Strongyloides

Question 1

What causes strongyloidiasis

Answer 1

Strongyloides stercoralis

Question 2

What are the symptoms of acute strongyloidiasis?

Answer 2

1 An itchy eruption at the site of larval penetration (patients seldom recollect this).
2 Cough and wheeze because of larvae in the lungs (also uncommon).
3 Abdominal pain and diarrhoea. Pain is usually vague and ill-defined. Diarrhoea can be marked. Occasionally, steatorrhoea and even bloody diarrhoea occurs.
4 Weight loss (usually associated with diarrhoea).

Question 3

What is ‘larva currens’?

Answer 3

• A characteristic of chronic strongyloidiasis
• Larva currens (‘creeping eruption’). This is a characteristic, virtually pathognomonic skin eruption.
• Caused by the migration of larvae through the skin during autoinfection. The eruption is typically:
• a serpiginous wheal (a raised line) surrounded by a flare;
• evanescent (comes and goes in a few hours);
• very itchy;
• confinedtothetrunkbetweentheneckandthe
  knees; and
• tends to appear in crops at irregular and unprdictable intervals.

Question 4

Describe the lifecycle of S. stercoralis

Answer 4

The Strongyloides stercoralis life cycle is complex, alternating between free-living and parasitic cycles and involving autoinfection. In the free-living cycle:
1. Rhabditiform larvae are passed in the stool of an infected definitive host
2. Rhabditiform larvae develop into either infective filariform larvae (direct development) (see #6) or free-living adult males and females
3. Females and males mate and produce eggs
4. Rhabditiform larvae hatch from eggs
5. Rhabditiform larvae eventually become infective filariform (L3) larvae.
6. Filariform larvae penetrate the human host skin to initiate the parasitic cycle (see below). This second generation of filariform larvae cannot mature into free-living adults and must find a new host to continue the life cycle.
Parasitic cycle:
6. Filariform larvae in contaminated soil penetrate human skin when skin contacts soil,
7. Filariform larvae migrate to the small intestine. It has been thought that the L3 larvae migrate via the bloodstream and lymphatics to the lungs, where they are eventually coughed up and swallowed. However, L3 larvae appear capable of migrating to the intestine via alternate routes (e.g. through abdominal viscera or connective tissue).
8. In the small intestine, the larvae molt twice and become adult female worms.
9. The females live embedded in the submucosa of the small intestine and produce eggs via parthenogenesis (parasitic males do not exist) , which yield rhabditiform larvae.
10. The rhabditiform larvae can either be passed in the stool image (see “Free-living cycle” above), or can cause autoinfection.

Question 5

How is S. stercoralis infection diagnosed?

Answer 5

• Usually diagnosed by microscopic identification of Strongyloides stercoralis larvae (rhabditiform and occasionally filariform) in the stool, duodenal fluid, and/or biopsy specimens
• Stool specimens may need to be repeated up to 7-12 times to increase sensitivity due to infrequent shedding of rhabditiform larvae in chronic infection
• Aspiration of duodenal fluid or use of the less invasive Entero-test (commonly called a string test) may be useful to detect larvae in patients with negative stool samples
• In disseminated strongyloidiasis, filariform larvae may be detected in sputum, bronchial washings or pleural fluid
• Antibody tests may be used

Question 6

What is the treatment for S. stercoralis infection? In hyper-infection?

Answer 6

• Ivermectin 3-7 days
• Albendazole 3-7 days

Hyper-infection, chronic infection, and those with HTLV-1 may require longer treatment

Question 7

Describe S. stercoralis rhabditiform larvae

Answer 7

• Rhabditiform larvae can be found in stool, as the eggs embryonate and hatch in the mucosa of the small intestine of the host.
• They may also be found in soil and cultured feces
• The first-stage rhabditiform larvae (L1) of Strongyloides stercoralis are 180—380 µm long
• Have a short buccal canal
• A rhabditoid esophagus (divided into three sections) extending 1/3 of the body length
• A prominent genital primordium.
• Second-stage rhabditiform larvae (L2) are longer and have a smaller esophagus/intestine ratio.

Question 8

Describe S. stercoralis filariform larvae

Answer 8

• Infective, third-stage filariform larvae (L3) of Strongyloides stercoralis are up to 600 µm long.
• The tail is notched
• The esophagus to intestine ratio is 1:1, which helps distinguish it from hookworm filariform larvae (which have a short esophagus and pointed tail).
• Infective L3 larvae are found in soil and invade the human host by direct penetration of the skin.
• They may be found in respiratory specimens during cases of autoinfection.

Question 9

Describe parasitic and free-living adult S. stercoralis worms

Answer 9

• Strongyloides stercoralis adult worms may be found in the human host or soil.
• Parasitic males do not exist
• Parasitic females are long, slender and measure 2.0—3.0 mm in length
• In the environment, rhabditiform larvae may develop into infective filariform (L3) larvae (direct cycle) or free-living male and female adult worms (indirect cycle).
• Free-living adult males measure up to 0.75 mm long;
• Free-living females measure up to 1.0 mm long.

Question 10

What are the 2 situations most associated with S. stercoralis hyperinfection?

Answer 10

• HTLV-1 infection
• Steroid immunosuppression