Fecal direct smear in cats and dogs

What it shows

Some intestinal protozoa live in the gut as active, moving forms called trophozoites. Flotation solutions do not reliably show them, and they are best seen moving in very fresh stool. A direct smear looks for them while they are still alive:

• How it is done — a speck of faeces on the end of a stick is mixed with a drop of saline on a slide and examined at once. Stained smears or a fixative help identify protozoa such as Giardia and trichomonads.
• What it can show — moving trophozoites, recognised by their shape, size and the way they move: Giardia drifts like a “falling leaf”, while Tritrichomonas moves in a jerky, erratic way. Worm eggs can also be seen now and then.
• What it is not — it is not a concentration test. Only flotation and sedimentation concentrate eggs and cysts; the smear looks at a tiny unconcentrated sample.

Higher than reference — what it may be a sign of

• Positive — trophozoites seen — the form names what was seen, for example Giardia or trichomonad trophozoites. This shows the organism is present in the stool. Giardia and trichomonads can be mistaken for each other under the microscope, so the vet may confirm the finding with other tests (antigen, culture or PCR).
• Positive — eggs seen — worm eggs are occasionally seen in a smear, but the test for eggs is flotation, which examines much more stool.
• Interpretation — finding an organism does not by itself prove that it causes the diarrhoea; the vet decides together with the other tests and the animal’s condition.

Lower than reference

A negative smear is a weak result. False negatives are common because such a small amount of faeces is examined. In one teaching exercise with a hookworm-positive sample, only about a quarter of students found eggs by direct smear (more in the flotation article). Stool that is not fresh or has been refrigerated is less suitable for finding moving trophozoites. Giardia is also shed intermittently, so repeat testing on different days may be needed.

Look at together with

• Fecal flotation — the routine screening test for eggs and cysts, ideally with centrifugation; zinc sulfate is preferred for Giardia cysts.
• Giardia antigen — finds Giardia proteins even when no cysts are seen under the microscope.
• Tritrichomonas — stool culture increases the number of organisms and makes them easier to find, and PCR tells trichomonad species apart.

Units and conversion

The result has no unit. It is recorded as text: “negative”, “no parasites seen” or the name of what was found (for example “Giardia trophozoites”). In the app it is stored as text.

Reference range

The reference range is on your lab’s form; for this test the expected result in a healthy animal is “no parasites seen”. A positive smear shows the organism is present; a negative one does not rule infection out. What to do next, the vet decides together with other stool tests and the animal’s condition.

Cats and dogs

Sources

1. Merck Veterinary Manual — Parasitology in veterinary practice (fecal examination methods)
2. CAPC (Companion Animal Parasite Council) — Microscopic fecal exam procedures
3. CAPC (Companion Animal Parasite Council) — Avoiding common pitfalls in microscopic fecal examinations
4. CAPC (Companion Animal Parasite Council) — Giardia guidelines
5. CAPC (Companion Animal Parasite Council) — Trichomoniasis guidelines