Pythiosis is one form of oomycosis, an infection caused by water molds. These organisms look a lot like fungi under the microscope, but they are not true fungi: they are closer to algae and belong to a different group of living things [1]. They live in soil and in still, fresh water such as ponds, swamps and flooded ground [1].
If your dog has a lump or sore on the skin that grows or will not heal, see your vet, and tell them if your dog has been in standing water.
Where it occurs
Most cases come from tropical and subtropical areas, which are hot, humid parts of the world [1]. In the United States, most dogs with the disease have lived in the states along the Gulf of Mexico, but cases have also been found further north and west, for example in Missouri, Tennessee, the Carolinas, Kansas, New Jersey and California [1]. Most cases show up in summer and autumn [1].
How dogs get the skin form
Dogs usually pick up the infection by standing in or drinking stagnant water [1]. The mold seems to need broken or damaged skin to get in [1]. Its spores can swim in water and are drawn to plant material and to injured tissue, where they attach and start to grow [1]. Spores are tiny seeds that the mold spreads by. Dogs that get pythiosis usually have a normal immune system [1].
Skin signs
The skin form causes sores and lumps that do not heal. Open lumps can grow, often quickly, into large masses that feel firm or soft and spongy, with channels that leak fluid onto the skin [1]. Some lumps stay closed, and the lumps are often itchy [1]. The sores tend to keep spreading into the tissue around them [1]. Because they can look like a growth, only a vet can tell this infection from a tumour.
Where the lumps appear
There can be one lump or several, and they are usually in one part of the body, most often the legs, the face or the base of the tail [1]. An early sore low on a leg can look like a lick sore from a dog that keeps licking one spot [1].
Signs beyond the skin
In dogs the infection affects the gut more often than the skin [1]. Large, young adult dogs, males and outdoor working dogs are affected most, probably because they spend more time near water, and German Shepherds and Labrador Retrievers seem to be at higher risk [1]. Signs of the gut form include diarrhea, fever, loss of appetite, tiredness and weight loss [1]. Pythiosis is a serious disease that is often fatal, so early vet care matters [1].
How vets diagnose it
Vets look at the history and the signs, then confirm the cause with lab tests on a sample [1]. These can include looking at cells from the lump under a microscope, a tissue biopsy, growing the mold from a fresh piece of tissue, a blood test for antibodies, and a DNA test [1].
Treatment
Your vet will advise on treatment for your dog.
At home, as directed by a vet:
- do not open, squeeze or drain 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.
When to see your vet
See your vet if your dog has:
- a lump that grows fast or does not heal [1];
- a wound that does not heal [1];
- lumps that break open into sores that drain fluid [1];
- a lump or sore on the legs, face or near the tail base [1];
- diarrhea, weight loss, fever or a poor appetite [1];
- any lump or sore you are unsure about, especially if your dog has been in swampy or standing water.
The bottom line
Pythiosis is an infection by a water mold that can get into a dog’s skin through a wound and cause lumps and sores that do not heal, for example on the legs or near the tail. 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 a lump or sore that will not heal, see your vet.
Sources
- Torres S. 4.12 Oomycosis: Small and Large Animals. In: Small and Large Animal Dermatology Handbook, Vol. 1. University of Minnesota, CC BY 4.0. https://open.lib.umn.edu/animaldermatology/chapter/subcutaneous-mycosis-pythiosis-small-and-large-animals/