PARASITES AND PARASITIC DISEASES OF
Preparasitic development from egg to L3 is typically strongyloid, though earthworms may
intervene as transport hosts. There are three modes of infection:
by ingestion of the free L3s (A),
ingestion of earthworms carrying L3s (B),
and skin penetration by L3s (C).
larvae exsheath in the intestine (D),
enter lymphatic vessels and pass to the mesenteric lymph nodes where the
first molt occurs. Molting larvae can be found in the nodes anywhere from 1 to 9 days
after infection. They continue on to the liver (E),
where the final molt takes place. Following ingestion of infected earthworm transport
hosts, L3s are released in the gut as the earthworm is digested. The released L3s migrate
to the liver as described above.
Click anywhere on the S. dentatus life
cycle image to see the percutaneous migration route. () Larvae infecting pigs
via skin penetration probably molt to L4s in subcutaneous tissues and reach the liver via
the lungs (H) and
systemic circulation (I).
Click anywhere on the life cycle to return
to the first image.
In the liver young adults wander in the parenchyma for three months or more before
piercing the capsule and migrating through the peritoneal cavity to the perirenal region (F). There they become enclosed
in a cyst by host reaction, and complete their development. The cyst communicates
with the ureter either directly or, if it is more distant, by a fine connecting canal,
allowing the worm eggs to be excreted in the urine via the bladder (J).
Occasionally, abberrant migration occurs with worms ending up in the pancreas (G), muscle, and other organs of
the host where they are trapped by encapsulation and never reach the perirenal area.
The prepatent period ranges from 6 to 19 months and adult worms have a longevity of
about two years .
There are reports in the literature supporting a case for transmission of this nematode
to piglets in utero.