Phaeohyphomycosis is a skin infection caused by dark-colored molds. A veterinary dermatology handbook from the University of Minnesota describes the cause as a group of fungi with dark pigment that live freely in soil and in rotting plant material [1].
If your dog or cat has a slow-growing lump under the skin, or a lump that opens and drains, see your vet.
How pets get it
The handbook explains that these molds get into the body when a wound is contaminated with them [1]. In dogs, the lumps turn up most often on the nose and the lower legs and feet, the parts of the body that touch the ground and soil most [1].
The infection has been reported in dogs, cats and horses [1]. A weak immune system, including one weakened by some medicines, may raise the risk, but many affected pets are not on such medicines when they are diagnosed [1].
Skin signs in dogs
In dogs, the handbook describes one lump or several lumps in or under the skin [1]. Their edges are often hard to define, and they often have open sores on top [1]. Over time, the lumps can form draining tracts, which are small channels that leak fluid onto the skin [1].
Skin signs in cats
In cats, the lumps grow slowly and can feel firm or soft and fluid-filled [1]. Most cats have a single lump, usually on a foot, leg, ear, the nose or the body [1]. Like in dogs, the lump can break open into a sore and drain [1].
How far it spreads
In most pets the infection stays in the skin and the tissue just below it. The handbook says that spread through the body is rare in cats, and that spread into nearby bone or through the body is uncommon in dogs, though it happens more often in dogs than in cats [1]. Pets with a weakened immune system are more likely to have the infection spread [1].
A lump that can look like a skin cancer
The handbook notes that the infected lumps can look visibly dark, so they may be mistaken for melanoma [1], a skin cancer that can also be dark. Only a vet can tell these apart.
How vets diagnose it
Your vet may take a sample of cells with a fine needle, a smear of the fluid from a draining sore, or a small piece of skin (a biopsy) [1]. Under the microscope, the lab looks for dark fungal threads, and it may also grow the fungus or use a DNA test to name it [1].
Treatment
Your vet will advise on treatment for your pet.
At home, as directed by a vet:
- do not open, squeeze or drain a lump 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.
When to see your vet
See your vet if your dog or cat has:
- a slow-growing lump or mass under the skin [1];
- a lump on the nose, legs or feet of a dog [1];
- a lump on a foot, leg, ear or the nose of a cat [1];
- a lump that breaks open into a sore or drains fluid [1];
- a dark-colored lump [1];
- any lump you are unsure about.
The bottom line
Phaeohyphomycosis is an infection by dark molds that can get into a pet’s skin through a wound and cause slow-growing lumps, for example on a dog’s nose or legs or a cat’s foot, ear or nose. 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 under the skin, see your vet.
Sources
- Torres SMF. 4.10 Phaeohyphomycosis: Small Animals. Small and Large Animal Dermatology Handbook, Vol. 1. University of Minnesota Libraries Publishing, 2025. CC BY 4.0. https://open.lib.umn.edu/animaldermatology/chapter/subcutaneous-mycosis-phaeohyphomycosis-small-and-large-animals/