INTESTINAL PARASITES (Helminths)
NEMATODA Order: Rhabditata

STRONGYLOIDES STERCORALIS

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Endemic in tropics, subtropics and warm moist climates.
Widespread in Eastern Europe and in the mediterranean region.
About 1% of the world population infected.
Rhabditoid larva in faeces: wet mount preparation.

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The infection is acquired by skin contact with contaminated soil.

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Adult females measure 2-3 mm in lenght by 30-50 µm
and have a long (1/3 of the lenght of the worm) cylindrical esophagus;
they inhabit the crypts of the small intestine and lay eggs that hatch within
the epithelium liberating the rhabditoid larvae (L1) in the lumen.
Rhabditoid larvae (L1) (180-380 by 14-20 µm)
excreted with faeces in the external environment,
transform into either filariform larvae (L3, the infective stage)
(the direct development cycle) or into free-living adult males
and females in the soil (the indirect development cycle).
The intestinal mucosa during S.stercoralis hyperinfection:
a) histological section of the mucosa showing an adult worm, larvae and eggs;
b) at higher magnification larvae and eggs within the mucosa;
c) cross section of an adult female in the crypt: 2 reproductive tubes
and the intestine are visible;
d) egg (50-60 by 30-35 µm) of S.stercoralis within the mucosa;
e) longitudinal section of an adult female showing a reproductive tube the intestine.

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In the indirect development cycle several
reproductive cycle of the free-living forms can occur;
they produce eggs (40-70 µm) which can develop
into rhabditiform larvae and into infectious third stage larvae;
this life-cycle takes several weeks to be completed.
The free living females measure about 1 mm in lenght by 50-75 µm:
a) a free living female containing embryonated eggs;
b) the free living females have a rhabdithoid esophagus;
c) particular of the buccal cavity;
d) the rhabdithoid esophagus;
e) eggs within the free-living female.

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Filariform larvae (L3) penetrate the skin by releasing hydrolitic enzymes,
enter dermal vessels and migrate through the blood
or lymphatic channels to the hearth and lungs.
Filariform larvae measure 300-600 µm in lenght by 10-20 µm.
Fecal culture of S.stercoralis larvae, filter paper technique, wet mount preparation.

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In the lungs the larvae break out of the capillaries into alveolar spaces,
migrate up the respiratory tract into the pharynx;
they are then swallowed, reaching the small intestine.
During this process, larvae moult to the fourth stage (L4)
and only females develop into adults.
Filariform larvae have a long non bulbous esophagus
of about 2/5 of the lenght of the body.
Fecal culture of S.stercoralis larvae, filter paper technique, wet mount preparation.

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The prepatent period (the time between infection and the recovering of larvae in faeces)
lasts about 1 month; infections may persist for over 30 years.
In the intestinal lumen rhabditiform larvae may directly transform in filariform larvae;
here they may penetrate the colonic mucosa or the perianal skin causing autoinfection.
Autoinfection explains the long duration of the infection
and the possible multiplication within the host (hyperinfection).
Filariform larvae are slender and have a notched tip of the tail.
Fecal culture of S.stercoralis larvae, filter paper technique, wet mount preparation.

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Symptoms are partially dependent on the nematode burden
which appears to be regulated by two factors:
the host immune system and the moulting regolatory mechanisms of the parasite.
The infection with the nematode may lead to many different clinical pictures.
Two different conditions can be distinguished:
chronic infection and hyperinfection (with or without dissemination).
Light infections may be asymptomatic usually with eosinophilia;
patients may complain about mild gastrointestinal symptoms
(light intermittent abdominal pain and diarrhea),
or pruritus of anal skin or larva currens
(a serpiginous intermittent pruritic rash).
Heavy infections are characterized by more severe intestinal symptoms
(nausea, vomiting, diarrhea), frequently with malabsorption,
gastrointestinal bledding, jejunal perforation, paralytic ileus,
granulomatous hepatitis, eosinophilic ascites often with marked eosinophilia.
Hyperinfection is an extremely dangerous condition.
Rhabditoid larvae in faeces.
Wet mount preparation of unconcentrated faeces during hyperinfection.

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S. stercoralis infection is diagnosed by the presence
in faeces of first-stage rhabditoid larvae.
They measure 180-380 by 14-20 µm.
Baermann concentration technique is the method
of choice for the detection of the parasite.
Rhabditoid larvae have a short buccal cavity,
an attenuated tail and a prominent genital primordium.
On the basis of these morphologic features,
they can be differentiated from Hookworm rhabditoid larvae.
Rhabditoid larva in faeces: wet mount preparation.

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S. stercoralis: the short buccal cavity is a characteristic feature
of the rhabditoid larva of S.stercoralis.

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S. stercoralis: oesophageal cavity in a rhabditoid larva.

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S. stercoralis: buccal and oesophageal cavity of a S.stercoralis first-stage larva.

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S. stercoralis: detail of the prominent genital primordium
in a first-stage larva of S.stercoralis.

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S. stercoralis: the attenuated tail of a first-stage larva of S.stercoralis.

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S. stercoralis: diagnosis of S.stercoralis infection is usually obtained
by faecal examination using concentration methods.
Antibody detection is usefull in patients with hypereosinophilia and negative stool examination.
Immunodiagnosis by indirect immunofluorescence.
Antigen: whole Strongyloides ratti larvae.

Thiabendazole (25 mg/Kg twice a day for 2 days)
and Ivermectine (200 micrograms/Kg in a single dose)
have both been reported to be effective in the treatment of chronic disease.