A mycetoma is a lump caused by an infection. When a fungus is behind it, vets call it a fungal (eumycotic) mycetoma. The fungi involved normally live in soil, and they usually get under the skin through a wound that has been dirtied with soil [1]. These lumps have been reported in dogs, cats and horses, and they are rare in the US and Europe [1].
If your dog or cat has a lump on the legs, feet or face, or a lump that opens into a sore, see your vet.
What makes them different: grains
Inside the lump, the fungus clumps together with dead tissue into tiny grains, also called granules [1]. Depending on which fungus is involved, the grains can be black or white, and they differ in size and shape [1]. These grains can turn up in the fluid that leaks from the lump [1].
Vets look for three main signs: a swelling, grains in any fluid that comes out, and draining tracts, though tracts are not always there [1]. A draining tract is a small channel that carries fluid from deep in the lump out through the skin.
Where the lumps appear
Most skin mycetomas are caused by the black-grain fungi [1]. The legs, feet and face are where they show up most often, and there is usually just one lump [1]. The lump sits under the skin at the spot where the fungus got in, and it is often sore to the touch [1].
Fluid can drain from it. The fluid may be clear and watery, look like pus, or contain blood [1]. Over time the lump may seem to heal and then flare up again, which can leave the area firm, swollen and scarred [1].
How deep they go
When the infection has been there for a long time, it can reach the muscle, joint or bone underneath [1]. In some animals it can also spread further through the body [1].
The white-grain type tends to behave differently. It usually forms a long-lasting lump inside the belly or in the belly wall, months or even years after a wound or surgery site was contaminated, especially one that opened up again [1].
How vets diagnose it
Many other problems can look like this kind of lump, including other fungal infections, abscesses, a reaction to something stuck in the skin, some bacterial infections and tumors [1]. So your vet will ask about your pet’s history, look at the lump, and use one or more lab tests [1]:
- looking at cells from the lump or its fluid under the microscope;
- a biopsy, which is a small piece of tissue taken for lab tests;
- growing the fungus in the lab from grains or tissue;
- a DNA (PCR) test that names the exact fungus.
Treatment
Your vet will advise on treatment for your pet. Treatment is usually led by removing the lump where that is possible, and the choice of any medicine works best when it is based on lab tests of the fungus [1].
At home, as directed by a vet:
- do not squeeze or pick at a lump or sore yourself;
- do not give your pet human or leftover medicines unless your vet says so;
- do not stop or change any medicine without talking to your vet;
- give any treatment exactly as your vet prescribes, for as long as your vet says;
- go back for any recheck your vet books.
Outlook
These infections can be hard to clear. One reason is that medicine on its own often cannot reach the grains well enough, and the fungus lives inside them [1]. Whether a lump can be fully removed depends on its size and where it is [1]. Every pet is different, so your vet can explain what this means for yours.
When to see your vet
See your vet if your dog or cat has:
- a lump or swelling in or under the skin [1];
- a lump on the legs, feet or face [1];
- a lump that leaks fluid or has a small opening in the skin [1];
- a sore or swelling that seems to heal and then comes back [1];
- a lump you are unsure about, or one that does not go away.
The bottom line
A fungal mycetoma is a lump under the skin in which a soil fungus grows in small grains, most often on a pet’s legs, feet or face. It is rare and can be hard to clear, and only a vet can find the cause with lab tests.
This article is for general information only and does not replace advice from your veterinarian. If your dog or cat has a lump, see your vet.
Sources
- Torres SMF. 4.9 Subcutaneous Mycosis: Eumycotic Mycetoma, Small and Large Animals. Small and Large Animal Dermatology Handbook, Vol. 1. University of Minnesota, 2025. CC BY 4.0. https://open.lib.umn.edu/animaldermatology/chapter/subcutaneous-mycosis-eumycotic-mycetoma-small-and-large-animals/