Strongyloidiasis is a common parasitic disease caused by infection with the roundworm Strongyloides stercoralis. Transmission occurs through skin penetration, most commonly from walking barefoot. Strongyloides has a unique life cycle that can be entirely completed in the human host, migrating from the skin to the pulmonary system and then to the GI system. Symptoms include cutaneous irritation, constipation, diarrhea, dry cough, and wheezing, depending on where the parasite is in its life cycle. Effective eradication of the parasite can be obtained with anthelmintic medications, usually ivermectin.

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Epidemiology and Etiology


  • Worldwide prevalence: 100 million cases currently
  • Most common in tropical and warm temperate regions, with ¾ of cases in Southeast Asia, Africa, and Western Pacific countries


Strongyloidiasis is caused by a parasitic infection by the roundworm (nematode) Strongyloides stercoralis. Roundworms are nonsegmented worms that can infect the GI tract, blood, or skin.

  • The ability to “auto-infect” its host leads to chronic cases.
  • Adult males: 0.9 mm (0.04 in) long, can be distinguished from females by size and presence of spicules (needle-like mating structure)
  • Adult females: 2.0–2.5 mm (0.08–0.10 in) long
Strongyloides sp.

Strongyloides sp.:
Photomicrograph with a magnification of 128X of a Strongyloides sp. filariform, a larval-staged organism

Image: “1547” by Dr. Mae Melvin. License: Public Domain


Life cycle of S. stercoralis

  1. Filariform larvae in contaminated soil penetrate human skin.
  2. Larvae migrate through bloodstream and lymphatics to lungs.
    • In lung parenchyma, larvae penetrate alveoli and migrate up trachea and larynx.
    • Larvae are then swallowed by the host.
  3. Swallowed larvae enter GI tract.
    • Larvae mature within mucosa of small intestine.
    • Adult larvae produce eggs, which yield noninfectious, rhabditiform larvae that migrate to intestinal lumen and are excreted in feces.
  4. Autoinfection can occur in hosts with compromised immune systems:
    • Noninfectious, rhabditiform larvae mature into infectious, filariform larvae within the intestine.
    • Filariform larvae penetrate intestinal mucosa or perianal skin to migrate through bloodstream and lymphatics to restart infectious cycle.
Strongyloides Life Cycle

The life cycle of S. stercoralis:
The roundworm S. stercoralis has a unique lifecycle that can be completed within its definitive human host. The parasite is transmitted to other hosts through excreted feces, but in immunodeficient patients, autoinfection can occur, leading to more severe, chronic forms of the disease.

Image: “Strongyloides LifeCycle en (01)” by Alexander J. da Silva, Melanie Moser. License: Public Domain


  • Humans (most common)
  • Cats and dogs


  • Most common: larvae found in contaminated sand or soil penetrate skin 
  • Fecal-oral transmission (less common)

Host risk factors

  • Walking barefoot on contaminated soil 
  • Contact with human waste or sewage
  • Occupations that increase contact with contaminated soil (farming and coal mining)
  • Immunosuppression

Clinical Presentation

Phases of strongyloidiasis infection

Strongyloidiasis can cause both acute and chronic infection. Symptoms correlate with the migration of the larvae from the skin, lung, and intestine.

  • Cutaneous phase: secondary to penetration of larvae into skin
    • Swelling
    • Erythema
    • Pruritus
    • Maculopapular rash with serpiginous lesions (larva currens)
  • Pulmonary phase: secondary to migration of larvae through bronchial tree and alveoli
    • 1 week after skin penetration
    • Löffler syndrome (pulmonary eosinophilia): transient respiratory disorder characterized by accumulation of eosinophils in lungs due to parasitic infection
      • Dry cough
      • Throat irritation
      • Wheezing
  • GI phase: secondary to adult larvae within mucosa of small intestine
    • 3–4 weeks after skin penetration
    • Symptoms:
      • Abdominal pain
      • Diarrhea or constipation
      • Anorexia
      • Nausea/vomiting

