Aspergillus antibody and antigen tests in dogs and cats

What it shows

Two kinds of blood test are used. Antibody tests show that the immune system has reacted to the mould. In agar gel immunodiffusion (AGID) the serum and the mould’s proteins meet in a gel and form a visible line if antibodies are present; ELISA measures the antibodies more sensitively. The antigen test (galactomannan) looks for a substance from the mould’s cell wall in serum or urine.

Because the spores are everywhere, the mould can also be found in the nose of healthy dogs. So the diagnosis rests on imaging (CT is more sensitive than X-rays), looking into the nose (rhinoscopy) with fungal plaques seen, and examining tissue under the microscope; blood tests are supporting evidence.

Higher than reference — what it may be a sign of

• Positive antibody test — together with fitting signs, such as long-lasting nasal discharge, it supports aspergillosis. A positive result can also reflect contact with the mould in the environment without infection.
• Positive antibody test plus mould found in culture — this combination is highly suggestive of infection.
• Positive antigen test — in dogs it is sensitive for the spread (disseminated) form, but it also reacts to other fungi, so it shows “a fungal infection” rather than Aspergillus specifically.

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

Lower than reference

A negative result does not rule aspergillosis out: false negatives are frequent, and in dogs blood tests for Aspergillus are generally considered unreliable for this reason. If the signs fit, the vet relies on imaging, rhinoscopy and tissue samples. In cats the antigen test finds only a small share of nasal and eye-socket cases and is generally not useful for diagnosis.

Look at together with

• Imaging and rhinoscopy — CT and a look inside the nose with tissue samples from the plaques.
• Tissue under the microscope — branched fungal threads in the lesion confirm a fungus; identifying Aspergillus needs special stains, culture or PCR.
• Culture — a single positive culture without other evidence is read with caution, because the mould is a common contaminant.
• Other causes of chronic nasal signs, which the vet chooses — for example cryptococcosis; for fungi spread through the body, also histoplasmosis.

Units and conversion

The result has no unit. AGID and many antibody tests are reported as “positive” / “negative”; if the lab prints a number next to the result, it is read only against that lab’s own cut-off. 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: a negative result is expected in a healthy animal. If the lab prints a number, it states from which value it calls the result positive; a positive result supports, but does not confirm, the diagnosis.

Cats and dogs

Sources

1. Merck Veterinary Manual — Aspergillosis in animals
2. ABCD (European Advisory Board on Cat Diseases) — Guideline for aspergillosis
3. Merck Veterinary Manual — Rhinitis and sinusitis in dogs and cats