Ringworm (dermatophyte) PCR in cats and dogs

What it shows

The fungus grows on hairs and produces spores that spread by direct contact and, less efficiently, from the environment. PCR copies a short piece of the fungus’s DNA, so it can find even small amounts and often shows the type of fungus (for example Microsporum canis or Trichophyton). The answer usually comes in a few days, while a fungal culture can take up to two to three weeks.

The test answers one question: is the fungus’s DNA present on the coat? It does not say whether the spores are alive, and it does not separate an infected animal from one that simply carries spores picked up from another animal or from the home.

Higher than reference — what it may be a sign of

• Positive with typical skin lesions — patchy hair loss, scaling or crusts. Together with fitting signs, a positive PCR supports ringworm.
• Positive without lesions — may be an early infection, a carrier without signs, or an animal with spores on its coat from the environment (a “fomite carrier”). The vet decides whether repeat tests are needed.
• Positive after a course of antifungal care — false positives are common at this stage, because PCR also finds DNA of dead spores. A positive result then does not on its own mean the animal is still ill; culture can help to separate an active infection from leftover spores.

The vet weighs the result together with the examination and other tests and decides on the next steps.

Lower than reference

A negative result makes ringworm less likely. If the sample was small — only a few hairs, or hairs taken away from the lesion — a false negative is possible. In an animal whose lesions have healed, a negative PCR supports the conclusion that the fungus has been cleared; the vet decides how many negative tests are needed.

Look at together with

• Wood’s lamp — an ultraviolet lamp under which hairs infected with M. canis can glow apple-green. Not all strains glow, other fungi do not, and dust or some ointments can glow too, so it is a screening step.
• Fungal culture — growing the fungus from hairs or a brushing; it shows whether the spores are alive and identifies the species, but takes longer.
• Hairs under the microscope — fast, but does not always find the fungus.
• Tests for other itchy or scaly skin causes, which the vet chooses — skin parasites, a skin impression smear.

Units and conversion

The result has no unit. It is reported as “detected” / “not detected” (or “positive” / “negative”), often separately for each fungus the panel looks for. A positive result is a finding on the coat, not a measure of how ill the animal is. In the app this result is stored as text, as printed on the form.

Reference range

The reference range is on your lab’s form: “not detected” is expected in a healthy animal. A “detected” result is read together with the skin, the history of contact with other animals and, after antifungal care, together with culture.

Cats and dogs

Sources

1. Merck Veterinary Manual — Dermatophytosis in dogs and cats
2. ABCD (European Advisory Board on Cat Diseases) — Guideline for dermatophytosis (ringworm) in cats
3. IDEXX — Ringworm (dermatophyte) RealPCR panel, diagnostic update