The Baermann test: Try this parasitology test in your practice

The centrifugal fecal flotation test remains the best technique for detecting the most common nematode parasites in dogs and cats. Some less common nematode infections, however, are more efficiently and accurately detected with a Baermann test. This test is used when the diagnostic stage of infection is a first-stage larva rather than an egg. The Baermann test is probably the easiest morphology-based parasitologic test to perform and evaluate but still is rarely carried out in veterinary practice.

What the test detects

The most common nematode parasites detected with the Baermann test are Aelurostrongylus abstrusus in cats and Strongyloides stercoralis in dogs. Aelurostrongylus abstrusus adults are found in the bronchioles and alveolar ducts and are transmitted through ingestion of a snail intermediate host or a bird or rodent paratenic host. Infection in cats can cause signs ranging from cough to severe bronchopneumonia.

Strongyloides stercoralis is found in the small intestine and can infect dogs by several routes, including skin penetration and transmammary transmission. Strongyloides stercoralis infection may cause diarrhea, but because larvae migrate through the lungs respiratory disease may also occur.

The Baermann test is also used to detect the canine nematode parasites Crenosoma vulpis and Angiostrongylus vasorum, but these parasites have very limited distribution in North America (Table 1).

Table 1: Canine and feline parasitic infections diagnosed by the Baermann test*

Why fecal centrifugation is less efficient

Aelurostrongylus abstrusus and S. stercoralis female worms produce eggs that hatch quickly, resulting in the passage of larvae from the host. These larvae may be found on a routine fecal flotation test, but the hyperosmotic flotation solution rapidly distorts them, making specific identification difficult. The Baermann test allows recovery of these larvae without exposing them to damaging solutions.

The Baermann test also allows a larger amount of feces to be used than in a fecal flotation test. Since larvae may be present in low numbers or intermittently, it is helpful to examine larger samples.

How the Baermann test works

In the past, the Baermann test required elaborate equipment and was never routinely used in practice. Today the only special equipment required is a disposable plastic wine glass with a hollow stem. A fecal sample is suspended in water in the bowl of the glass for a period of at least eight hours, giving larvae in the sample an opportunity to move out of the feces and into the water through random movement. The larvae are unable to swim and fall to the bottom of the hollow stem of the glass where they can be collected and examined microscopically (see "How to perform a Baermann test" on the next page).

It is imperative to use a fresh fecal sample (collected immediately after passage from the animal) in a Baermann test. When feces comes into contact with the ground it can be contaminated with free-living nematodes that will also be recovered in the test and can be hard to distinguish from parasite larvae.

In addition, hookworm eggs can embryonate and hatch in a short time in warm weather. First-stage hookworm larvae will also be found by a Baermann test and can be difficult to differentiate from Strongyloides larvae. A sample that was collected fresh but then refrigerated for a lengthy period (days) is also not recommended, since larvae may die in that time, and the test requires the presence of live larvae.

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.

Supplies

• 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

• Water

• A transfer pipette or other means of collecting larvae

• A microscope slide and cover slip
• Lugol's iodine solution

Procedure

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 solution.

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.

Lungworms not detected by the Baermann test

The Baermann test is often thought of—correctly—as a primary test for lungworms, but it is also important to remember that in both dogs and cats there are helminths of the respiratory system for which the Baermann test is not the test of choice. The capillarid lungworm species, Eucoleus boehmi and Eucoleus aerophilus, produce bipolar plugged eggs that are best diagnosed with a fecal flotation test.

Finally, two other nematode parasites of the respiratory system rarely seen in the United States—Oslerus osleri and Filaroides species—are best diagnosed with a 33% zinc sulfate flotation test. Even though O. osleri and Filaroides species pass larvae and not eggs into the environment, these first-stage larvae do not move vigorously and so are not easily recovered with a Baermann test.

Anne M. Zajac, DVM, PhD, DACVM (parasitology)

Meriam Saleh, BS

Department of Biomedical Sciences and Pathobiology

Virginia-Maryland Regional College of Veterinary Medicine

Virginia Tech

Blacksburg, VA 24061-0442

Suggested Reading

1. Bowman, DD. Georgis' parasitology for veterinarians. 9th ed. St. Louis MO; Saunders, 2009.

2. Bowman DD, Hendrix CM, Lindsay DS, et al. Feline clinical parasitology. Ames, Iowa: Wiley-Blackwell, 2002.

3. Conboy G. Helminth parasites of the canine and feline respiratory tract. Vet Clin North Am Small Anim Pract 2009;39 (6):1109-1126.

4. Zajac AM, Conboy GA. Veterinary clinical parasitology. 8th ed. Ames, Iowa: Wiley-Blackwell, 2012.