subject: Strongyloidiasis aetiology and clinical features [print this page] Strongyloidiasis aetiology and clinical features
Strongyloidiasis is an infection of the small intestine withthe nematode parasite Strongyloides stercoralis, which isdistributed worldwide but flourishes particularly in a hothumid climate. Most people are asymptomatic. A smallnumber have cutaneous manifestations, and immunosuppressedpatients may develop massive systemic invasionwith filariform larvae.A particular characteristic of thishelminth is its capacity to produce prolonged infections fordecades after exposure as a result of autoinfection (e.g.former prisoners of war of the Japanese in Thailand andMyanmar).
Aetiology Strongyloides stercoralis adult females are small (2mmlong) and are found in the intestinal lumen, where theyinsinuate their anterior ends between the villi.A variation of the lifecycle involves rhabditiform larvae developing into filariform larvae during transit down thegut. These larvae penetrate the rectal or anal mucosa orthe perianal skin, and then enter the usual migratory tissuephase. It is this variation of the lifecycle that gives rise to prolonged infection, because filariform larvae develop intonew adults that maintain infection. When it occurs inimmunosuppressed patients very large numbers of filariformlarvae develop and give rise to the massive autoinfectionsyndrome.
Transmission and epidemiology Transmission and epidemiology are broadly the same asthose of hookworm but tend to be focal, depending on soilconditions. The occurrence of autoinfection in Strongyloidiasisallows infection to persist for 30 years or more,successive generations of adults maintaining infection. Ithas wide distribution in the tropics but is particularlycommon in Asia.
Pathology The migrating larvae in the lung evoke some degree ofeosinophilic pneumonitis. Markedly raised eosinophilcounts are usually seen during the phase of migration, andlesser but elevated counts persist in established infection.Adult larvae provoke a mild inflammation in the gut. Maiabsorptionmay occur but the mechanism is not understood.Diarrhoea, hypoproteinaemia and oedema of thesmall bowel wall occur in children.Massive autoinfection with filariform larvae can occur inimmunosuppressed patients who are taking steroids or aretaking steroids with cytotoxics as part of chemotherapy fortumours, with dissemination of larvae to the gut, lungs andCNS, and concurrent septicaemia from the gut.
Clinical features Most patients are asymptomatic and the diagnosis is madeby stool microscopy finding rhabditiform larvaeor in the investigation of eosinophilia. Somepatients present with a transient, linear or serpiginousurticaria that comes and goes within 40-60 minutes.Lesions are seen on the trunk, buttocks and upper thighs.This is caused by subcutaneous migration of infective filariformlarvae, larva currensIntermittently rhabditiform larvae develop into infectivefilariform larvae during their passage down the intestine,and these then invade the rectal wall and perianal skincausing autoinfection.The massive autoinfection syndrome occurs most oftenin people on steroids or immunosuppressive drugs. Fever,abdominal distension, diarrhoea, vomiting and ileus arecommon presenting symptoms. There may be signs ofmeningitis. Dyspnoea, cyanosis and pulmonary infiltratesindicate pulmonary involvement. Septicaemia with organismsfrom the colonic microflora is common. There is arange of disease severity, and the earlier it is recognizedand treated the better the outlook though, overall theprognosis tends to be poor.
Diagnosis Diagnosis is usually made by finding rhabditiform larvaein stools or juice obtained from the duodenum by intubation.Eosinophilia is usual in normal persons but absent inpatients with massive autoinfection. Larvae are readilyfound in sputum, jejunal aspirate or faeces in massive autoinfection.
Management Albendazole (400mg twice daily for 3 days) is effective andcauses few side-effects. There is evidence to suggest thativermectin, 200ug/kg as a single dose may be more effective.Treatment with albendazole is continued for 7 days orlonger in massive autoinfection. There is no experiencewith ivermectin given over days, but adverse reactionsto the drug are not common. Occasionally in those veryseverely affected the parenteral veterinary preparation ofivermectin has been given with good effect, though relapseis all too common and some form of maintenance therapyis often needed, e.g. intermittent ivermectin. Intensivecare is also needed for these patients.
Prevention Safe disposal of faeces would prevent S. stercoralis infection,and wearing sandals would help to prevent skin-soilcontacts. Treatment with cytotoxic or immunosuppressivedrugs for leukaemia, lymphoma and other malignancies,reactional states in leprosy or organ transplantation shouldbe preceded by albendazole or ivermectin treatment inpatients from or in the tropics
