Fecal flotation and fecal egg count in cats and dogs

What it shows

Parasite eggs and cysts are lighter than the flotation solution, while most of the stool debris is heavier. The eggs rise to the surface, stick to a coverslip and are identified by their shape. On a form this test can appear in three rows:

• Fecal flotation (qualitative) — answers “what was found”: the type of eggs, cysts or oocysts, or nothing. Centrifugal flotation, where the tube is spun in a centrifuge, is far more sensitive than simple flotation, where the tube just stands. In one teaching exercise with a hookworm-positive sample, about a quarter of students found eggs by direct smear, about 70% with simple flotation and all of them with centrifugal flotation.
• Fecal egg count (quantitative) — counts eggs per gram of stool (EPG) in a counting chamber, for example by the McMaster method; more sensitive variants exist. It shows how many eggs are being shed, not how many worms there are.
• Zinc sulfate centrifugal flotation — the same idea with a zinc sulfate solution. It is considered the best flotation for Giardia cysts: sugar solution makes the cysts collapse.

Flotation does not reliably find moving single-celled forms (trophozoites), worm larvae and heavy fluke eggs. For trophozoites a direct smear is used, for larvae the Baermann test, and for fluke eggs sedimentation.

Higher than reference — what it may be a sign of

• Positive — the form names what was found: for example roundworm, hookworm or whipworm eggs or Giardia cysts. A positive result means the parasite is present; whether it explains the signs the vet decides.
• Eggs that are not the animal’s own — dogs often eat faeces, and eggs of other hosts’ parasites can simply pass through.
• Higher egg count — more eggs per gram means heavier shedding in that sample. The sources describe EPG cut-offs only for horses and farm animals, not for cats and dogs.

Some parasites, such as roundworms, can pass to people; questions about the risk for people are for their own doctor.

Lower than reference

A negative result lowers the likelihood of infection but does not rule it out:

• Prepatent period — young worms do not lay eggs yet. For the dog roundworm this period is 2–4 weeks, and puppies infected before birth do not shed eggs until 2.5–3 weeks of age. Antigen tests can find some such infections because antigen is not tied to egg laying.
• Intermittent shedding — eggs and cysts can be shed with breaks, so testing is repeated on several days. For Giardia, see its article; in general, three samples over consecutive days or over 7–10 days are examined.
• Sample — too little stool, old stool (more than 6 hours at room temperature is unsuitable) or a poorly prepared solution can miss eggs.

A zero egg count means no eggs were seen in the counted volume; with the McMaster method each egg seen stands for 50 EPG, so lower numbers cannot be shown.

Look at together with

• Giardia antigen — used alongside zinc sulfate flotation because cysts come and go.
• Direct smear — for moving protozoa in fresh loose stool, such as Giardia or Tritrichomonas.
• Baermann test — for lungworm and Strongyloides larvae, which flotation finds poorly.

Units and conversion

Qualitative flotation and zinc sulfate flotation have no unit: the result is text — “negative”, “no parasites seen” or the name of what was found. The egg count is a number in EPG (eggs per gram); “eggs/g” on a form is the same unit, so nothing needs converting.

Reference range

The reference range is on your lab’s form; for these tests the expected result in a healthy animal is “negative” or zero eggs. A positive result shows that the parasite is present. What it means for the animal, the vet decides together with its age, signs and lifestyle.

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) — Ascarid guidelines
6. CAPC (Companion Animal Parasite Council) — Aelurostrongylus abstrusus guidelines