Infectious Disease and Microbiology – Strongyloidiasis

Strongyloidiasis is an intestinal nematode infection caused mainly by Strongyloides stercoralis. Infection begins when infective larvae in contaminated soil penetrate the skin. Unlike many other intestinal helminths, Strongyloides can complete its life cycle within the human host, allowing autoinfection and persistence of infection for decades.

The disease occurs worldwide but is most common in tropical and subtropical regions, especially in areas with poor sanitation. High prevalence has been reported in parts of South America, Southeast Asia, and other resource-limited regions.

A particularly important feature is that chronic infection may remain asymptomatic for many years after a person leaves an endemic area because continuous autoinfection maintains the parasite within the host.

Major risk factors include residence or travel in endemic areas, walking barefoot or having direct skin contact with contaminated soil, and impaired immunity.

Severe disease is strongly associated with immunosuppression, particularly systemic corticosteroid therapy, solid-organ or hematopoietic stem-cell transplantation, hematologic malignancy, cytotoxic chemotherapy, and TNF-α inhibitors.

Even relatively short courses of corticosteroids can precipitate Strongyloides hyperinfection syndrome in an infected patient.

Coinfection with HTLV-1 is another important risk factor for severe, persistent, and disseminated strongyloidiasis.

Prevention depends primarily on adequate sanitation, avoidance of barefoot exposure to contaminated soil, and recognition of chronic infection before immunosuppressive therapy.

Patients with a history of residence or substantial exposure in endemic areas should be considered for screening before transplantation or major immunosuppressive treatment, particularly before corticosteroid therapy when clinically appropriate.

Life Cycle and Pathophysiology

Adult female worms reside within the mucosa of the duodenum and jejunum.

Eggs produced by the adult worms hatch within the intestine and release rhabditiform larvae.

These larvae may pass in the stool and continue their life cycle in the soil.

Some rhabditiform larvae instead transform into infective filariform larvae while still inside the host.

These infective larvae can penetrate the intestinal mucosa or the perianal skin and re-enter the circulation.

This process is called autoinfection and explains why Strongyloides infection can persist for decades without repeated environmental exposure.

After skin penetration, filariform larvae enter the bloodstream and migrate to the lungs.

They pass through the pulmonary circulation, enter the alveoli, ascend the respiratory tract, are swallowed, and eventually reach the small intestine where they mature into adult worms.

Hyperinfection Syndrome

In immunosuppressed patients, autoinfection can accelerate dramatically.

Large numbers of larvae migrate through the intestine and lungs, producing hyperinfection syndrome.

If larvae spread beyond their usual gastrointestinal and pulmonary life cycle into organs such as the brain, liver, or kidneys, the condition is termed disseminated strongyloidiasis.

Larval migration through the intestinal wall can carry enteric bacteria into the circulation.

This can produce severe gram-negative bacteremia, polymicrobial sepsis, meningitis, or other metastatic bacterial infections.

Clinical Manifestations

Many patients with chronic strongyloidiasis are asymptomatic or have only mild intermittent symptoms.

Skin Manifestations

During acute infection, patients may develop pruritic erythematous papules at the site of larval penetration, commonly on the feet.

A characteristic manifestation of chronic autoinfection is larva currens.

Larva currens consists of a rapidly moving, serpiginous, intensely pruritic urticarial eruption, usually beginning around the perianal region and extending onto the buttocks, thighs, or trunk.

Its rapid migration helps distinguish it from classic cutaneous larva migrans.

Gastrointestinal Disease

Patients may experience intermittent diarrhea, abdominal cramping, diffuse abdominal discomfort, nausea, anorexia, or weight loss.

Rarely, chronic infection can produce malabsorption or nutritional deficiencies.

In severe hyperinfection, gastrointestinal manifestations may include mucosal ulceration, gastrointestinal bleeding, bowel-wall edema, ileus, and intestinal dysfunction.

Pulmonary Disease

Larval migration through the lungs can produce cough, wheezing, bronchospasm, or transient pulmonary infiltrates, resembling a Löffler-type eosinophilic pulmonary syndrome.

In hyperinfection syndrome, pulmonary disease can progress rapidly to diffuse pneumonitis, hypoxemia, respiratory failure, or acute respiratory distress syndrome (ARDS).

