Strongyloides stercoralis - MegaMicro

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Strongyloides stercoralis

Microbiology > Parasitology > Nematodes
Structure:  Helminth, nematode (roundworm)
Epidemiology:
• Endemic in tropical and subtropical regions; sporadically seen in temperate regions (such as southeastern USA).  Associated with agricultural activities involving soil contaminated with infected human feces.
Pathobiology:  
• The infection begins when a skin penetrating filariform larvae enters the circulation and migrates to the lungs.  The larvae are coughed up and swallowed; then mature into adults in the small intestine.  Adult females reproduce parthenogenetically.  Eggs hatch into rhabditiform larvae (noninfective) which are released into the lumen of the bowel and passed into the stool.  The rhabditiform larvae may either develop into infective filariform larvae (direct cycle) or develop into free-living adult worms and initiate an indirect cycle, completing its life cycle within the human host (autoinfection).  
• Autoinfection occurs when the rhabditiform larvae are not passed in the stool, but develop into filariform larvae in the intestine.  They enter the circulation, complete the pulmonary cycle, and then develop into adults, leading to an increase in the intestinal worm burden. This cycle can persist for years and lead to hyperinfection and dissemination.


Disease Manifestations:
• Most infections have no prominent symptoms
• Cutaneous reactions at the site of penetration and around the perianal area (autoinfection)
• Intermittent gastrointestinal symptoms
• Pneumonitis from migrating larvae similar to that seen with ascaris and hookworm infections  
• Heavy worm burdens may result in abdominal  inflammation and ulceration
• Hyperinfection of immunocompromised hosts may lead to systemic infection which is often fatal


Laboratory Diagnosis:
  • Stool exam: collect 3 samples on separate days as passage of rhabditiform larvae in the stool is intermittent (eggs rarely seen in contrast to hookworm infections).
  • If stool exams are negative, duodenal aspirate or biopsy
  • The larvae are 180-380 um long and 14-24 um in diameter.  Differ from hookworm larvae by the length of the buccal cavity and esophagus and by the structure of the genital primordium.
  • Serology may be useful as a screening tool, but false positives may occur with other helminth infections.
  • Symptoms mimicking peptic ulcer disease with peripheral eosinophilia should suggest the diagnosis of strongyloides.

Differential Diagnosis:
• Peptic ulcer disease, ulcerative colitis

Treatment:
  • Ivermectin (2 days) Many clinicians repeat a course of tx after two weeks. For hyperinfection, tx x 7-14 days.
  • Albendazole - lower cure rate
  • taper steroids/immune suppression

Prevention and Control
• Proper sanitation, prompt treatment of existing infections, wearing of shoes to avoid contact with contaminated soil.

Related concepts
1. Parasite
2. Strongyloides
3. S. stercoralis
4. Rhabditiform larvae
5. Filariform larvae

 
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