Page 176 - parasitology for medical and clinical laboratoryprofessionals

156 CHAPTER 6

MICROSCOPIC DIAGNOSTIC FEATURE

General Classification—Nematodal threadworm
Organism Strongyloides stercoralis
Specimen Required Feces
Stage Most often the adult larval form is the primary infective
form as the rhabditiform larva
Size Two forms of larva; rhabditiform larva are 200–250 μm, which show
short buccal capsule (mouth), prominent genital primordium;
filariform larva are approximately 500 μm in length and possess
notched tail with equal lengths of esophagus and intestinal tract
Shape Larva are elongated and slender, hence the term threadworms as
the filariform larvae are more slender than the rhabditiform stage
Motility Rhabditiform larvae are motile in the soil and penetrate skin of
those coming in contact
Other Features Free-living cycle in the soil may revert to production of infective
filariform larvae

S. stercoralis
Larvae
Bulb

Long buccal
cavity
Genital
Delmar/Cengage Learning
primordium
Rhabdifiform

are able to penetrate the walls of the colon, ileum, or pe- and S. stercoralis larvae are made by comparison of the
rianal skin, giving access to the circulatory system. These forms based on anatomic differences in the larvae of the
larvae migrate in the blood to the lungs, are coughed up, species (Table 6-2). In widely disseminated cases larvae
and then swallowed where they repeat the cycle. This have also been recovered from sputum for identification of
autoinfective ability leads to long-standing and lifelong the larvae. Threadworm ova are rarely seen in the feces.
infection unless effective treatment destroys all adult par- Eggs must be allowed to hatch before the larvae are avail-
asites as well as the migrating infective larvae. able for identification. When found, the eggs (ova) are thin-
shelled, measure 54 3 32 μm, and may be segmented.
Laboratory Diagnosis It is necessary to distinguish subtle morphologi-
cal characteristics of these larvae. Sometimes the course
The diagnosis for S. stercoralis is most frequently made of treatment is predicated upon the species of parasite
by identification of the rhabditiform larvae appearing in inhabiting the body. The few species of hookworms
human feces. Occasionally filariform larvae may also be that commonly infect the human can be differentiated
seen in feces. Differentiation between hookworm larvae through the use of these tables.