Histoplasma antigen test in cats and dogs

What it shows

Animals usually breathe in spores from contaminated soil. The infection starts in the lungs and chest lymph nodes and can spread through the body to the gut, liver, spleen, lymph nodes, bone marrow, skin and eyes, and can cause lameness. In the USA the fungus is most common in the Mississippi and Ohio river valleys; it also occurs in other parts of the Americas and in Africa, and cases in cats have been reported outside the usual areas.

The antigen test (a laboratory ELISA) looks for a piece of the fungus’s cell wall that passes into the blood and urine. Some labs report a number, which also helps follow the course of the disease over time. Rapid in-clinic versions exist but are less sensitive and less specific than the laboratory test.

Higher than reference — what it may be a sign of

• Positive result — in an animal with fitting signs (weight loss, fever, breathing problems, diarrhoea, enlarged lymph nodes) it supports a fungal infection such as histoplasmosis.
• Cross-reaction — the test also reacts to other fungi, especially Blastomyces, so it is a screening test for both infections rather than proof of one.
• Changing results over time — in cats that responded well the antigen level fell, and a rise may come before or with a relapse; there are still few data, so the vet reads this together with the animal’s condition.

The diagnosis is confirmed by finding the yeast under the microscope in a needle aspirate, a smear or tissue. The vet weighs everything together and decides on the next steps.

Lower than reference

A negative result makes widespread histoplasmosis less likely, but does not rule it out: false-negative results do happen. If the signs fit, the vet looks for the fungus in cells or tissue. During follow-up, negative urine antigen results can support the vet’s decision about when to stop antifungal care.

Look at together with

• Cytology — a fine-needle aspirate of lymph nodes, skin nodules or organs, or a bone marrow sample in cats, often shows the small yeasts inside cells.
• Blood smear — in widespread disease the yeasts are sometimes seen inside white blood cells (see blood smear for parasites).
• Chest X-rays — the lungs are often involved.
• Other fungal infections with similar signs — for example cryptococcosis or aspergillosis; the vet chooses which to test.

Units and conversion

In the app this result is stored as text, without a unit: as “positive” / “negative”, or as the value and comment exactly as printed on the form. If the lab gives a number, it is compared only with that lab’s own cut-off and with earlier results from the same lab.

Reference range

The reference range is on your lab’s form: a negative result is expected in a healthy animal. The lab states from which value it calls the result positive; repeat results are best compared within the same lab.

Cats and dogs

Sources

1. Merck Veterinary Manual — Histoplasmosis in animals
2. ABCD (European Advisory Board on Cat Diseases) — Guideline for histoplasmosis
3. VCA Animal Hospitals — Histoplasmosis in cats