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Chapter 9.5 : Additional Techniques for Stool Examination
Category: Clinical Microbiology
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Strongyloides stercoralis larvae are usually the only larvae found in stool specimens. Depending on bowel transit time and the condition of the patient, rhabditiform and, rarely, filariform larvae may be present. If there is delay in examination of the stool, then embryonated ova and larvae of hookworm may be present. Culture of feces for larvae is useful to (i) reveal the presence of larvae when they are too scanty to be detected by concentration methods, (ii) distinguish whether the infection is due to Strongyloides or hook-worm on the basis of rhabditiform larval morphology by allowing hookworm eggs to hatch and release first-stage larvae, and (iii) allow development of larvae into the filariform stage for further differentiation.
Diagram of the Baermann apparatus used for recovery of larval-stage nematodes (from reference 2 ).
Diagram of Harada-Mori culture system (from reference 1 ).
Diagram of petri dish-filter paper slant (from reference 1 ).
Agar culture method for S. stercoralis. (1) Agar plates are prepared; (2) agar is dried for 4 to 5 days on the bench top; (3) plates are stored in plastic bags; (4) fresh stool is submitted to the laboratory; (5) approximately 2 g of stool is placed onto an agar plate; (6) the plate is sealed with tape; (7) the culture plate is incubated at 26 to 33°C for 2 days; (8) the plate is examined microscopically for the presence of tracks (bacteria carried over agar by migrating larvae); (9) 10% formalin is placed onto agar through a hole made in the plastic with hot forceps; and (10) material from the agar plate is centrifuged and (11) examined as a wet preparation for rhabditiform or filariform larvae (high dry power; magnification, ×400). (Illustration by Sharon Belkin; from reference 2 ).
Tracking of Strongyloides stercoralis larvae on agar plate culture.
Diagram of a sidearm hatching flask (from reference 2 ).
Diagram of a schistosomal miradicium within the egg shell (from reference 2 ).