Protothecosis is an infection caused by Prototheca, a group of tiny, colourless algae that behave a bit like fungi [3]. These algae live almost everywhere in the environment, from damp natural places to sewage systems [1][2]. In cats the infection has only rarely been reported [1].
If your cat has a firm lump in or under the skin, especially on the face, head or legs, see your vet.
How cats get it
A review of Prototheca infections in people and animals explains that these algae usually cause disease only when they get a chance, for example when they are pushed into the body through an injury [1]. In cats, the skin lumps tend to appear at the spot where something pierced the skin [1].
The algae do well in wet places, such as natural water and sewage, and seem to grow more where the climate is warm and humid [1].
Skin signs
In cats, the most common sign is one or more lumps (nodules) in the skin [1]. They are usually firm swellings in or just under the skin. They can be easy to mistake for other causes of lumps, so a vet needs to check them.
Where the lumps appear
The review describes the lumps mostly on the face and head and on the lower legs and feet [1]. Reported cat cases also include swelling and damage on the nose [1][2].
Signs beyond the skin
Sometimes the infection goes deeper. A cat described by researchers in Georgia, USA, had months of sneezing, wheezing and a blocked nose, and its nose slowly became swollen and misshapen. Tests after its death showed the algae had spread through the inside of the nose and the tissue around it [2].
How vets diagnose it
Vets confirm protothecosis by taking a tissue sample and having it examined under a microscope, and by growing the alga from the sample in a lab [1]. Under the microscope it can look like a fungal infection: in the Georgia cat, the first biopsy was read as a likely fungal infection, and the alga was only identified later with culture and genetic testing [2]. Only a vet can tell these causes apart.
Treatment
Your vet will advise on treatment for your cat.
At home, as directed by a vet:
- do not open, squeeze or drain a lump yourself;
- do not give your cat 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 cat has:
- firm lumps or swellings in or under the skin [1];
- lumps on the face, head or lower legs, or a nose that looks swollen or changed in shape [1][2];
- sneezing or a blocked nose that does not settle [2];
- any lump or skin change you are unsure about.
The bottom line
Protothecosis is a rare infection caused by an alga that can get into a cat’s skin through an injury and cause firm lumps, mostly on the face, head or legs. 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 cat has a lump in or under the skin, see your vet.
Sources
- Libisch B, Picot C, Ceballos-Garzon A, et al. Prototheca Infections and Ecology from a One Health Perspective. Microorganisms, 2022;10(5):938. Open access, CC BY 4.0. https://pmc.ncbi.nlm.nih.gov/articles/PMC9144699/
- Maboni G, Elbert JA, Stilwell JM, Sanchez S. Genomic and Pathologic Findings for Prototheca cutis Infection in Cat. Emerging Infectious Diseases (CDC), 2021;27(3):979-982. Public domain. https://pmc.ncbi.nlm.nih.gov/articles/PMC7920651/
- Masuda M, Jagielski T, Danesi P, et al. Protothecosis in Dogs and Cats: New Research Directions. Mycopathologia, 2021 (abstract, paraphrased). https://pubmed.ncbi.nlm.nih.gov/33206310/