Epidemiology

Two to 4 million cat, dog, and human bites are reported by the Centers for Disease Control and Prevention every year, as well as approximately 300,000 emergency department (ED) visits secondary to mammalian bites. The exact number of mammalian bites is difficult to determine because many cases are not reported. Although dog bites outnumber cat bites, 20% to 80% of cat bites become infected, whereas just 3% to 18% of dog bites will become infected.^1–4

Pathophysiology

Wound infections from mammalian bites have a complicated microbiology profile. The majority of infections are polymicrobial with a mix of aerobic and anaerobic bacteria. Patients with acute cat bites (<24 hours from the time of the bite) most often have wound cultures that grow Pasteurella organisms, especially Pasteurella multocida. Patients with subacute cat bites (>24 hours) or who return to the ED more than 24 hours after initial treatment of an acute bite typically have cultures that grow mixed aerobic and anaerobic species, with Pasteurella, Staphylococcus, and Streptococcus species being the predominant pathogens. Other pathogens include the aerobe Moraxella and the anaerobes Fusobacterium, Bacteroides, and Porphyromonas.^2,3,5,6

Dog bites are the most common mammalian bites seen in the ED. Dog bites tend to cause more damage, injure deeper structures, and crush tissues because of the animal’s powerful jaws, which can exert a pressure of 200 to 450 psi when biting.¹ Damage to deeper structures is more common with police dogs.² Dogs trained specifically for guard duty or for fighting, such as pit bulls, rottweilers, German shepherds, and chows, tend to cause a disproportionate number of bites resulting in serious injury and death.²

Current research reveals that Pasteurella species are the most common organisms present in culture isolates from infected dog bite wounds, most frequently Pasteurella canis.³ Other common aerobic pathogens in infected dog bite wounds include Staphylococcus and Streptococcus species.^2,3,5 Aerobes less frequently associated with cat and dog bite wound infections include Moraxella and Neisseria. Causative anaerobic organisms include Fusobacterium, Bacteroides, Porphyromonas, Prevotella, and Capnocytophaga. Capnocytophaga canimorsus causes an opportunistic infection that can result in severe infection and sepsis in immunocompromised patients. Mortality in patients with C. canimorsus infection approaches 30%.^3,6

When compared with cat bite victims, patients with dog bites tend to be seen later after injury but have a lower infection rate. Conversely, patients whose injuries become infected secondary to cat bites tend to seek treatment earlier than those with infected dog bite wounds, at a mean time of 12 hours.¹ It is speculated that the infection rate is higher because wounds from cats are more likely to be punctures, involve the hand, and become infected with Pasteurella species.^1,2

Human bites have long been associated with high rates of infection and complications such as septic joints, osteomyelitis, tenosynovitis, and fractures. However, recent studies show that human bites do not have a higher rate of infection than other bite wounds do.^1,2,5

Human bites can be divided into two categories. The first category is referred to as occlusional bites, defined as intentional bites in which the teeth actually close down on the victim’s skin. The second category is referred to as clenched-fist injuries (also called “fight bites”); these bites occur when the teeth hit or puncture the dorsum of the metacarpophalangeal region of a clenched hand. Clenched-fist injuries have a high propensity for causing injury to deep structures of the hand. Open fractures, joint involvement, and tendon injuries are common. Human bites to the hand, especially clenched-fist injuries, are at high risk for infection.^2,4,5

Wound infections caused by human bites are usually polymicrobial. Streptococci are present in 60% to 80% of isolates. Staphylococcus aureus is found in 37% to 46% of isolates, and anaerobic species are present in 44% to 60% of isolates, specifically Eikenella corrodens (20% to 25%). Herpes simplex virus can also be transmitted through infected saliva and can cause herpetic whitlow, as well as wound infections (Table 138.1).²

Table 138.1 Pathogens in Mammalian Bites