Clinical and Health Affairs
Woman’s Best Friend?
By Katherine T. Franklin, M.D., and Thomas W. Day, M.D.
Abstract
Dog bites are commonly thought to be less infectious than similar injuries from other domestic or wild animals. Many people who are bitten by a dog treat themselves at home or go to a medical site to have their wounds simply “cleaned and watched.” This report describes a serious delayed consequence of a dog bite that was missed by common bacteriological testing.
An otherwise healthy 58-year-old woman from rural northern Wisconsin presented to the emergency department in December with a diffuse blotchy rash and a five-day history of increasing arthralgias. After hanging Christmas lights in trees outside, she had noticed a scratch on her finger. The finger became swollen, and a red streak extended up her hand. However, it was pain in her shoulder, hips, and knees aggravated by movement that brought her to the hospital. The morning before admission, she noticed a nonpruritic rash across her trunk and all four extremities.
The woman, who works as a nurse with Head Start, had not previously experienced such an episode. She had undergone back surgery at L4-5 more than 10 years ago; was taking atenolol, hydrochlorothiazide, and venlafaxine; and had no known allergies. She denied using alcohol, illicit drugs, or tobacco, and she had not traveled to any exotic location. The woman said she owned two dogs and a cat, all of which lived outside.
Her family medical history indicated that a cousin was diagnosed with Lupus and an uncle with Sjogren syndrome.
On admission, she was afebrile and had a BP of 126/68. She had difficulty moving from a chair to the bed because of the pain. Her right shoulder and hip were tender to palpation but did not appear swollen. The erythema and swelling from the scratch on her finger had resolved. A diffuse macular rash was noted on her face, trunk, arms, and legs. Her mucus membranes were normal, and she had no lymphadenopathy. Her initial lab results included a WBC of 13.8, potassium of 2.3, and CRP of 45.
Because the most likely diagnosis of a rash with arthralgias is a viral syndrome with exanthem, a number of possible pathogens were considered including Cytomegalovirus, Epstein-Barr virus, Coxsackie viruses, parvovirus B19, and Herpes zoster. Mycoplasma, anaplasmosis, and Lyme disease were also considered, as was autoimmune disease because of the patient’s family history. In addition, vasculitis was considered.
Blood was drawn for cultures, viral panels, and ANA testing. A Lyme spot, EKG, and troponin test were all negative. The patient’s HIV screen was normal. Hip X-rays were negative.
The patient was given supportive therapy without antibiotics. Her potassium was supplemented, and her antihypertensives held. She was discharged to home with narcotic analgesia.
After a few days’ incubation, one of the blood cultures was positive for a slow-growing gram-negative rod; it was sent to a research lab for further analysis. The woman returned to the hospital because her narcotic analgesia regimen was not adequately managing her pain. On readmission, her sed rate was greater than 100 and her C-reactive protein remained elevated.
The patient then told her doctor that three days before hanging her Christmas lights, her youngest dog, a Dachshund puppy, had choked on a piece of meat. She performed several finger sweeps of the glottis, the Heimlich maneuver, rescue breaths, and chest compressions to save the puppy. At approximately the same time the treatment team heard this, the reference lab called identifying the organism as Capnocytophaga canimorsus.
Capnocytophaga canimorsus: Infection and Treatment
Capnocytophaga canimorsus is part of the normal flora found in the oral cavities of cats and dogs. It was identified in 1976 as a slow-growing gram-negative rod.1,2 The organism requires specific growth media and typically doesn’t grow for the first four or five days (it has been identified between one and eight days of culture).1,3 Since many labs discard cultures after four days, presence of C. canimorsus may be missed.
The organism is typically transmitted to humans through a bite or by an animal licking an existing wound. Presenting signs are often not specific: fever, malaise, myalgias, vomiting, diarrhea, abdominal pain, headache, and confusion.1 Dermatologic manifestations with macular, maculopapular, or purpuric features are commonly reported.1 The mean period from exposure to presentation is about three days. Nearly half of infections occur without any predisposition, but a patient who is immunosuppressed because of alcohol abuse, splenectomy, or other causes is much more vulnerable to clinical infection.1
Infection can present with many different manifestations including septicemia, meningitis, or endocarditis. There are case reports of myocardial infarction, arthritis, localized eye infection, mycotic aortic aneurysm, dermatologic lesions, gangrene, and complications of sepsis (acute renal failure, DIC, TTP).1,2 Although the mortality rate from infection with C. canimorsus is reported to be as high as 30%,1-3 the organism is actually very sensitive to several common antibiotics including penicillin G or other beta-lactams. Amoxicillin/clavulanate (Augmentin) given orally is used to treat milder cases. More severe cases may call for IV antibiotics.
Our patient received piperacillin/tazobactam (Zosyn) intravenously, which was effective. Her organism was negative for beta-lactamase, whereas some that are positive require additional treatment with a beta-lactamase inhibitor.
Other Considerations
Only 5% to 10% of reported dog bites become infected. If the wound appears to be at low risk for infection, is superficial, is examined soon after it is inflicted, and is carefully debrided and disinfected, it is reasonable to withhold antibiotics. When infection does ensue, it is often polymicrobial.
Pasteurella and Staphylococcus species are the aerobes most commonly encountered in bite wound infections. Pasteurella is found in 75% of infected cat bites and 50% of infected dog bites. Actinomyces bacteria also may be present. These other pathogens must be treated as well. With that in mind, physicians often will prescribe a prophylactic course of antibiotics for three to seven days for most dog-bite wounds; the course may be somewhat longer if cellulitis is present. Reference manuals recommend augmentin/clavulanate for bat, raccoon, skunk, cat, dog, human, pig, and rat bites.
In some cases, it is wise to obtain blood cultures before starting treatment. The possibility of rabies must also be considered. In most cases, the treating clinician can describe signs and symptoms of infection to the patient and ask them to call or return if any occur. If a patient is considered unlikely to notice new signs or symptoms, the clinician may choose to re-examine the bites in 24 to 48 hours. MM
Katherine Franklin is a resident in the Duluth Family Medicine Residency Program. Thomas Day is the program director.
References
1. Lion C, Escande F, Burdin JC. Capnocytophaga canimorsus infections in humans: review of the literature and cases report. Eur J Epidemiol. 1996;12(5):521-33.
2. Nelson MJ, Westfal R. Case report: vertebral osteomyelitis/discitis as a complication of Capnocytophaga canimorsus bacteremia. J Emerg Med. 2008;35(3):
269-71.
3. Janda JM, Graves, M, Lindquist D, Probert W. Diagnosing Capnocytophaga canimorsus infections. Emerg Infect Dis. 2006;12(2):340-2.