All major intestinal helminth infections are still solely dependent on microscopy for diagnosis. Helminth eggs are often easier to find and identify because of their size and their distinctive morphological features. In some cases, the helminthes (eggs/ova) are present in sufficient quantities to be found by direct examination of a small amount of faeces, i.e. the direct smear.
It is usually more efficient for laboratories to do a simple concentration
to avoid overlooking parasites that may be present in very small numbers.
A modification of the direct smear procedure, the Kato-Katz technique
, is specially useful for field surveys aimed to detect schistome or soil-transmitted nematode (round worm, whipworm and hookworm) because it also gives an estimation of the intensity of infection.
Direct faecal Smears-Saline and Iodine wet mount preparations for intestinal parasites detection
1. With a wax pencil or other marker, write the patient’s name or identification number and the date at the left-hand end of the slide.
2. Place a drop of saline in the centre of the left half of the slide and place a drop of iodine solution in the centre of the right half of the slide (Fig .1).
(Note. Iodine wet mount preparations are most useful for protozoa, less so for helminths.)
3. With an applicator stick or match, pick up a small portion of faeces (approximately 2 mg which is about the size of a match head) and add it to the drop of saline: add a similar portion to the drop of iodine. Mix the faeces with the drops to form suspensions (Fig. 2). .
4. Cover each drop with a cover slip by holding the coverslip at an angle, touching the edge of the drop, and gently lowering the coverslip onto the slide so that air bubbles are not produced (Fig. 3).
(Note ideal preparations containing 2 mg of faeces are uniform – not so thick that faecal debris can obscure organisms, nor so thin that blank spaces are present.)
Examine the preparations with the 10X objective or, if needed for identification, higher power objectives of the microscope in a systematic manner (either up and down or laterally) so that the entire coverslip area is observed (Fig. 4).
When organisms or suspicious objects are seen, switch to higher magnification to see the more detailed morphology of the object in question.
Source: The procedure mentioned above is reproduced from Bench Aids for the Diagnosis of Intestinal Parasites published by World Health Organisation.