Baermann test (lungworm larvae) in cats and dogs

What it shows

Some worms that live in the lungs, airways or gut are passed in the stool not as eggs but as first-stage larvae. Flotation finds such larvae poorly and can distort their shape, so a different method is used:

• How it works — stool wrapped in a double layer of gauze sits in a funnel of warm water for at least 8 hours. Live larvae crawl out of the stool, swim into the water and sink to the bottom. The fluid from the bottom is spun down and examined; larvae are identified by their shape.
• Which parasites — cat lungworm (Aelurostrongylus abstrusus), and in dogs the heart and lung worm Angiostrongylus vasorum, the fox lungworm Crenosoma vulpis and the intestinal worm Strongyloides stercoralis.
• Limits — larvae of some dog lungworms (Filaroides hirthi, Oslerus osleri) are too sluggish to swim out, so the test is not reliable for them; airway samples are used instead.

Higher than reference — what it may be a sign of

• Positive — larvae found — the form names the larvae seen, if they could be identified. Finding first-stage lungworm larvae in the stool confirms the infection.
• Cat lungworm — can occur without signs, but may also cause coughing, laboured or open-mouth breathing, wheezing and sneezing, with weight loss and lethargy. These signs can resemble feline asthma and other lung diseases.
• Angiostrongylus vasorum in dogs — associated with chronic cough, breathing difficulty, tiring easily, poor appetite and sometimes bleeding, such as bleeding into the lungs or coughing up blood. Early diagnosis matters because the infection can be serious.
• Strongyloides — an intestinal worm whose larvae are also found by this test. It can infect 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. Infected animals do not always pass larvae, and when they do, there may be only a few. Larvae appear in the stool only after the prepatent period: about 6 weeks after infection for the cat lungworm, and 28–108 days for Angiostrongylus vasorum. The test also needs live larvae, so old or badly stored stool can give a false negative. Three Baermann tests on samples collected on 3 consecutive days, or every 2–3 days, may be needed.

Look at together with

• Fecal flotation — finds worm eggs and cysts; larvae can turn up on flotation too, but less reliably and sometimes distorted.
• Direct smear — for moving protozoa in fresh loose stool, not the main method for larvae.
• Angiostrongylus antigen test — a blood test available for dogs, used alongside the Baermann; PCR on a lung wash sample can also help.
• Chest X-rays and airway examination — can help when lungworm is suspected.

Units and conversion

The result has no unit. It is recorded as text: “negative”, “no larvae seen” or “larvae found”, sometimes with the name of the parasite. 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 larvae seen”. Finding larvae confirms infection; a negative result does not rule it out. What it means for the animal, the vet decides together with its signs, X-rays and other tests.

Cats and dogs

Sources

1. Merck Veterinary Manual — Parasitology in veterinary practice (Baermann technique)
2. CAPC (Companion Animal Parasite Council) — Aelurostrongylus abstrusus guidelines
3. CAPC (Companion Animal Parasite Council) — Angiostrongylus vasorum guidelines
4. CAPC (Companion Animal Parasite Council) — Strongyloidiasis guidelines
5. Merck Veterinary Manual (Cat Owners) — Lungworm infection in cats