Quantitative Faecal Flotation - McMaster Egg Counting Technique

This technique is usually applied to faeces, or sometimes intestinal contents, to recover and count eggs of helminth parasites and cysts and oocysts of intestinal protozoa.


Purpose

This technique is usually applied to faeces, or sometimes intestinal contents, to recover and count eggs of helminth parasites and cysts and oocysts of intestinal protozoa. The resulting data, usually recorded as eggs per gram of feces, provides an estimate of parasite infection intensity within a host.

Principle

The McMaster technique uses a special counting chamber which enables a known volume of faecal suspension (0.30 ml) to be examined microscopically. If a known weight of faeces and a known volume of flotation solution are used to prepare the suspension, then the number of eggs per gram of faeces can be calculated.

Advantages of this technique: fast, eggs floated free of debris.

Disadvantages: requires special counting chamber (slide), which are available in Canada. Sources include;

Equipment

  • Scale
  • Syringe or other instrument capable or measuring 60 ml of fluid
  • Saturated NaCl flotation solution
  • Sieve or cheesecloth
  • Beakers or flasks
  • Pasteur Pipette (glass)
  • Microscope

Procedure

1. Weigh out 2 grams of faeces.

2. In a clean glass beaker or flask mix faeces with 60 ml of saturated NaCl (or other flotation solution) until the mixture is homogeneous.

3. Filter the mixture with a sieve or cheesecloth (~0.15mm opening) and collect the filtrate in a new beaker or flask.

4. While mixing the filtrate vigorously take a sample with a pipette and transfer it to one of the chambers of the McMaster slide.

5. Repeat the procedure of mixing and drawing off a sample and fill the other chamber.

6. Wait 30 seconds then count the total number of eggs under both of the etched areas on the slide using a microscope. The eggs can be a bit difficult to find. To make certain that one is in the correct focal plane first focus on the etched lines of the grid, then focus down a bit; the eggs will be floating just below the top of the chamber.

7. Calculate the total eggs per gram for the sample. The total number of eggs in the 2 chambers multiplied by 100 is the eggs per gram (e.p.g.). The volume under the etched area of each chamber is 0.15 ml (the etched area is 1 cm X 1 cm and the chamber is 0.15 cm deep) so the volume examined is 0.3 ml. This is 1/200 of 60 ml. Since you started with 2 g of faeces and then multiplied by 100, the final result is eggs per gram of faeces.

Variations

There are other modifications of the McMaster method including the use of centrifugation to facilitate the removal of fine particles from the filtrate or using different starting sample weights and starting flotation solution volumes.

Flotation solutions

The most common flotation solution used when conducting a McMaster egg count is a saturated salt solution.

Saturated Salt Solution: 1.20 S.G.

  • 400g Sodium Chloride
  • 1 liter of water
  • Dissolve salt and water in the top of a double boiler or with gentle heat
  • Check the specific gravity with a hydrometer and adjust accordingly
Other flotation solutions will also work when determining egg counts using this method.

Problems

If the faeces are not properly stored between collection and examination, nematode eggs may hatch, releasing larvae, and any of the parasite stages in the faeces may degenerate, making detection and identification difficult or impossible. Problems may also arise from inadequate mixing of faeces and flotation solution.

The McMaster is less sensitive than other egg counting techniques.  In the technique described above each egg seen represents 100 eggs per gram (e.p.g) of feces.  Often, in western Canada, animals have infection levels of fewer than 100 e.p.g.
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