href="#ulink_69ab37ca-6e72-54cd-8ec1-09fc3fe93274">1, 12). Larvae migrate through the gauze into the water and settle to the bottom of the funnel, where they can be collected and examined. The main difference between this method and the Harada-Mori and petri dish methods is the greater amount of fresh stool used, possibly providing a better chance of larval recovery in a light infection. Besides being used for patient fecal specimens, this technique can be used to examine soil specimens for the presence of larvae.
Figure 4.2 (Upper) Baermann apparatus. (Illustration by Nobuko Kitamura.) (Lower) Baermann apparatus set up in the laboratory. doi:10.1128/9781555819002.ch4.f2
Quality Control for the Baermann Technique
1. Follow routine procedures for optimal collection and handling of fresh specimens for parasitologic examination.
2. Examine known positive and negative samples of stools (from laboratory animals), if available, to make sure that the procedure is precise.
3. Review larval diagrams for confirmation of larval identification.
4. The microscope should be calibrated, and the objectives and oculars used for the calibration procedure should be used for all measurements on the microscope. The calibration factors for all objectives should be posted on the microscope for easy access (multiplication factors can be pasted on the body of the microscope).
5. Record all QC results.
Procedure for the Baermann Technique
1. If possible, use a fresh fecal specimen that has been obtained after administration of a mild saline cathartic, not a stool softener. Soft stool is recommended; however, any fresh fecal specimen is acceptable.
2. Set up a clamp supporting a 6-in. glass funnel. Attach rubber tubing and a pinch clamp to the bottom of the funnel. Place a collection beaker underneath (Fig. 4.2).
3. Place a wire gauze or nylon filter over the top of the funnel, followed by a pad consisting of two layers of gauze.
4. Close the pinch clamp at the bottom of the tubing, and fill the funnel with tap water until it just soaks the gauze padding.
5. Spread a large amount of fecal material on the gauze padding so that it is covered with water. If the fecal material is very firm, first emulsify it in water.
6. Allow the apparatus to stand for 2 h or longer; then draw off 10 ml of fluid into the beaker by releasing the pinch clamp, centrifuge the fluid for 2 min at 500 × g, and examine the sediment under the microscope (magnification, ×100 and ×400) for the presence of motile larvae. Make sure that the end of the tubing is well inside the beaker before slowly releasing the pinch clamp. Infective larvae may be present; wear gloves when performing this procedure.
Results and Patient Reports from the Baermann Technique
Larval nematodes (hookworm, S. stercoralis, or Trichostrongylus spp.) may be recovered. Both infective and noninfective Strongyloides larvae may be recovered, particularly in a heavy infection.
1. Report “No larvae detected” if no larvae could be detected at the end of incubation.
2. Report larvae detected by fecal culture.
Example: Strongyloides stercoralis larvae detected by fecal culture
Procedure Notes for the Baermann Technique
1. It may be difficult to observe morphologic details in rapidly moving larvae; a drop of iodine or formalin or slight heating can be used to kill the larvae.
2. Infective larvae may be found any time after the fourth day and occasionally after the first day in heavy infections. Caution must be exercised in handling the fluid, gauze pad, and beaker to prevent infection. Wear gloves when using this technique.
3. Remember to make sure that the pinch clamp is tight until you want to release some of the water.
4. Preserved fecal specimens or specimens obtained after a barium meal are not suitable for processing by this method; fresh stool specimens must be obtained.
Procedure Limitations for the Baermann Technique
1. The Baermann technique allows both parasitic and free-living forms of nematodes to develop. If specimens have been contaminated with soil or water containing these forms, it may be necessary to distinguish parasitic from free-living forms. This distinction is possible since parasitic forms are more resistant to slight acidity than are free-living forms. Proceed as follows (9, 10).
Add 0.3 ml of concentrated hydrochloric acid per 10 ml of water containing the larvae (adjust the volume accordingly to achieve a 1:30 dilution of acid). Free-living nematodes are killed, while parasitic species live for about 24 h.
2. Specimens that have been refrigerated or preserved are not suitable for culture. Larvae of certain species are susceptible to cold environments.
3. Gloves should be worn when this procedure is performed.
4. Release the pinch clamp slowly to prevent splashing; have the end of the tubing close to the bottom of the beaker for the same reason.
Modification of the Baermann Method
A simple modification of the Baermann method for diagnosis of strongyloidiasis has been developed (13). For this modification, the funnel used in the original version is replaced by a test tube with a rubber stopper, perforated to allow insertion of a plastic pipette tip (Fig. 4.3). The tube containing the fecal suspension is inverted over another tube containing 6 ml of saline solution and incubated at 37°C for at least 2 h. The saline solution from the second tube is centrifuged, and the pellet is observed microscopically as a wet mount. Larvae of S. stercoralis can be found in the pellet. Although the method is almost identical to the original Baermann method, the amount of stool used in the modified method is smaller.
Figure 4.3 Baermann apparatus modification. (Drawing by Sharon Belkin; adapted from reference 13.) doi:10.1128/9781555819002.ch4.f3
Agar Plate Culture for Strongyloides stercoralis
Agar plate cultures are also recommended for the recovery of S. stercoralis larvae and tend to be more sensitive than some of the other diagnostic methods (14–22). It is important to remember that more than half of S. stercoralis-infected individuals tend to have low-level infections (23). The agar plate method continues to be documented as a more sensitive method than the usual direct smear or fecal concentration methods (
