Background

Strongyloides stercoralis is a worm that infects humans, estimated to affect at least 370 million people worldwide. Presentations range from an asymptomatic carrier state (detected on screening) to disseminated disease (hyperinfection) in the immunocompromised host. It is endemic in tropical and subtropical areas where sanitation standards are poor.¹

Life cycle - see CDC. Human infection occurs when skin comes in contact with filariform larvae of Strongyloides, usually in soil contaminated by human faeces. The larvae penetrate the skin and migrate via the bloodstream to the lungs, where they are coughed up and swallowed, moving to the small bowel, where they develop into adult worms. They may also migrate directly to the intestine. The adult worms may live for up to 5 years in the host, laying eggs which develop into rhabditiform larvae that are excreted in the stool. These larvae contaminate soil where sanitation facilities are poor, developing into filariform larvae that can infect others. Strongyloides are also able to complete their entire lifecycle in the human host (auto-infection), meaning that infection can persist for several decades.²

A person with chronic asymptomatic Strongyloides infection may suddenly become acutely unwell with hyperinfection syndrome, if their T-cell immunity becomes depressed (e.g. due to steroids, biologic therapy, chemotherapy, HTLV-1, and to a lesser extent, HIV).³

Epidemiology

Strongyloides stercoralis is endemic in tropical and subtropical areas, including the northern parts of Australia, and many source countries/transit countries for current Humanitarian entrants and asylum seekers in Australia. Rates of Strongyloides spp. infection are high in indigenous communities in Northern Australia.^4,5 Strongyloidiasis is also a common infection among new arrivals to Australia, data from refugee health clinics suggest a positivity rate by serology of 20-40% in African and Karen groups.^6-8

Assessment

Many people with chronic Strongyloides infection are asymptomatic.

• Initial infection may cause mild itchiness at the site of larval skin entry (‘ground itch').
• Chronic infection may cause upper abdominal pain, skin itch, ‘larva currens’ (serpiginous raised rash commonly on the buttocks) or transient pulmonary symptoms (as worms are coughed up). Full blood examination (FBE) commonly shows eosinophilia.
• Hyperinfection can occur decades after initial infection, and presents with fever, vomiting, diarrhoea, abdominal pain and respiratory symptoms (cough, wheezing, shortness of breath and haemoptysis). Symptoms may be due to direct worm burden (due to filariform larval migration and organ invasion) or due to secondary gram negative bacterial infection. A chest X-ray may demonstrate pulmonary infiltrates. Hyperinfection is associated with a mortality of 60%, even with treatment.

Screening

• Screening should be completed in all refugee and asylum arrivals if not completed previously (even many years after arrival).
• Screening should also be completed in people from endemic areas who have haematological malignancy or who are starting immunosuppressive therapy (including corticosteroids) if their screening or treatment status is unknown (also consider on-spec treatment in this setting - see below).
• Screening is by serology - ELISA (for IgG to filariform larvae) - positive serology represents active infection. Serology is highly sensitive and specific (estimated 95% and 99%).

Stool examination is not sensitive for detecting Strongyloides infection.⁹ The rhabditiform larvae can sometimes be detected in stool using specialised laboratory techniques (e.g. Harada culture - NB not available at RCH). In cases of hyperinfection syndrome, filariform larvae may be found in respiratory or gastrointestinal specimens. Faecal PCR for Strongyloides is specific, but less sensitive.¹⁰

Management

• All patients with positive or equivocal Strongyloides serology (or strongyloides identified on stool by either microscopy or PCR) should receive treatment.
• Treatment is with Ivermectin 200 mcg/kg orally on day 1 and day 14 (2 doses total) (for weight 15kg and above, no upper dosing limit, tablet strength = 3mg).²
• This regimen is estimated to be more than 90% effective. 
  • Ivermectin is not recommended for children <15kg, but has been used in mass treatment campaigns in younger children with no reported adverse events.¹¹
  • Treatment with ivermectin can precipitate encephalopathy in people with Loa loa infection; consider testing for Loa loa in people from West/Central Africa prior to ivermectin.²
  • Ivermectin is used as daily dosing in patients with hyperinfection syndrome - see CDC notes.
  • Ivermectin comes in 3 mg tablets (dispersible), and is pregnancy B3.
• Albendazole in an alternative antibiotic with activity against Strongyloides but is less efficacious than ivermectin.² Albendazole may be used in younger children (6 months and older) - seek specialist advice for this age group.¹²  The recommended dose is: (weight >10 kg) Albendazole 400mg oral bd 7 days (consider 200 mg oral bd 7 days for children over 6 months up to 10kg - seek specialist advice for this age group).¹²
• Pregnancy - neither ivermectin [B3] or albendazole [D] are recommended during pregnancy. In asymptomatic patients with positive serology, withhold treatment until after delivery. Seek specialist advice in symptomatic patients.

Follow-up

• All patients treated for Strongyloides infection should be followed up with FBE (to confirm eosinophilia has resolved) and repeat serology 6 months after therapy.
• Serological titres usually fall significantly after successful treatment¹³ - repeat serology at 6 months, (and at 12 months if serology remains positive at 6 months).
• Stool samples should also be repeated 2-4 weeks after therapy, if positive on microscopy (not PCR) prior to therapy.

References

• References

_{Immigrant health clinic protocols. Author: Vanessa Clifford and Georgie Paxton, January 2014. Updated June 2020. Contact georgia.paxton@rch.org.au}