Manual of basic techniques for a health laboratory - libdoc.who.int

4. Parasitology 157differentiated from larvae of Ancylostoma duodenale and Necator americanus (hookworm),which may also hatch in stools 12–24 hours after passage. The appearanceof filariform larvae of S. stercoralis may indicate a systemic hyperinfection.The genital primordium will be more visible in preparations stained with iodine.The iodine kills the larvae and makes the features easier to see. You will need to usethe ¥ 40 objective to see these structures.●If you see a larva with a short mouth opening and a prominent (clearly visible)genital primordium, it is S. stercoralis.● If you see a larva with a long mouth opening and do not see a genital primordium,it is A. duodenale or N. americanus.The main distinguishing features of S. stercoralis and A. duodenale or N. americanuslarvae are summarized in Table 4.6 and illustrated in Fig. 4.109.4.6 Chemical test for occult blood in stoolsThis test is used for screening for parasitic infection, for example intestinal schistosomiasis,or for detection of bleeding in the intestine caused by polyps, tumoursor inflammation. It was originally developed using benzidine. However, theuse of benzidine is no longer recommended because it has been shown to becarcinogenic.Note: For 1 day before the examination, the patient should not:— eat any meat;— take any drugs containing iron compounds;— brush his or her teeth vigorously.4.6.1 PrincipleOxygen is produced when the haemoglobin in blood comes into contact with hydrogenperoxide. The liberated oxygen reacts with aminopyrine (aminophenazone)to yield a blue colour.4.6.2 Materials and reagents●●CentrifugeConical centrifuge tubeTable 4.6 Characteristics of larvae of Strongyloides stercoralis andAncylostoma duodenale or Necator americanusLarval stage S. stercoralis A. duodenale or N. americanusRhabditiform Buccal cavity short (4 mm) Buccal cavity long (15 mm)Oesophagus one-third of body Oesophagus one-third of bodylength with 2 swellingslength with 2 swellingsGenital primordium large (22 mm) Genital primordium small (7 mm)Anal pore 50 mm from posterior Anal pore 80 mm from posteriorendendFilariform Size 200–500 mm ¥ 15–20 mm Size 200–500 mm ¥ 14–20 mmUnsheathedSheathedTail forked or bluntTail taperedOesophagus half of body length Oesophagus one-third of bodywith no swellinglength with no swelling