Strongyloidiasis is an infection caused by the parasitic roundworm Strongyloides stercoralis. This parasite, sometimes called a threadworm, infects humans through direct skin contact with contaminated soil. The infection is most common in tropical and subtropical regions, but it can persist in a person’s body for decades, with symptoms appearing long after exposure. An infection can range from having no symptoms to a life-threatening illness.
The Life Cycle of Strongyloides stercoralis
The life cycle of Strongyloides stercoralis is distinct among parasitic worms because it involves both a free-living and a parasitic cycle. The cycle in humans begins when infectious larvae, known as filariform larvae, in contaminated soil penetrate the skin. Once inside, they travel through the bloodstream to the lungs.
From the lungs, the larvae are coughed up, swallowed, and then travel to the small intestine. In the small intestine, the larvae mature into adult female worms and embed themselves in the intestinal lining. These females produce eggs that hatch into a different larval form, called rhabditiform larvae, within the gut. Most of these larvae are then passed out of the body in the stool, where they can mature in the soil and infect another person.
A unique feature of this parasite is its ability to cause autoinfection, where reinfection occurs without the larvae leaving the host. Some rhabditiform larvae in the intestine transform into infectious filariform larvae. These larvae can penetrate the intestinal wall or the skin around the anus to re-enter the bloodstream and repeat the migration process. This autoinfection cycle allows the parasite to persist in the body for decades, leading to chronic infections.
Symptoms and Diagnosis
The symptoms of strongyloidiasis vary depending on whether the infection is acute or chronic. During the initial, or acute, phase, a person might notice an itchy, red rash where the larvae entered the skin. As the larvae migrate through the lungs, a dry cough or throat irritation can develop. Once the worms establish themselves in the intestines, symptoms can include upper abdominal pain, nausea, bloating, and diarrhea.
In its chronic phase, the infection is often asymptomatic for long periods. When symptoms appear, they are frequently vague, such as recurring abdominal discomfort or alternating bouts of diarrhea and constipation. A characteristic skin manifestation is called larva currens, which is a rapidly moving, linear, itchy rash caused by migrating larvae under the skin.
Diagnosing strongyloidiasis is challenging because the number of larvae passed in the stool is often low and inconsistent. The primary method is the microscopic examination of multiple stool samples to identify larvae, as a single sample is often insufficient. Another diagnostic tool is serology, a blood test that detects antibodies produced by the body in response to the infection. In some cases, larvae may be found in samples of sputum or fluid from the duodenum.
Medical Treatment and Potential Complications
Treatment for strongyloidiasis aims to eliminate the worms from the body. The primary medication used is the antiparasitic drug ivermectin, with albendazole as an alternative. For uncomplicated infections, a standard course of treatment is effective at killing the worms residing in the small intestine.
The most severe outcomes are Hyperinfection Syndrome and Disseminated Strongyloidiasis. These conditions are medical emergencies and carry a high mortality rate if not treated promptly. Hyperinfection occurs when autoinfection accelerates uncontrollably, leading to a massive increase in the number of worms. This results in severe symptoms affecting the lungs and digestive tract, such as respiratory failure and intestinal blockage.
Disseminated strongyloidiasis occurs when migrating larvae spread to organs outside of the usual life cycle, including the brain, liver, or heart. These severe forms of the disease almost always happen in individuals with compromised immune systems. These conditions can also lead to secondary bacterial infections as larvae carry bacteria from the gut into the bloodstream.
Prevention and High-Risk Groups
Prevention focuses on avoiding contact with contaminated soil. The most effective method in endemic areas is wearing shoes or other protective footwear. Proper sanitation and sewage disposal systems are also necessary to reduce soil contamination with human waste.
Certain populations are at a higher risk of becoming infected and of developing severe disease. This includes people who live in or travel to tropical and subtropical regions, particularly in rural areas where sanitation may be inadequate. The group at highest risk for life-threatening complications are individuals with compromised immune systems. This includes:
- Patients taking immunosuppressive drugs like corticosteroids
- Organ transplant recipients
- Individuals with co-infections such as the human T-lymphotropic virus 1 (HTLV-1)