Animal Bite-associated Infections

Microbiology and Treatment

Nicole Thomas; Itzhak Brook


Expert Rev Anti Infect Ther. 2011;9(2):215-226. 

In This Article

Dog Bites

According to the American Pet Product Manufacturers Association there are approximately 77.5 million dogs in the USA,[204] and CDC records indicate that approximately 2% (4.5 million people) of the population is bitten annually.[14] Children from the ages of 5–9 years (especially boys) are at the highest risk of being bitten, and most of these bites are from a household dog or a dog that is known to the child.[15] Half of these bites are considered unprovoked and the incidence seems to be higher in dogs who have not been neutered.[16] Dog breeds more likely to bite include the Pitt Bull, Rottweiler and German Shepherd, although any dog is capable of biting.[17] Injuries to children are usually located on the head, neck and face, because of the relationship of the dog's mouth to the child's height, whereas adults are most likely bitten on the hand or upper extremity.[18] On an international level, the lack of standard reporting in many countries makes accurate estimates of dog bite incidence difficult to determine.

Bite wound infections are usually polymicrobial, with a mix of animal oral flora, recipient skin flora and environmental organisms. The most common pathogens in dog bites are Pasteurella spp. (both Pasteurella multocida and Pasteurella canis), Staphylococcus and Streptococcus spp., and the fastidious Gram-negative rod Capnocytophaga canimorsus (previously known as the CDC and Prevention Group Dysgonic Fermenter-2). Capnocytophaga has been identified as part of the normal oral flora in anywhere from 16–41% of dogs.[19–21] Since this organism is fastidious, early notification of the microbiology laboratory by the provider evaluating the bite victim is very helpful in cases of possible exposure to the organism. C. canimorsus infection usually affects older individuals, and may be especially debilitating for asplenic individuals, those with other forms of immunocompromise or those with a history of alcoholism, although this bacteria has also been associated with severe disease in healthy individuals.[22] In addition to wound infections and gangrene, the organism has been associated with dissemination and associated sepsis, meningitis, endocarditis and ocular infection.[23–26]

Other organisms associated with dog bite infection include anaerobes, which may be present in up to 75% of dog bite infections, especially those with abscess formation. The most frequently isolated anaerobic pathogens include Porphyromonas,[27]Bacteroides and Fusobacterium spp.[28] Of note, Corynebacterium auriscanis was recently isolated from a localized dog bite infection in an immunocompetent host.[29]

Most C. canimorsus isolates are sensitive to penicillin (PCN), extended-spectrum PCNs such as amoxicillin/clavulante, and third-generation cephalosporins, as well as carbapenems. For PCN-allergic patients, alternatives include a fluoroquinolone or doxycycline.[30]

The dog is the most common transmitter of rabies to humans worldwide, with greater than 95% of reported cases being due to these animals.[31] In the USA, however, animal control and vaccination programs have effectively eliminated domestic dogs as reservoirs of rabies.[32] In the USA, recommendations for postexposure prophylaxis depend on the health of the dog. If the animal is healthy and available for 10 days of observation, then prophylaxis is only required if the dog develops any sign of rabies (along with testing the dog). If the animal is rabid or suspected of being rabid, the animal should be euthanized and tested for rabies, and postexposure initiated. Immunization may be discontinued if the immunoflourescent test result for the animal is negative.[33] The WHO recommends, in addition to local cleaning of the wound (15 min with either soap and water, or a virucidal antiseptic), postexposure guidelines based on the category of exposure to the suspect rabid animal. If there are only minor scratches or abrasions without bleeding, then immediate vaccination and treatment of the wound are recommended, whereas if any further injury occurs, if licks on broken skin, or contamination of mucous membranes with saliva from licks occurs, then administration of rabies immunoglobulin is added to the regimen.[205] Other factors that the WHO recommends considering include the likelihood of the animal being rabid, the clinical features of the animal, and its availability for monitoring and lab testing. In developing countries, they do not recommend that the vaccination status of the suspected animal be used to determine whether to initiate prophylaxis or not.


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