1. Wet preparation (thick smear method): Examine a drop of fresh (anticoagulated) blood under a coverslip on a microscope slide (bearing in mind the periodicity in microfilarial activity, see p. 403). The highly motile organisms are clearly visible even at low magnification (250 x).
2. Concentrating the sample: To 3 - 5 mL of drawn venous blood, add 10 -15 mL of a mixture of 95 mL formalin (5 %), 5 mL acetic acid, and 2 mL of an alcoholic gentian violet solution (4 g per 100 mL 96 % alcohol). Centrifuge the mixture, and examine the sediment for stained microfilar-iae. (Membrane filtration methods provide a particularly good yield.)
(Onchocerca volvulus). Place a large drop of physiologic saline solution onto a slide. Immerse in the saline a pinhead-size piece of skin excised with a Walser dermatome (if that is not available, use a razor blade). Cover with a coverslip, let stand 20 min, then examine with the microscope at low power (^300 x). The organisms will pass from the skin into the saline medium and will move vigorously in the fluid.
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