Bacteriologic analysis of infected dog and cat bites. Emergency Medicine Animal Bite Infection Study Group

To define better the bacteria responsible for infections of dog and cat bites, we conducted a prospective study at 18 emergency departments. To be eligible for enrollment, patients had to meet one of three major criteria for infection of a bite wound (fever, abscess, and lymphangitis) or four of five minor criteria (wound-associated erythema, tenderness at the wound site, swelling at the site, purulent drainage, and leukocytosis). Wound specimens were cultured for aerobic and anaerobic bacteria at a research microbiology laboratory and, in some cases, at local hospital laboratories.

The infected wounds of 50 patients with dog bites and 57 patients with cat bites yielded a median of 5 bacterial isolates per culture (range, 0 to 16) at the reference laboratory. Significantly more isolates grew at the reference laboratory than at the local laboratories (median, 1; range, 0 to 5; P<0.001). Aerobes and anaerobes were isolated from 56 percent of the wounds, aerobes alone from 36 percent, and anaerobes alone from 1 percent; 7 percent of cultures had no growth. Pasteurella species were the most frequent isolates from both dog bites (50 percent) and cat bites (75 percent). Pasteurella canis was the most common isolate of dog bites, and Past. multocida subspecies multocida and septica were the most common isolates of cat bites. Other common aerobes included streptococci, staphylococci, moraxella, and neisseria. Common anaerobes included fusobacterium, bacteroides, porphyromonas, and prevotella. Isolates not previously identified as human pathogens included Reimerella anatipestifer from two cat bites and Bacteroides tectum, Prevotella heparinolytica, and several porphyromonas species from dog and cat bites. Erysipelothrix rhusiopathiae was isolated from two cat bites. Patients were most often treated with a combination of a beta-lactam antibiotic and a beta-lactamase inhibitor, which, on the basis of the microbiologic findings, was appropriate therapy.

Infected dog and cat bites have a complex microbiologic mix that usually includes pasteurella species but may also include many other organisms not routinely identified by clinical microbiology laboratories and not previously recognized as bite-wound pathogens.