Biomedical Engineering Reference
In-Depth Information
Strongyloides stercoralis is known as threadworm; in the UK, it is known as
pinworm.
Nematoda
Secernentea
Rhabditida
Strongyloididae
*Strongyloides
Ancylostomatidae
*Ancylostoma
*Necator
Metastrongylidae
*Angiostrongylus
Trichostrongylidae
*Trichostrongylus
Class Rhabditida is characterized by juvenile forms that are adept at free-
living (usually in soil) and are endowed with an impressive skill-set; the abil-
ity to penetrate skin, to migrate through tissues, and to survive for extended
periods.
Strongyloides stercoralis is a nematode that primarily infects humans;
naturally occurring infection by Strongyloidies stercoralis in animals other
than humans, is not known at this time. Humans become infected when lar-
vae, passed from human feces, and free-living in contaminated soil, penetrate
exposed skin and enter the circulation system. No secondary host is involved
in the life cycle of Strongyloides stercoralis. Once in the bloodstream,
Strongyloides stercoralis larvae move to the lungs, where they invade the
bronchial tree, advance to the pharynx, and drop into the esophagus. When
they reach the small intestine, they are adults, and female worms begin to
lay eggs, thus renewing the infectious life cycle. Infections in humans are
uncommon and occur in countries with poor sanitation.
Strongyloides has one important biological trick that contributes to its
clinical presentation. Larvae that develop from eggs laid in the small intes-
tine are capable of autoinfecting the host, by invading though the intestinal
mucosa, or by invading through the anal skin (when they emerge from the
large intestine). The larvae pass to the lung, renewing the infection cycle
within their original human host. This step short-circuits the step wherein lar-
vae dwell as free-living organisms in soil.
The consequences of auto-infection are several: infection can continue
for a long time, sometimes extending throughout the lifespan of the host; the
infectious load (i.e. number of organisms in the body) can be immense.
The term applied to a high infectious load of Strongyloides stercoralis is
“hyperinfection syndrome” [93]. Immune-compromised patients are at high-
est risk for hyperinfection syndrome. When hyperinfection occurs, the dis-
ease can be fatal.
