A study on the bacteria of dog bite wounds in dogs and their susceptibility to antimicrobials

Abstract

To investigate the bacterial composition of infected and non-infected dog bite wounds (DBW), a prospective study was performed on dogs with various grades of bite wounds presenting at the Onderstepoort Veterinary Academic Hospital, University of Pretoria, and a nearby animal shelter. Fifty dogs with bite wounds inflicted within the previous 72 hours were selected. This represented 104 wounds. Wounds were clinically graded according to severity. Swabs were collected from all wounds for bacterial culture and cytology. Infection was diagnosed if 2 of the following 3 criteria were met: macroscopic purulence, microscopic presence of phagocytosed bacteria, or pyrexia. Non-infected wounds were either classed as sterile (established by culture) or contaminated (culture positive but bacteria not phagocytosed on cytology). To determine the origin of the bacteria, swabs were collected from the skin near the wounds and gingiva of 15 bite victims. All swabs were cultured aerobically and anaerobically and all aerobic cultures were evaluated for antimicrobial susceptibility using the Kirby Bauer disk diffusion test. The victims were predominately male, uncastrated, small-breed dogs. Of the 104 wounds studied, 21 were judged to be infected and 83 non-infected. Infected wounds were significantly more likely to culture positive (Fisher's exact test: p = 0.02). Sixteen per cent of wounds did not culture bacteria, 67% grew aerobes only, 1% anaerobes only and 67% a mixture of aerobes and anaerobes. A total of 213 isolates were cultured representing a mean of 2 isolates per wound. Of the aerobe species cultured, 22%, 19% and 17% belonged to the genera of Pasteurella, Streptococcus and Staphylococcus respectively. The species of Pasteurella multocida (66%) and Staphylococcus intermedius (70%) were predominant. Pasteurella canis and pyogenic streptococci were common in infected wounds, whereas Bacillus spp., Actinomyces spp. and oral streptococci were usually found in contaminated wounds. Three anaerobic genera were cultured, namely, Prevotella, Clostridium and Peptostreptococcus, and were usually associated with wounds with dead space. This study also describes the first documented case of Capnocytophaga canimorsus in an infected dog bite wound. Notably clinical and cytological assessment was capable of establishing whether antimicrobials were required or not. Although no single antimicrobials was considered to be effective against all the bacteria, amoxycillin plus clavulanic acid, 1st and 3rd generation cephalosporins, ampicillin or amoxycillin and potentiated sulphonamides gave the best in vitro sensitivity results.