will take fluid from the duodenum, the first section of your small intestine. They’ll then examine the fluid under a microscope for the presence of S. stercoralis.
  • Sputum culture. Your doctor can use a sputum culture to analyze fluid from your lungs or airways for S. stercoralis.
  • Stool sample for ova and parasites. Your doctor can use a stool sample to check for S. stercoralis larvae in the feces. You may need to repeat the test to get accurate results.
  • Complete blood count (CBC) with differential. A CBC test with differential may help to rule out other causes of symptoms.
  • Blood antigen test. A blood antigen test can help your doctor look for antigens to S. stercoralis. It’s performed when your doctor suspects you have an infection but they can’t find the parasite in a duodenal aspiration or in several stool samples. However, the test results can’t be used to tell the difference between a past and current S. stercoralis infection.

The most common methods of diagnosis are microscopic examinations of duodenal or stool samples.

The goal of treatment is to eliminate the worms. The medicine of choice to treat strongyloidiasis is a single dose of the antiparasitic medication ivermectin (Stromectol). This drug works by killing the worms in your small intestine.

Your doctor may also prescribe two courses of albendazole (Albenza), to be taken 10 days apart. Taking thiabendazole (Tresaderm) twice per day for two or three days is also an effective treatment.

You may need longer or repeated courses of medication if the infection is widespread.

An S. stercoralis infection can cause the following complications:

Eosinophilic pneumonia

Eosinophilic pneumonia occurs when your lungs swell due to an increase in eosinophils. Eosinophils are a type of white blood cell (WBC) that your body produces when the worms enter your lungs.


Malnutrition occurs if your intestines can’t properly absorb nutrients from the foods you eat while you’re infected with the worms.

Disseminated strongyloidiasis

Disseminated strongyloidiasis involves widespread distribution of the parasite to other organs of your body. This can occur if you’re taking immunosuppressive medicines or if you have an immune deficiency caused by a virus. It happens when S. stercoralis changes its lifecycle, enters the intestines, and re-enters the bloodstream.

The symptoms include:

With proper medical treatment, the prognosis for strongyloidiasis is very good. You can expect to make a full recovery, and the parasites should be fully eliminated. Occasionally, the treatment will need to be repeated.

However, severe or widespread infections in people with a weakened immune system are very serious. Those at risk of a more severe infection include people who use oral or intravenous (IV) steroids, recipients of transplants, and those with certain blood disorders. The infection can be fatal in these people if a diagnosis is delayed.

Strongyloidiasis can’t always be prevented.

However, exercising good personal hygiene, using sanitary facilities, and not walking barefoot when traveling to warm or tropical climates can reduce your risk of infection.