Actinomycosis is a bacterial infection. The germs behind it, Actinomyces species, belong to a group of thread-like bacteria that can look a lot like a fungus under the microscope [1]. They live in healthy animals, in the mouth and along the airways and gut, and they only cause trouble when they get into damaged tissue [1]. The disease is uncommon in dogs [1].
If your dog has a swelling or abscess under the skin after a bite or other wound, see your vet. If your dog has trouble breathing, see your vet now.
How dogs get it
The bacteria take advantage of a break in the skin or the lining of the mouth. A wound that goes through the skin is one of the ways in [1]. That is why a deep puncture, such as a bite, deserves a vet check even when it looks small on the surface. For how bite wounds can hide damage under the skin, see bite wounds in dogs and cats.
Dogs that hunt or work in the field in warm southern climates are more likely to get it [1]. In the United States, most reports come from the West and the Midwest, although the disease is found worldwide [1].
Skin signs
In dogs and cats the typical sign is a swelling or an abscess under the skin [1]. An abscess is a pocket of pus. It tends to show up on the head, the neck, the chest, the flank or the belly [1].
Some swellings open and leak through small channels in the skin, called draining tracts [1]. The fluid can be thick and yellow-grey, or thin and bloody, and it usually smells bad [1]. Sometimes it holds tiny gritty grains, often called sulfur granules [1].
Signs beyond the skin
The infection does not always stay in the skin. It can reach the bone, and pus can build up inside the chest [1]. Pus in the chest can make breathing hard, which is an emergency.
How vets diagnose it
Your vet starts with the history and what the swelling looks like [1]. To find the cause, they may look at a sample of the fluid or a needle sample under the microscope, take a small piece of tissue (a biopsy) for special stains, or send a sample to the lab to grow the bacteria (a culture) [1]. The gritty grains turn up in only about half of cases, so their absence does not rule it out [1].
A similar infection called nocardiosis looks almost the same in a dog, so the lab work matters for telling the two apart [1].
Treatment
Your vet will advise on treatment for your dog. Treatment is often long, and the infection can come back [1], so follow-up visits matter.
At home, as directed by a vet:
- do not open, squeeze or drain a swelling 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 swelling or abscess under the skin, especially on the head, neck, chest or belly [1];
- a sore that leaks fluid through small holes in the skin [1];
- any wound or skin change you are unsure about.
See your vet now if your dog has trouble breathing.
The bottom line
Actinomycosis is an uncommon bacterial infection that can start when bacteria get into a wound, and it often shows up as an abscess or leaking swelling under a dog’s skin. 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 swelling or abscess under the skin, see your vet.
Sources
- Torres SMF. 1.7 Actinomycetic Infections: Small and Large Animals. In: 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/actinomycetic-infections-small-and-large-animals-2/