Strongyloidiasis (Threadworms) | Causes, Pathophysiology, Signs and Symptoms, Diagnosis, Treatment

Strongyloidiasis is a gastrointestinal condition caused by infection with parasitic nematodes, commonly known as threadworms. The most common species is Strongyloides stercoralis, with Strongyloides fuelleborni also being significant. The prevalence is often underestimated, with global estimates ranging from 7-9% of the population, and higher rates in certain regions like the East Mediterranean, tropics, subtropics, and Southeast Asia. Many infections are asymptomatic, leading to underdiagnosis.

Strongyloides nematodes have two life cycles: a free-living cycle as rhabditiform larvae and a parasitic cycle as filariform infective larvae. Humans (and dogs) shed rhabditiform larvae in stool, which develop into free-living adults in the soil. These adults produce more rhabditiform larvae, which then mature into infective filariform larvae. These filariform larvae penetrate human skin, migrate to the intestines, mature into adults, lay eggs, and hatch into rhabditiform larvae, which are then excreted. Autoinfection can occur within the host, where rhabditiform larvae develop into filariform larvae in the large intestine and re-infect the host. This process is usually minor in immunocompetent individuals but can be severe in immunocompromised patients. While transcutaneous penetration is the most common route of infection, fecal-oral transmission and organ transplants are also possible, though rare.

Strongyloidiasis can manifest acutely or chronically. Acute infection often begins with larval invasion of the skin, leading to dermatological findings like larva currens, a rapidly migrating (5-15 cm/hour), itchy, serpiginous rash. Urticarial rashes, sometimes called 'ground rash,' frequently appear on the feet and are intensely pruritic. Gastrointestinal symptoms include profuse, mucoid watery diarrhea, crampy epigastric pain, nausea, vomiting, weight loss, malabsorption, pruritus ani, and bloating. In children, it can lead to malnourishment and failure to thrive. Respiratory symptoms (in about 10% of patients) include a non-productive cough, dyspnea, wheezing, fever, eosinophilia, Löffler-like syndrome, pneumonitis, and migratory pulmonary infiltrates. Chronic strongyloidiasis involves prolonged, often milder symptoms, maintained by continuous autoinfection with a lower parasite load.

In immunocompromised patients, Strongyloidiasis can lead to life-threatening complications known as hyperinfection syndrome or disseminated strongyloidiasis. In these conditions, excessive autoinfection causes rapid multiplication and spread of organisms to multiple organ systems, leading to severe symptoms and a high mortality rate (60-85%). Key features include constant fever, bacteremia, septicemia (due to bacterial translocation from intestinal damage, often E. coli or Klebsiella), hyponatremia, severe respiratory manifestations (hemoptysis, pleural effusions, pleuritic chest pain, severe dyspnea, ARDS), and severe gastrointestinal issues (ileus, small bowel dilation, strictures, ulcerations, bloody diarrhea). Dermatological signs like larva currens may also be more pronounced. Risk factors for severe disease include corticosteroid therapy, other immunosuppressants, diabetes mellitus, HIV/AIDS, chronic kidney disease, advanced age, malignancy, chronic alcohol consumption, and HTLV-1 infection.

Diagnosis involves screening for risk factors, especially before initiating immunosuppressive therapy, as latent infections can reactivate. Stool examination for rhabditiform larvae is crucial, often requiring multiple samples over several days due to low and intermittent excretion. Blood work may show eosinophilia (acute phase), anemia, thrombocytopenia, and elevated PT/PTT/INR. ELISA and PCR can also be used. Imaging (chest X-ray, CT scan) can reveal pulmonary involvement. Endoscopy, duodenal aspiration (EnteroTest), lumbar puncture, skin biopsy, and body fluid analysis are used in disseminated cases. Treatment is essential for all cases, including asymptomatic ones, to prevent future severe complications. The primary drug is ivermectin (200 mcg/kg PO for 1-2 days). Alternatively, albendazole (400 mcg PO BID for 7 days) can be used. For hyperinfection and disseminated disease, ivermectin is given daily until stool/salivary samples are negative for two weeks, along with broad-spectrum antibiotics for co-occurring bacterial infections. Intensive care is often required. Post-treatment stool examinations at 6 and 12 months are vital to confirm eradication. Prevention includes wearing protective footwear in endemic areas and improved sanitation and hygiene.