MONITORING FOR GASTROINTESTINAL HELMINTHS
Regular fecal examinations should be performed at least annually as part of the routine physical examination of every adult
dog and cat. Pups and kittens, which are more likely to harbor intestinal parasites, should have at least two and as many
as four fecal examinations performed in the first year of life. However, young pups and kittens should be presumptively dewormed;
it is inappropriate to wait for a positive fecal examination result before treating a newborn dog or cat.
To ensure that you reach an accurate diagnosis, include gross inspection of the feces as part of the fecal examination, noting
blood, mucus, intact worms, or the presence of cestode proglottids. Be sure not to mistake fibrous material commonly found
in feces for a parasite. Next, perform a centrifugal flotation procedure. This concentrates any eggs present and allows their
detection by microscopic examination. A direct smear may also be prepared from fluid diarrheic fecal samples and examined
microscopically to detect motile protozoa or nematode larvae. To be effective, the fecal flotation must evaluate an adequate
amount of material, which may be as little as 1 g of formed feces or as much as 6 g of fluid feces. The small samples collected
by a fecal wand are not appropriate for diagnosis because they do not provide enough material to allow the detection of a
low number of eggs and, thus, may yield false negative results.
Centrifugal flotation is a more sensitive and accurate means of detecting parasite eggs in fecal samples than the commonly
used passive tabletop flotation method. Veterinary clinics that switch from passive to active fecal flotation protocols often
note increased detection of eggs in fecal samples. Centrifugation of feces suspended in a solution (1.18 to 1.2 g/ml) of appropriate
specific gravity to float nematode eggs forces the eggs to the top of the solution and the debris to the bottom. Both Sheather's
sugar solution and zinc sulfate solution are commonly used in centrifugal fecal flotation techniques.21 The sugar solution is viscous, and frequent cleaning of laboratory areas where it is used is required. It also collapses
some protozoal cysts (e.g. Giardia species), altering their morphology. However, Sheather's sugar solution allows efficient flotation of eggs, and slides can
be stored for examination for longer periods. Zinc sulfate is not as viscous and also allows efficient flotation of all common
gastrointestinal parasites, but the salt crystallizes on the slides fairly quickly, thereby obscuring examination of a slide
at a later time. Disposal of zinc sulfate solution may be problematic in some areas.
Regardless of the solution used, the preparation of a sample for centrifugal flotation requires suspending about 1 g of feces
in 20 to 30 ml of flotation solution, straining that slurry through a sieve or cheesecloth, and then pouring the strained
fluid into a centrifuge tube. If you use a centrifuge with a swinging bucket rotor, you can fill the tubes to form a reverse
meniscus on top, add a coverslip, and spin the tubes with the coverslip in place. When using a fixed-rotor centrifuge, fill
the tubes to just below the top to avoid spillage, spin the tubes without a coverslip, and then add more solution to form
a reverse meniscus. You can then add a coverslip and allow the tube to stand for another five minutes so that the eggs can
passively float to the top of the solution. Then remove the coverslip and examine the entire area microscopically (Figure 2).21
Figure 2: Ova of Toxocara canis (lower right) and Ancylostoma caninum (upper left), gastrointestinal helminths commonly found on canine fecal examination.