Key facts:

• Strongyloidiasis is a chronic parasitic infection of humans caused by Strongyloides stercoralis.
• Transmission occurs mainly in tropical and subtropical regions but also in countries with temperate climates.
• An estimated 30–100 million people are infected worldwide; precise data on prevalence are unknown in endemic countries.
• Infection is acquired through direct contact with contaminated soil during agricultural, domestic and recreational activities.
• Like other soil-transmitted helminthiases, the risk of infection is associated with hygiene, making children especially vulnerable to infection.
• Strongyloidiasis is frequently underdiagnosed because many cases are asymptomatic; moreover, diagnostic methods lack sensitivity.
• Without appropriate therapy, the infection does not resolve and may persist for life.
• Infection may be severe and even life-threatening in cases of immunodeficiency.
• No public health strategies for controlling the disease are active at the global level.

©WHO / Strongyloides stercoralis threadworm in stool, analyze by microscope.

Strongyloidiasis is an infection caused by Strongyloides stercoralis (and rarely S. fülleborni), a helminth present mainly in tropical and subtropical regions but also in temperate climates. Some 30–100 million people are estimated to be infected worldwide (probably an underestimate).

Transmission cycle

Strongyloidiasis is transmitted through direct penetration of human skin by infective larvae when in contact with soil; walking barefoot is therefore a major risk factor for acquiring the infection. Strongyloides spp. larvae penetrate the human host and reach the intestine where they mature into adults and produce eggs; the eggs hatch in the gut lumen and yield larvae that are evacuated in faeces. The peculiarity of this worm is that some larvae are not excreted but reinvade the intestine or perianal skin to perpetuate the infection (“autoinfection cycle”).

Symptoms

Strongyloidiasis may cause intermittent symptoms that mostly affect the intestine (abdominal pain and intermittent or persistent diarrhoea), the lungs (cough, wheezing, chronic bronchitis) or skin (pruritus, urticaria). Asymptomatic cases may host the parasites for years unaware of the infection. Although strongyloidiasis has usually mild manifestations, the infection may be severe and life-threatening in cases of immunodeficiency (haematological diseases, immunosuppressive therapies). For this reason it is extremely important to suspect, diagnose and treat the infection.

Diagnosis and therapy

Diagnosis of strongyloidiasis is not standardized. The most frequent procedure entails direct examination of stool specimens with enrichment and/or stool culture, but this often does not yield positive results even when the disease is present. Alternative tests (serology and polymerase chain reaction) are more efficient.

Ivermectin, thiabendazole and albendazole are the most effective medicines for treating the infection. Albendazole is considered the least effective. Ivermectin, the drug of choice, is not available in all endemic countries. Moreover, the optimal schedule has yet to be defined.

Prevention and control

No public health strategy has been developed to control strongyloidiasis.

Strongyloidiasis has almost disappeared in countries where sanitation and human waste disposal have improved.

In areas where mass treatment with ivermectin has been used to control onchocerciasis or lymphatic filariasis, the prevalence of strongyloidiasis is probably reduced, but further investigation is needed.