Definition and Epidemiology

Every year there are millions of people throughout the world who sustain bite wounds. Although many of these bites are minor, there is significant risk of injury and infection. In the United States, there is an annual incidence of 2 million to 5 million occurrences, which account for about 1% of all visits to emergency departments. Animal and human bites have a large health care cost, with dog bites alone estimated to cost $165 million dollars per year.¹

Domestic animals inflict the majority of bite wounds. Dog bites account for 80% to 90% of those bites that require medical care, yet have the lowest incidence of wound infection (2% to 13%).^2,3 Even though the majority of injuries that are caused by dog bites are relatively minor, severe injuries can occur. These can include destructive soft tissue injuries, neurovascular injuries, orthopedic injuries, and even death.⁴ Severe dog bite wounds most commonly affect the extremities, are seen more often in children and young adults, and are more frequent when the animal is provoked. Cat bites are the second most common type of mammalian bite, accounting for 3% to 15% of bite wound cases per year. However, the infection rate is much higher (30% to 50%) as a result of the deep puncture wounds from the animal’s sharp teeth.¹ The most common sites of injury are the arm, forearm, and hand.

Human bites can occur from physical altercations, often with closed-fist injuries, in the setting of alcohol ingestion. These can also occur accidentally during sexual activity, or from self-inflicted causes such as nail biting. Human bites have overall infection rates of around 10%.⁵ The most common sites of infection are the nose, lip, and ear. Bites not located on the hand have an infection rate similar to that of routine lacerations, but the clenched-fist injury, or “fight bite,” has a much higher complication rate because of the high penetrating force causing local tissue destruction and potentially osteomyelitis, tendinitis, and septic arthritis.

Physician consultation is indicated for suspected rabid animal bites; facial, hand, or extensive bites; tendon, bone, or joint involvement; or significant infectious complications.

Pathophysiology

The morbidity and mortality associated with mammalian bites are mostly related to tissue injury or, more accurately, polymicrobial infection near the bite site. The pathogens involved reflect the oral flora of the culprit, the skin flora of the victim, and the environment in which the bite took place. Infections involving aerobes alone (24% to 44%) or mixed aerobes and anaerobes (54% to 66%) are the most common.² The risk factors for infection are listed in Box 44-1.

Animal bites can contain a variety of pathogens, both bacterial and viral. The most common aerobic bacteria are Pasteurella, Streptococcus, Staphylococcus, and Corynebacterium species.^1–3,5 Bacteroides, Actinomyces, Porphyromonas, and Fusobacterium species are common anaerobic isolates and may produce β-lactamase.^1–3,5 A rare but serious bacterial infection caused by Capnocytophaga canimorsus causes overwhelming sepsis and disseminated intravascular coagulation with mortality rates from 25% to 30%. It is more common in patients with predisposing conditions, such as asplenia, liver disease, or immunosuppressive therapy.⁶

Among viral diseases, rabies is the most common and well known. The rabies virus infects the central nervous system of the bite victim, which eventually leads to disease in the brain and death. Animals such as bats, raccoons, skunks, and foxes are the most common carriers in the United States, whereas dogs and cats are the most predominant carriers in other countries. Other pathogens rarely transmitted through animal bites include those that cause tularemia, leptospirosis, cat-scratch disease, rat-bite fever, tetanus, plague, sporotrichosis, and blastomycosis.

Human bites are also polymicrobial, with similar pathogens; however, there are some important differences. Staphylococcus aureus is the organism most often isolated in human bite cases if the studies did not use anaerobic methodology. Methicillin-resistant Staphylococcus aureus (MRSA) is an increasingly recognized pathogen in bite wounds.⁷ Penicillin-resistant gram-negative rods have been reported in 24% to 43% of human bite cultures. Streptococcus pyogenes is also generally found in human bites.⁸ Pasteurella and Capnocytophaga species are not transmitted through humans, but Eikenella corrodens is present in 30% of infected human bite injuries (particularly clenched-fist injuries), is often resistant to certain antibiotics, and can lead to a serious indolent infection.² Many of the isolates (24% to 43%) produce β-lactamase. Rare organisms from humans include herpes simplex virus types 1 and 2, hepatitis B and C viruses, and Mycobacterium tuberculosis. Human immunodeficiency virus (HIV) has a biologic possibility of transmission through a bite wound, but the risk of transmission is extremely low.⁹

Clinical Presentation

The history must include the location and time of the bite; the breed and behavior of the animal; the domestication and rabies vaccine status of the animal; whether the animal was provoked; drug allergies; current immunization status for tetanus and rabies; alcohol use; current medications; and past medical history with an emphasis on immunocompetence, history of splenectomy, chronic edema, or liver disease. With human bites, the presence of infectious diseases in the attacker should be investigated. Patients may be unwilling to admit to human bite wounds, particularly in a clenched-fist injury.

Physical Examination

Physical examination should document the location, extent, and depth of the wound; type of wound (puncture, scratch, tear, or avulsion); and tenderness and other signs of infection (e.g., erythema, streaking, warmth, fluctuation, adenopathy, purulent discharge). There should be careful testing for involvement of underlying tendons, joints (range of motion), and nerves and for signs of compartment syndrome (disproportionate pain, paresthesia, pallor, paralysis).

Diagnostics

Culture of uncomplicated fresh bite wounds is rarely beneficial, but wounds with signs of infection should be cultured for aerobic and anaerobic bacteria.¹ Blood cultures and complete blood count (CBC) may be indicated if there are signs of systemic infection, but they have a low sensitivity.¹⁰ C-reactive protein level and erythrocyte sedimentation rate (ESR) can be used to monitor response to treatment.¹¹ In cases of bites that occur near joints, or those that look severely infected, radiographs should be obtained to look for fractures, the presence of foreign bodies, soft tissue injury, subcutaneous gas, and osteomyelitis.⁸ Careful examination is warranted to assess the extent of the injury, which may include tendon laceration or joint space penetration.¹²