Actinomycosis is an infection caused by bacteria. These bacteria can survive both in the presence and in the absence of air, which makes them quite hardy.
Actinomyces are a normal inhabitant of dogs’ mouths, and are also present as contamination on natural objects such as grass awns (bristles). Dogs become infected when actinomyces get into a cut or scratch via dog bites or a migrating grass awn. The bacteria set up a localized infection that then develops into an abscess.
Dogs with a roughy-toughy outdoor lifestyle who are constantly getting nicks and scratches, or dogs who get into fights, are most likely to pick up actinomycosis. When a dog bites another, this bacterium is effectively injected into the skin. An abscess develops as the body tries to ward off infection.
Once the cause of the infection is identified, it is easily treatable with penicillin and, if necessary, surgical drainage of the abscess.
Another means of infection is when a contaminated foxtail grass awn gets caught in the dog’s fur and pierces the skin. If the grass awn gets stuck in the webbing between the toes, then a soft, fluid swelling develops that irritates the dog and makes him want to lick the paw.
If the awn is picked up on the chest and migrates inward into the chest cavity, then infection and pus develop around the lung (a condition known as pyothorax). These dogs have rapid, shallow breathing, cough and poor appetites.
Likewise, if the awn works its way into the dog’s belly, free pus can be found inside the dog’s abdomen, which causes it to become distended. Dogs in these cases, and those with pyothorax, can become ill, run a fever and go off their food.
Actinomyces are gram-positive (this refers to Gram staining) filamentous bacteria found in a normal dog’s mouth flora and fauna. They also commonly contaminate grass and grass awns.
Actinomyces can be picked up on a culture of purulent discharges. The pus associated with actinomyces has a gritty, granular appearance. If these granules are smeared onto a slide and examined under a microscope, the typical branching nature of actinomyces can be seen (although another infection, Nocardia, also has a similar appearance).
Blood screens tend only to provide a general picture of infection and inflammation.
In cases with pus in either the abdomen or chest cavity, an MRI or CT scan can help track down the migrating foreign body, which needs to be removed or it will act as a focus for further reinfection.
Around 90 percent of cases are cured with penicillin antibiotics and surgical drainage of pus when needed. The most common cause of treatment failure is an inadequate dose of antibiotic, or the treatment course not being long enough.
An abscess is a pus-filled pocket of tissue. It is difficult for antibiotics to penetrate deeply enough into an abscess to be effective, and so surgical drainage of the pus (cutting down on the work of the antibiotic) helps the cure.
If a grass awn is found within the abdomen or chest on a scan, then surgical exploration of the cavity is required to find and remove the foreign body.
It’s a great idea to thoroughly check your dog’s coat for grass awns after every outing. Timely removal can prevent deep migration of the bacteria and the risk of serious, costly complications.
- “Actinomyces canis sp. isolated from dogs.” Hoyles, Falsen, Foster, Pascual, Greko & Collins. Int J Syst Evol Microbio, 50(4): 1547–1551.
- Actinomycosis and Nocardiosis: Infections Diseases of the Dog and Cat. Publisher: WB Saunders. 2nd edition.
This pet health content was written by a veterinarian, Dr. Pippa Elliott, BVMS, MRCVS. It was last reviewed Oct. 1, 2015.