Severe manifestations

  • Hyperinfection syndrome: occurs due to acceleration of Strongyloides life cycle
    • Excessive worm burden within traditional reproductive route (skin, lungs, and intestines)
    • Occurs most often in immunosuppressed patients
  • Disseminated strongyloidiasis: due to widespread dissemination of larvae outside traditional reproductive route, often involving liver, brain, heart, and urinary tract
    • CNS: 
      • Headache 
      • Altered mental status
      • Focal seizures
      • Disorientation
      • Coma
    • Respiratory: 
      • Choking
      • Hemoptysis
      • Chest pain
      • Palpitation
      • Dyspnea (can progress to acute respiratory distress syndrome)
    • Immune: 
      • Sepsis
      • Anaphylaxis
      • Shock
    • GI: 
      • Malabsorption
      • Abdominal pain 
      • Distension
      • GI inflammation and hemorrhage
      • Small bowel obstruction

Diagnosis and Management


 Diagnosis requires a high clinical suspicion test, based on symptoms. 

  • Eosinophilia 
    • Common finding in chronic infections
    • Nonspecific finding
  • Skin biopsy: may provide microscopic identification of mobile rhabditiform larvae
  • Serologic studies 
    • ELISA
    • Indirect immunofluorescence microscopy
    • Immunoblot
  • Stool studies: identify parasite in feces
  • Endoscopy not routinely performed
Strongyloides larvae

Wet mount of stool showing Strongyloides larvae

Image: “Strongyloides Hyperinfection in a Renal Transplant Patient: Always Be on the Lookout” by Case Reports in Infectious Diseases. License: CC BY 4.0


  • Anthelmintic medication:
    • 1st line: ivermectin
    • For complicated infections in immunosuppressed individuals, albendazole may be added.
  • Treatment may need to be repeated for persistent infections.


  • Use of footwear when walking outdoors, especially on soil or sand in endemic regions
  • Proper sewage disposal and fecal management

Differential Diagnosis

  • Pulmonary phase:
    • Tuberculosis: pulmonary bacterial infection caused by Mycobacterium tuberculosis, which may present asymptomatically or with fever, weight loss, night sweats, hemoptysis, and productive cough.
    • Ascariasis: infection caused by a parasitic roundworm, Ascaris lumbricoides. May present asymptomatically or with abdominal discomfort, bowel obstruction, and Löffler syndrome.
  • GI phase:
    • Crohn’s disease: inflammatory bowel disease characterized by inflammation of GI tract, presenting with signs of malabsorption, abdominal pain, and non-bloody, chronic diarrhea.
    • Peptic ulcer disease: presence of 1 or more ulcerative lesions in stomach or lining of duodenum, presenting with dyspepsia. May progress with signs of GI bleeding.


  1. La Hoz RM, Morris MI; AST Infectious Diseases Community of Practice. (2019). Intestinal parasites including Cryptosporidium, Cyclospora, Giardia, and Microsporidia, Entamoeba histolytica, Strongyloides, Schistosomiasis, and Echinococcus: Guidelines from the American Society of Transplantation Infectious Diseases Community of Practice. Clin Transplant. 2019 Sep;33(9):e13618. doi: 10.1111/ctr.13618. Epub 2019 Jun 23. Erratum in: Clin Transplant. 2020 Mar;34(3):e13807. PMID: 31145496. Retrieved from
  2. Chu E, Whitlock WL, Dietrich RA. (1990). Pulmonary hyperinfection syndrome with Strongyloides stercoralis. Chest. 1990 Jun;97(6):1475-7. doi: 10.1378/chest.97.6.1475. PMID: 2347234. Retrieved from
  3. Krolewiecki A, Nutman TB. (2019). Strongyloidiasis: A Neglected Tropical Disease. Infect Dis Clin North Am. 2019 Mar;33(1):135-151. doi: 10.1016/j.idc.2018.10.006. PMID: 30712758; PMCID: PMC6367705. Retrieved from
  4. Mehta RK, Shah N, Scott DG, Grattan CE, Barker TH. (2002). Case 4. Chronic urticaria due to strongyloidiasis. Clin Exp Dermatol. 2002 Jan;27(1):84-5. doi: 10.1046/j.0307-6938.2001.00951.x. PMID: 11952686. Retrieved from

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