Macroscopically examine the feces in all cases. Note the fecal color, and look for mucus, undigested food, and blood. This
information, along with the history, should be enough to classify the diarrhea as large bowel, small bowel, or both (Table 1).
Fecal smear and flotation
Perform fecal flotation and saline smear evaluations multiple times since some GI parasites may be missed on a single examination.
To perform a saline smear, mix fresh feces with a small amount of saline solution to form a thin slurry. Place a drop of the
mixture on a slide, and top it with a coverslip. Routine fecal cytology with Gram staining may reveal the gull-wing-shaped
rods of Campylobacter species (Figure 1). Examine the slide microscopically at low and high magnification to identify trophozoites and cysts. A fresh fecal sample
(less than 30 minutes old) is necessary to ensure optimal diagnostic sensitivity. Trophozoite motility decreases quickly after
removal from the host, so saline smears should be performed as promptly as possible. Direct smears may be preferable to flotation
to identify heavy eggs, delicate larvae, and trophozoites.1
Tritrichomonas and Giardia species trophozoites appear grossly similar and may be mistaken for each other on cursory examination. However, distinct
differences between the two organisms will help correct identification. Tritrichomonads have a long, undulating membrane along
their entire bodies, and their movement is jerky and axial (Figure 2). Giardia species trophozoites demonstrate a more rolling motion commonly compared to falling leaves. They are pear-shaped and contain
a characteristic concave ventral disk (Figure 3).
Figure 1. A fecal smear showing several Campylobacter species organisms. Note the characteristic gull-wing shape of the organisms (Gram stain; 1,000X). (Photo courtesy of Michael
D. Willard, DVM, DACVIM.)
When performing fecal flotation, centrifugal flotation methods substantially improve the sensitivity of the process.2 Mix 2 to 3 g (about ½ tsp) of fresh feces with 5 to 10 ml of flotation solution. If the sample is very soft or watery, use
5 g (1 tsp) of feces to account for the dilution effect. Sheather's flotation method using sugar solution allows good parasite
recovery in most cases, but a zinc sulfate flotation test should be used if infection with Giardia species is suspected since this solution is less likely to shrink and distort protozoan cysts.1
Figure 2. A fecal smear showing five trichomonads. Note the anterior flagella (Giemsa stain; 1,000X). (Photo courtesy of Thomas
M. Craig, DVM, PhD.)
Strain the fecal mixture through cheesecloth into a container suitable for pouring. Pour the contents of the container into
a centrifuge tube. If a swinging bucket centrifuge is used, fill the tube with enough solution to create a meniscus, and place
a coverslip on top. Centrifuge for 10 minutes at 1,500 rpm. If a fixed-angle centrifuge is used, do not fill the centrifuge
tube to the top. Create a meniscus by adding flotation solution after centrifugation, and then place the coverslip on top.
Let the sample stand for three or four minutes, and then transfer the coverslip to a slide. Examine the sample at low magnification
to identify pathogenic ova, cysts, and oocysts. If time is limited, samples for flotation can be refrigerated and examined
later in the day.
Figure 3. A zinc sulfate flotation sample showing several Giardia species cysts and one trophozoite. Note the characteristic pear shape and bilateral symmetry (1,000X). (Photo courtesy of
Thomas M. Craig, DVM, PhD.)