Page 1456 - Small Animal Internal Medicine, 6th Edition

1428 PART XIV Infectious Diseases

not adversely affect cell morphology. The cotton swab is are available for use in veterinary clinics. Centrifugation
rolled on a microscope slide gently multiple times to give techniques are more sensitive than passive flotation tech-
VetBooks.ir areas with varying smear thickness (Fig. 91.1). After air- niques. Most eggs, oocysts, and cysts are easily identified
after centrifugation in zinc sulfate solution (Box 91.1) or
drying, the slide can be stained. White blood cells and bac-
teria morphologically consistent with Campylobacter spp.
passive flotation techniques for the demonstration of proto-
(spirochetes) or Clostridium perfringens (spore-forming Sheather sugar solution. These procedures are superior to
rods; Fig. 91.2) can be observed after staining with Diff-Quik zoan cysts (particularly Giardia spp.; Fig. 91.4). Many clini-
or Wright or Giemsa stains (see Cytology section). However, cians are now using big service laboratories for this service,
these findings do not confirm the diagnosis as there are but in one study, a smaller university laboratory had a greater
many spirochetes in feces and cytologic detection of sus- sensitivity than a commercial laboratory for detection of
pected C. perfringens spores that do not always correlate to Giardia cysts (Hascall et al., 2016). Fecal sedimentation
the presence of enterotoxins. Histoplasma capsulatum or Pro- recovers most cysts and ova but also contains debris.
totheca may be observed in the cytoplasm of mononuclear
cells. Methylene blue in acetate buffer (pH 3.6) stains tropho- Baermann Technique
zoites of the enteric protozoans. Iodine and acid methyl This technique is used to concentrate motile larvae from
green stains are also used for the demonstration of protozo- feces. The feces are diluted in water, placed in a funnel
ans. Modified acid-fast staining of a thin fecal smear can be clamped at the ventral end, and the larva concentrate by
performed in dogs and cats with diarrhea to aid in the diag- gravity. Some respiratory parasites are passed as larvated
nosis of cryptosporidiosis. Cryptosporidium spp. are the only eggs but release larvae shortly after being passed in feces.
enteric organisms of approximately 4 to 6 µm in diameter Eggs or larvae from respiratory parasites can also be detected
that will stain pink to red with acid-fast stain (Fig. 91.3). by cytologic evaluation of airway washings (Fig. 91.5).

Fecal Flotation
Cysts, oocysts, and eggs in feces can be concentrated to
increase the sensitivity of detection. A variety of techniques

FIG 91.3
FIG 91.1 Cryptosporidium parvum oocysts stained with a modified
Diff-Quik–stained fecal smear showing appropriate smear acid-fast stain. The oocysts are approximately 4 × 6 µm.
thickness.

BOX 91.1

Zinc Sulfate Centrifugation Procedure
1. Place 1 g fecal material in a 15-mL conical centrifuge
tube.
2. Add 8 drops of Lugol iodine and mix well.
3. Add 7 to 8 mL of zinc sulfate (1.18 specific gravity)*
and mix well.
4. Add zinc sulfate until there is a slight positive
meniscus.
5. Cover the top of the tube with a coverslip.
6. Centrifuge at 1500 to 2000 rpm for 5 minutes.
7. Remove the coverslip and place on a clean
microscope slide for microscopic examination.
8. Examine the entire area under the coverslip for the
FIG 91.2 presence of ova, oocysts, or larvae at ×100.
Wright-stained, thin fecal smear. A neutrophil and spore-
forming rods are present in the center of the field. *Add 330 g zinc sulfate to 670 mL distilled water.