How to perform a Baermann test
The Baermann test set-up. An inexpensive, common plastic disposable wine glass with a hollow stem is the only special equipment
required to perform the Baermann test.
- A plastic disposable wine glass with a hollow stem (I prefer a wine glass with its sloping sides to a disposable champagne
glass, which is flatter at the bottom of the bowl.)
- Material to hold the feces such as gauze or cheesecloth
- A rubber band
- A pencil, applicator sticks, or another means of suspending the fecal packet
- A transfer pipette or other means of collecting larvae
- A microscope slide and cover slip
- Lugol's iodine solution
Step 1: Place a 10-g or larger fecal sample in the center of a double layer of cheesecloth (from the cooking supply section of grocery
stores) or gauze.
Step 2: Wrap the edges around the fecal sample to make a pouch and secure it with the rubber band.
Step 3: Pass a pencil, applicator sticks, or a similar object through the elastic band and suspend the pouch containing the fecal
sample over the bowl of the wine glass.
Step 4: Fill the wine glass completely with tepid tap water. Be sure not to let the corners of the fecal packet hang over the sides
of the wine glass because they act as a wick for water.
Step 5: Allow the glass to sit for at least eight hours and preferably overnight.
Step 6: Remove the feces and discard.
Step 7: Using a transfer pipette or 1-ml syringe with a needle attached, aspirate a small amount water from the very bottom of the
hollow stem of the glass.
Step 8: Place a few drops on a slide, add a cover slip, and examine the slide under a microscope. The slide can be scanned with the
4X objective lens for the presence of larvae, which can then be examined more closely at a higher power. Since the morphologic
features of rapidly moving larvae can be hard to appreciate, place one or more drops of Lugol's iodine solution at the edge
of the cover slip. It will diffuse across the slide and kill the larvae in a straight position and also provide some staining
to help visualize the larvae. Lugol's iodine is widely available commercially.
What you will see
In general, if a known fresh fecal sample is used and motile larvae are detected in the Baermann test they will be parasitic
larvae. In cats, A. abstrusus is the only species producing larvae that is routinely seen, and it can be easily confirmed by identification of the kinked
tail with subterminal spine (Figure 1). In dogs, the most common larva encountered is S. stercoralis, which is slightly more difficult to specifically identify, but in most parts of the United States other parasites producing
larvae are rare (Figure 2).
Figure 1. An Angiostrongylus vasorum larva. The tails of larvae of Aelurostrongylus abstrusus and A. vasorum end in an S-shaped
kink and have a subterminal spine. The arrow indicates the location of the spine, which is not easily seen in this photograph
but can be seen easily by using the fine focus of the microscope.
Occasionally eggs from other parasite species are found in a Baermann test. As the fecal sample begins to disintegrate in
the water, eggs will be released and sink through the water to the bottom of the glass. When eggs are seen in a Baermann test,
it usually indicates that large numbers are present in the sample.
Figure 2. A Strongyloides stercoralis larva. One of the major characteristics used to identify Strongyloides species larvae
in the Baermann test is the prominent genital primordium (black arrow). This large body is in the midsection of the larva
(the bulb at the posterior end of the esophagus is indicated by the red arrow). The larva has been stained with Lugol's iodine
If confirmation of identification is desired, additional fluid from the Baermann test can be placed in a small tube with an
equal volume of 5% to 10% formalin solution and sent to a reference laboratory. If no additional material is available, larvae
can be recovered from a slide by removing the cover slip and rinsing both the slide and cover slip with formalin solution
into a tube. Slides should not be sent since the larvae cannot be preserved.