Lagenidiosis is a deep skin infection caused by a water mold, a fungus-like organism that lives in warm water. Researchers first identified it in 2003 in dogs from the southeastern United States, where it caused invasive skin disease that looked a lot like pythiosis [1]. Most reported cases in the US come from the Gulf Coast states, although cases have turned up in other regions too [2].
If your dog has lumps or sores that spread, or sores that drain, see your vet.
How dogs get it
Water molds of this kind release tiny swimming spores into slow-moving or still fresh water. Experts think the spores get in through skin that is already broken, such as a scratch or small wound [2]. That is why dogs that swim or wade in warm ponds, lakes or swamps are the ones usually linked to the disease.
Where the lumps appear
The infection mainly affects the skin and the tissue just under it, and it tends to get worse over time [4]. Sores can show up in more than one place on the body.
What the skin looks like
Vets describe two main pictures [2]:
- firm lumps in the skin or just under it;
- open sores in skin that has become thick and swollen, with patches of dead tissue and small channels that leak fluid.
Because these sores look like other fungal and bacterial skin infections, they are easy to mistake for something more common at first [4].
Why it can be serious
Most of the dog cases in the medical literature have been fatal [3]. In the first group of six dogs described, two died when a large blood vessel ruptured, and the others were put to sleep because the disease kept spreading [3]. This is why early checks matter.
How vets diagnose it
Your vet will usually take small skin samples (biopsies) to look at under the microscope and to grow the organism in the lab [2]. Special lab tests, such as blood antibody tests and DNA (PCR) tests, help tell this water mold apart from similar organisms [2][4].
Treatment
Your vet will advise on treatment for your dog.
At home, as directed by a vet:
- do not squeeze or pick at a lump or sore yourself;
- do not give your dog 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
For infections with water molds of this group, the outlook is generally described as poor to grave [2]. Statements like this are about groups of dogs. They cannot predict how one dog will do. Your vet can explain what this means for your dog.
When to see your vet
See your vet if your dog has:
- skin problems that keep getting worse or show up in more than one place [4];
- firm lumps in or under the skin [2];
- open sores or thick, swollen skin with small channels that leak fluid [2];
- a sore that does not heal with the usual treatment;
- any lump or sore you are unsure about, especially after time in warm, still fresh water.
The bottom line
Lagenidiosis is an infection by a water mold that causes lumps and draining sores in and under a dog’s skin. It is linked to warm, still fresh water, mostly in the southern United States, and it can be very serious. 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 has spreading lumps or draining sores, see your vet.
Sources
- Vilela R, Taylor JW, Walker ED, Mendoza L. Lagenidium giganteum Pathogenicity in Mammals. Emerging Infectious Diseases (CDC), 2015;21(2):290-297. Public domain. https://pmc.ncbi.nlm.nih.gov/articles/PMC4313650/
- White AG, Smart K, Hathcock T, et al. Successful management of cutaneous paralagenidiosis in a dog. Medical Mycology Case Reports, 2020. CC BY 4.0. https://pmc.ncbi.nlm.nih.gov/articles/PMC7398966/
- Theophilopoulos J, King R, Citta A, et al. Cutaneous Lagenidium deciduum infection in a patient with relapsed acute myeloid leukemia. BMC Infectious Diseases, 2024. CC BY 4.0. https://pmc.ncbi.nlm.nih.gov/articles/PMC11112786/
- Tarannum A, Barua S, Mendoza L, et al. Highly sensitive multiplex PCR for canine Oomycota pathogens. Microbiology Spectrum, 2025. CC BY 4.0. https://pmc.ncbi.nlm.nih.gov/articles/PMC12131853/