Dogs

Bite wounds in dogs most often result from aggressive dog-dog interactions. The dog breeds affected depend on regional variation in dog breed popularity. Younger adult, male dogs and especially intact male dogs appear to be predisposed, but studies that compare affected dogs to a control (or “background”) population have not been reported.1–3 Bite wounds in dogs can also result from the bites of other domestic and wild animal species, but the epidemiology of infections that result from these wounds is not well described.

Approximately 20% of dog-dog bite wounds become infected, as opposed to contaminated. The likelihood that a wound becomes infected increases with the time lag between the aggressive interaction and when veterinary attention is sought.2,3 Infection develops 8 to 24 hours after the bite occurs and is less likely to be develop if the wound is limited to the dermis.³ The distribution of bacterial species involved varies from one study to another and may be influenced by whether infected or only contaminated wounds were evaluated. The most common bacterial species isolated from dog-dog bite wounds are Staphylococcus pseudintermedius, Enterococcus spp., Pasteurella spp., streptococci, and Escherichia coli.^1–3 Staphylococcus aureus and Capnocytophaga canimorsus, which are important pathogens in humans that are bitten by dogs, are rarely isolated from bitten dogs,^2,3 although the prevalence of C. canimorsus is probably underestimated because it is very difficult to grow in culture. In the author’s hospital, Pasteurella spp. accounted for 17% of 41 bacterial species cultured from 33 dog bite wounds, followed by a variety of staphylococci (some methicillin-resistant) (15%), E. coli (12%), Enterococcus spp. (7%), and Bacteroides spp. (7%). Other organisms were Peptostreptococcus spp., Clostridium spp., and Pseudomonas aeruginosa (each 5%); and Streptococcus viridans, Actinomyces spp., Acinetobacter spp., Serratia marcescens, Klebsiella pneumoniae, Myroides spp., Prevotella spp., Fusobacterium spp., Aeromonas spp., Mycobacterium smegmatis, and Mycoplasma spp. (one isolate each or 2%). Obligate anaerobes were isolated from 5 dogs (15%), and in all 5 of these dogs, multiple bacterial species were isolated.

Cats

Most bite wounds in cats result from aggressive interactions with other cats, but nonfatal dog bite wounds can also occur. Occasionally, cats are bitten by a variety of other small wildlife species that vary geographically based on the local fauna present. In contrast to dogs, which often crush and tear tissues with their teeth, cats deliver deep puncture wounds that create an environment where obligate and facultative anaerobes flourish. Thus, anaerobic bacterial infections are more prevalent in cat bite abscesses than in dog bite wounds, and accordingly, polymicrobial infections are present more often in closed cat bite abscesses than in dog bite wounds. In a study of 36 closed cat bite abscesses, 168 bacterial strains were isolated, of which 72% were obligate anaerobes and 28% were facultative anaerobes.⁴ The most prevalent anaerobes isolated from cat bite abscesses include Porphyromonas, Bacteroides, Prevotella, Peptostreptococcus, and Fusobacterium. Pasteurella multocida, a commensal of the oral cavity of virtually all cats, is the most common facultative anaerobe present.^4–6 Porphyromonas spp. appear to be particularly prevalent; in one study of 15 abscesses in Australian cats, they accounted for 92% to 99% of all the facultative and obligate anaerobes present.⁷ Less often Actinomyces, β-hemolytic streptococci, lactobacilli, and Propionibacterium spp. have been isolated.

Dogs

Among dogs, dog bite wounds consist of abrasions, lacerations, avulsions (i.e., skin flaps), crushing injuries, and deep puncture wounds (Figure 57-1). Abscesses can also develop. Dog bite wounds may also penetrate body cavities and cause pneumothorax or damage the esophagus, vertebral column, or gastrointestinal tract. Pyothorax or bacterial peritonitis can also occur. Most dogs have between 1 and 5 wounds. Rarely, more than 10 wounds are present.^1–3 The majority of bite wounds occur cranial to the diaphragm, especially on the head and neck (Figure 57-2). The location of the bite wounds also depends on the size of the bitten dog; large-breed dogs are more likely to have wounds on the neck and face, but small-breed dogs often have wounds on their dorsum. Injury to underlying tissue is frequently dramatically more severe than is apparent on the surface. Classification systems have been used to describe the severity of dog-dog bite wounds based on the type of injury (laceration vs. puncture wound) and the presence or absence of dead space or abscessation.1–3 Abscessation is rarely associated with dog bite wounds as compared with cat bite wounds.³ The presence of pus, fever, erythema, subcutaneous emphysema, and/or a foul odor suggests that infection has occurred. Uncommonly, hematogenous spread of bacteria leads to signs of severe sepsis or septic shock (see Chapter 86).

Cats

In cats, cat bite wounds may be difficult to identify on the surface, but infection can extend to penetrate bone, joints, tendon and muscle.⁵ Cat bite wounds are most often located on the forelimbs, lateral aspect of the face, and near the base of the tail (Figure 57-3).⁸ In contrast, scratches are often found on the bridge of the nose, pinna, and inguinal region. Cat bite abscesses are characterized by the presence of firm or fluctuant subcutaneous swellings or masses, with or without fever, lethargy, inappetence, and hyperesthesia. Some cats are brought to the veterinarian solely because of lethargy, and a careful physical examination is required before an abscess or cellulitis is detected. One or more scabs may be found on top of the abscess, or the overlying skin may lack hair and have a gray, necrotic appearance. If the abscess ruptures, cream-colored or red-brown purulent material may be identified on the haircoat, sometimes in association with a foul odor (Figure 57-4). Alternatively, the haircoat may be matted over the site. Lameness may be apparent if the limbs are involved, and especially if there is extension to the bone or a joint. Involvement of the thoracic cavity may be associated with fever, lethargy, tachypnea, and decreased lung sounds due to pyothorax (see Chapter 87). Bite wounds to the calvarium can result in neurologic signs such as circling, disorientation, head pressing, mental obtundation, delayed or absent placing reactions, tetraparesis, anisocoria, absent menace responses, and seizures.⁹ Penetration of the caudal vertebral column and spinal cord can lead to vertebral fractures, osteomyelitis, bacterial meningitis, and signs of pelvic limb paralysis (Figure 57-5). As in dogs, progression to severe sepsis or septic shock can occur, but most cat bite abscesses rupture and resolve spontaneously.

Cats with dog bite wounds often have life-threatening injuries. Fractures, severe hemorrhage, and hypovolemic shock can dominate the clinical picture. Penetrating injury of cervical structures, the thorax, abdominal organs, brain, or spinal cord can occur (Figure 57-6). Death most often results from trauma rather than infection, but occasionally deep-seated infections or bacteremia develops if the cat survives the initial traumatic episode.

Laboratory Abnormalities

Laboratory abnormalities in cats or dogs with wound infections are variable and depend on the degree of tissue trauma, the underlying organs involved (if any), and the severity and type of infection. The CBC may show nonregenerative or regenerative anemia, neutrophilia with bandemia, toxic neutrophils, monocytosis, and lymphopenia or lymphocytosis. Animals with severe sepsis or septic shock may be thrombocytopenic. Findings on the chemistry panel include hypoalbuminemia and evidence of muscle damage (increased activities of CK, AST, and sometimes mild increases in serum ALT activity). Increased muscle enzyme activities may be a clue to an underlying cat bite wound in cats with fever of unknown origin. Prolongations of the PT and/or APTT may be present in animals with septic shock and disseminated intravascular coagulation.

Diagnostic Imaging