Approximately 300,000 visits are made to emergency departments in the United States each year for management of animal bites, usually cat or dog bites. Up to 10,000 hospital admissions and 20 deaths may occur in cases of secondary infection, usually in young children. Noted bacterial pathogens include streptococcal and staphylococcal species, as well as Pasteurella multocida and Capnocytophaga canimorsus. Knowledge of the bacteriology of animal bites may have a significant impact on selection of empiric antibiotic therapy and subsequent morbidity and mortality. Talan and colleagues performed a multicenter, prospective study to identify the bacteria responsible for infections in cat and dog bites.

Eighteen university-affiliated emergency departments in the United States participated. Patients were included in the study if the cat or dog bite was large enough to perform a deep culture. Major criteria for infection included at least one of the following: the presence of fever, abscess formation or lymphangitis. Minor criteria (four of five were required for inclusion) were wound erythema greater than 3 cm (1.2 in), tenderness, swelling, purulent drainage or a white blood cell count of more than 12,000 per mm³ (12.0 × 10⁹ per L). Patients who had taken antibiotics within 72 hours of presentation or who had a fracture or radiographically confirmed osteomyelitis were excluded. Aerobic and anaerobic specimens were cultured from the wounds using swabs or needle aspiration. All specimens were sent to the hospital laboratory and a second, standard-reference laboratory. The specimens were incubated for two weeks to permit growth of fastidious organisms.

A total of 107 patients were evaluable, including 50 with infected dog bites and 57 with infected cat bites. The patients ranged from one to 82 years of age, and most bites occurred on the hands and arms. Eighty-five percent of the cat bites and 60 percent of the dog bites were puncture wounds.

The median time for evidence of infection for the cat bites was 12 hours compared with 24 hours for dog bites. The median interval for specimen collection was 25 hours after the bite occurred. The median number of bacterial isolates from the wounds was five, with a range of zero to 16. The most common pathogen was Pasteurella species, occurring in 50 percent of dog bites and 75 percent of cat bites. Other common isolates included Streptococcus, Staphylococcus, Moraxella and Corynebacterium species, as well as a large number of anaerobes. Eikenella corrodens, a pathogen found in human bite wounds, was isolated from one cat bite and one dog bite. A significantly larger number of bacterial isolates were identified at the reference laboratory compared with the hospital laboratories. A variety of antibiotics were used to treat these patients, but most regimens included a beta-lactam drug plus a beta-lactamase inhibitor.

The authors conclude that most cat and dog bites that become infected are complex, polymicrobial infections. Infected bites may also contain organisms not routinely identified by a hospital laboratory. It is therefore suggested that empiric antimicrobial therapy include a beta-lactam antibiotic and a beta-lactamase inhibitor, a second-generation cephalosporin with anaerobic activity, penicillin plus a first-generation cephalosporin, or clindamycin and a fluoroquinolone.