“Infections are bad.”
Now there’s a statement that seems self-evident, right? But as is always the case in veterinary medicine, exceptions to the rule exist. I know of at least one instance when a surgical site infection can be looked at as, if not exactly a good thing, at least a cloud that might very well have a silver lining.
Osteosarcoma is the most common type of bone cancer in dogs and usually affects a leg, although other locations are possible. The disease is most commonly diagnosed in middle-aged or older large and giant breeds of dogs. The first symptom that develops is usually a limp. Owners often think something relatively benign like arthritis is to blame, and leave the veterinary hospital heartbroken because their dog has just been diagnosed with a fatal disease.
Treatment for osteosarcoma is often worthwhile, however. Studies show that dogs who undergo amputation of the affected leg and no other form of treatment live, on average, another five months. When amputation isn’t possible (e.g., for pets whose other limbs are compromised by arthritis or neurological disease), limb-sparing surgery is a good, albeit expensive, alternative. Post-operative chemotherapy increases the median survival time after surgery to around one year. Radiotherapy can also play a role in treatment, either to eliminate cancerous tissue that can’t be surgically removed or simply to reduce pain.
I tell owners to make the decision for or against surgery and chemotherapy with that one year median survival number in mind. Of course, the very definition of “median” means that some dogs do worse and others do better. Is there something that the dogs who live longer than one year after diagnosis have in common? This is the question that a group of scientists recently tried to answer.
They combed through the medical records of 90 dogs with appendicular [affecting the limbs] osteosarcoma looking at a variety of parameters. Eighty-nine dogs (99%) underwent surgery, and 78 (87%) received chemotherapy. The median survival time beyond one year for these dogs was approximately 8 months (range 1 to 1,899 days). Nineteen dogs (21%) lived for more than 3 years, and 5 dogs (6%) lived for more than 3 years after diagnosis.
Of all the parameters the scientists evaluated that could potentially affect a dog’s survival time, the one that stood out was infection of the surgical site after limb-sparing surgery. The 20 dogs who had this complication had a median survival time after 1 year of 180 days (range 25 to 1,899 days) in comparison to the other dogs whose median survival time after 1 year was 28 days (range 8 to 282 days).
Two studies prior to this one had similar results, which makes one think this is a real effect, not an arbitrary finding. Veterinarians currently hypothesize that a type of “bystander effect” is at work in these cases. The immune system’s response to the infection inadvertently enhances its ability to recognize cancerous cells as a threat, thereby prolonging survival.
Postoperative infections aren’t all good news, of course. They increase the cost of treatment, cause discomfort for the patient, and can even shorten survival times if they don’t respond to antibiotics. So while no one is recommending that we intentionally contaminate the surgical site of a dog undergoing limb sparing surgery for osteosarcoma, if infection does develop, a small smile isn’t an irrational response.
Dr. Jennifer Coates
Evaluation of outcome and prognostic factors for dogs living greater than one year after diagnosis of osteosarcoma: 90 cases (1997-2008). Culp WT, Olea-Popelka F, Sefton J, Aldridge CF, Withrow SJ, Lafferty MH, Rebhun RB, Kent MS, Ehrhart N. J Am Vet Med Assoc. 2014 Nov 15;245(10):1141-6