Strongyloides Stercoralis

Strongyloides stercoralis is a soil dwelling nematode capable of producing infection in humans and other mammals.

• It is a parasite common in wet tropical areas and usually only seen in the UK in travellers who have returned from such areas.
• Infection is most often acquired by walking barefoot on contaminated soil.
• The infection caused by Strongyloides stercoralis (strongyloidiasis) can be either acute or chronic in nature. The chronic form of the infection is well documented in individuals who have been prisoners of war in the Far East.
• Infection can be asymptomatic for many years. It is therefore very important to eradicate the parasite prior to starting immunosuppressive treatment or chemotherapy as failure to do so may result in a hyperinfection syndrome due to dissemination of the larvae to several organs, which is associated with a high mortality rate.¹

• Incidence - Strongyloides stercoralis is thought to have infected 30-100 million people in 70 different countries.
• Infection mainly occurs in tropical and subtropical areas but pockets of infection exist in the USA and several areas of Western Europe.
• The prevalence of the infection is reported to be over 80% in some areas e.g. rural Argentina.
• Infection is commonly associated with rural areas and unsanitary conditions.

• This parasite has a complex life cycle involving both parasitic and free living stages.
• The adult form can survive and reproduce either in the human small intestine or in the soil. In the small intestine of the adult, eggs hatch into rhabditiform larvae which are passed in the stool.
• The larvae may either mature into filariform larvae that are capable of penetrating the skin, or develop into free living worms living in the soil independently of a human or mammalian host.
• The infective filariform larvae, having penetrated the skin, travel via the blood stream to the lungs, then migrate up the airways to the oesophagus and are swallowed back into the intestine, where mating occurs to continue the cycle.

• Symptoms and signs Infection with Strongyloides has a variable effect on the host. Individuals may be asymptomatic for many years, or may develop a series of acute or chronic non-specific symptoms.
• In other cases specific symptoms make the diagnosis more obvious, e.g. the migratory rash. Symptoms that may occur include:
  • Skin involvement:
    • Larva currens (a linear lesion which may advance at a rate of 10 cm/hour) often occurs on the buttocks, groin and trunk.
  • Gastrointestinal involvement:
    • Abdominal discomfort and bloating
    • Profuse watery diarrhoea, sometimes alternating with constipation
    • Malabsorption
    • Weight loss
  • Pulmonary involvement:
    • Cough
    • Bronchospasm
    • Haemoptysis
    • Shortness of breath

• In a massive hyperinfection, the larvae cause lung haemorrhage when passing from the capillaries to the alveoli.
• The vast numbers of larvae also penetrate other organs which are not normally involved in the usual life cycle, e.g. urinary tract, central nervous system and liver.
• Bacterial infection also occurs due to leakage of material through the damaged bowel. The bacteria then are carried on the surface of the larvae to produce meningitis, pneumonia and septicaemia, which are frequently the immediate cause of death in patients with hyperinfection syndrome.
• Hyperinfection syndrome may occur due to immunosuppression of the host and particularly by treatment of the host with steroids.²
• It is therefore particularly important to exclude and treat the infection in any person who has spent time in the tropics or subtropics prior to initiating treatment with immunosuppressive agents or steroids, as the mortality rate of hyperinfection syndrome is reported to be as high as 87%.¹

• Definite diagnosis may be difficult unless larvae are seen in the stool. Often a high index of suspicion is needed together with one or more suggestive investigations.
• Full blood count may show a mild eosinophilia.
• Examination of stool samples may show the presence of larvae, although several samples be required as the larval output is minimal in the majority of cases.¹
• Culture of stool using agar tracking may be more successful, but is not routinely available in clinical practice.
• An assay for immunoglobulins is currently being developed. Due to problems with specificity, it is likely to be only useful as a screening tool and not for diagnosis.
• Chest x-ray: may show pulmonary infiltrates, consolidation or cavitations.

All patients with Strongyloidiasis require treatment, whether or not they are symptomatic, due to the possibility of developing a hyperinfection syndrome at some stage in the future.

• Albendazole 400 mg twice daily for 3 days, repeated after 3 weeks if necessary.
• Ivermectin 200 micrograms/kg daily for 2 days may be the most effective drug for chronic Strongyloides infection.³
• Symptomatic treatment includes antihistamines for pruritus.
• Immunocompromised patients may require hospital admission and intensive care in disseminated infection.

• Appropriate antihelminthic treatment results in a cure in the majority of patients, although re-infection is common in endemic areas.
• Hyperinfection syndrome has a mortality rate of up to 87%.¹

Infection can be reduced by good sanitation and the wearing of footwear in endemic areas.