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Intestinal Nematodes 155

TABLE 6-2 Differentiation of S. stercoralis and Hookworm Larvae

BUCCAL CAVITY BULB TAIL GENITAL PRIMORDIUM
Hookworm—rhabditiform Long Yes Small
Hookworm—filariform Long No Pointed
Strongyloides—rhabditiform Short Yes Large
Strongyloides—filariform Short No Notched

and Ancylostoma duodenale (Table 6-2). This is neces- diarrhea and abdominal pain occur with vomiting and
sary because the clinical manifestations of an overwhelm- weight loss in some individuals. Pulmonary symptoms may
ing systemic infection with S. stercoralis are severe fever occur when the filariform larvae enter the lungs via the cir-
and abdominal pain, respiratory problems, shock, and culatory system, causing coughing and shortness of breath,
possibly death. These medical complications are in ad- particularly in heavy infective loads of organisms. This
dition to the common problems associated with a general condition may progress to bronchopneumonia in severe
common hookworm infection. In addition, the treatment cases. Sepsis and meningitis may develop where multiple
for the two groups is radically different. forms of bacteria develop with a spread of the organisms
into the blood stream and to the brain, a condition that is
Morphology more common in immunocompromised individuals.

The adult forms of the worm called Strongyloides stercora- Life Cycle
lis are also called threadworms. The adult female is rarely
seen in fecal specimens and is roughly 2 mm long, with a Most commonly the direct route is the transmission
short buccal cavity and a long and slender esophagus. The route where infective filariform larvae from the soil pen-
worm produces thin-shelled eggs that are a bit smaller than etrate the skin and then pass into the circulation. In the
those of hookworms, but in most respects are the same as blood stream the larvae are transported to the right heart,
and are indistinguishable from those produced by hook- the lungs, trachea, and pharynx where they are swal-
worms. The noninfective rhabditiform larvae are released lowed and mature into adult worms in only 2 weeks.
from the eggs in the intestine following ingestion but are The adult females then produce eggs that upon hatching
seldom found in stool specimens. The diagnostic stage of release rhabditiform larvae into the intestine. These non-
S. stercoralis larvae is that of the first stage, which is usu- infective larvae develop into an infective stage and are
ally passed in the feces and ranges from 200 to 400 μm capable of infecting a new host. An indirect cycle may be
and 15 to 20 μm in width. The buccal cavity is short and implemented where the rhabditiform larvae develop into
the organism has a prominent genital primordium, which free-living adult male and female worms. They mate and
is a primary means of differentiating it from the hookworm produce eggs and noninfective larvae that then develop
larvae. The third stage is the infective stage, where the fi- into infective larvae upon incubation in the soil.
lariform larvae develop from the rhabditiform larvae in the
soil in most instances. The third stage is somewhat larger Disease Transmission
than the rhabditiform stage and reaches a length of up to
680 μm. This form has a longer esophagus than does the Strongyloidiasis is transmitted from one host to an-
hookworm and has a notched tail in direct contrast to that other host when the skin is penetrated by the infective
of the pointed tail of a hookworm larva. filariform larvae living in contaminated soil. Because
hookworms are contracted in a similar manner, the
Symptoms S. stercoralis larvae must be differentiated from the hook-
worm larvae. S. stercoralis is equipped to undergo a
Some itching may be experienced during skin penetration unique process called autoinfection. This phenomenon
but there are few symptoms associated with this stage until involves the development of the first larval form into in-
the intestinal phase is reached. During the intestinal phase, fective larvae in the host’s intestine. The infective larvae