Disseminated Disease

Disseminated infection may involve the central nervous system, liver, kidneys, skin, and other organs.

An important diagnostic clue is the development of unexplained gram-negative or polymicrobial sepsis or meningitis, particularly with enteric organisms such as Escherichia coli or Klebsiella, in an immunosuppressed patient with epidemiologic risk for Strongyloides.

Diagnosis

Eosinophilia may occur in chronic uncomplicated infection, but its absence does not exclude strongyloidiasis.

In fact, eosinophilia may disappear in severe hyperinfection, so a normal eosinophil count can be falsely reassuring.

Diagnosis can be established by identifying Strongyloides larvae in stool.

Because larval shedding can be intermittent and low in chronic infection, a single stool examination has limited sensitivity.

Repeated stool examinations improve diagnostic yield.

More sensitive parasitologic methods include stool agar-plate culture, concentration techniques, and molecular detection such as PCR where available.

Serologic testing, commonly by ELISA, is useful for screening and has good sensitivity in immunocompetent patients, although sensitivity may be lower in immunosuppressed individuals.

Duodenal aspirates or biopsy specimens may demonstrate larvae when stool studies are negative but clinical suspicion remains high.

In hyperinfection, larvae may also be found in sputum, bronchoalveolar lavage fluid, or other specimens.

Chest imaging may demonstrate interstitial infiltrates, focal pneumonia-like changes, diffuse pulmonary opacities, or ARDS in severe disease.

Differential Diagnosis

Strongyloidiasis can resemble other helminth infections, including ascariasis, hookworm disease, and cutaneous larva migrans.

Pulmonary manifestations may resemble atypical pneumonia, eosinophilic lung disease, or tropical pulmonary eosinophilia.

Gastrointestinal disease can mimic other causes of chronic diarrhea, malabsorption, or inflammatory bowel disease.

Treatment

Ivermectin is the treatment of choice for uncomplicated strongyloidiasis.

A commonly used regimen is ivermectin 200 μg/kg orally once daily for 1–2 days, although treatment schedules may vary depending on the clinical situation.

Albendazole is less effective and is generally considered an alternative when ivermectin cannot be used.

Hyperinfection and Disseminated Strongyloidiasis

Hyperinfection syndrome is a medical emergency.

Treatment requires daily ivermectin, generally continued until clinical improvement occurs and parasitologic examinations remain negative for an adequate period.

Patients with ileus or severe gastrointestinal dysfunction may have poor absorption of oral ivermectin.

In exceptional life-threatening situations where oral or enteral therapy cannot be reliably absorbed, alternative routes of ivermectin administration have been used under specialist supervision and regulatory or compassionate-use arrangements.

Associated bacterial sepsis must be treated aggressively with appropriate antibacterial therapy and supportive care.

Whenever possible, immunosuppressive therapy should be reduced, especially corticosteroids.

Follow-Up

After treatment of uncomplicated infection, repeat stool testing may be performed to document parasitologic clearance, particularly when symptoms persist or the patient is immunocompromised.

Serologic antibody levels generally decline over months after successful treatment and can sometimes assist with follow-up.

In disseminated or hyperinfection disease, stool and other relevant specimens should be examined repeatedly during treatment until evidence of active infection has resolved.

Prognosis

Uncomplicated strongyloidiasis usually responds well to appropriate therapy.

By contrast, hyperinfection and disseminated strongyloidiasis are potentially fatal, particularly in patients receiving corticosteroids or other major immunosuppressive therapies.

Complications

The most serious complications include hyperinfection syndrome, disseminated larval infection, severe pneumonitis, ARDS, gram-negative bacteremia, polymicrobial sepsis, meningitis, gastrointestinal bleeding, and multiorgan failure.

High-Yield Pattern

Endemic soil exposure + chronic intermittent GI symptoms ± eosinophilia + rapidly migrating perianal rash (larva currens) → consider strongyloidiasis.

Steroids or major immunosuppression + unexplained pulmonary deterioration + gram-negative sepsis → urgently consider Strongyloides hyperinfection.

Diagnosis → repeated stool examination / agar culture / PCR / serology.

Treatment → ivermectin.

Hyperinfection → daily ivermectin + reduce immunosuppression + treat associated sepsis.