- Author: Jeffrey Barrett, MD; Chief Editor: John L Brusch, MD, FACP more...
Approximately 10%-15% of human bite wounds become infected owing to multiple factors. The bacterial inoculum of human bite wounds contains as many as 100 million organisms per milliliter and is made up of as many as 190 different species. Many of these are anaerobes that flourish in the low redox environment of tartar that lies between human teeth or in areas of gingivitis. Most injuries due to human bites involve the hands. Hand wounds, regardless of the etiology, have a higher rate of infection than do those in other a locations. (See Pathophysiology and Etiology.)
Infections associated with human bites are often far advanced by the time they receive appropriate care. Patients often wait until infection is well established before seeking medical treatment. These wounds are frequently more extensive than estimated on initial examination by the inexperienced observer and are frequently managed inadequately. (See Prognosis, Presentation, Treatment, and Medication.)
Evidence suggests that it is biologically possible, but quite unlikely, to transmit human immunodeficiency virus (HIV) through human bites. (See Pathophysiology, Presentation, and Workup.)
The goals of therapy are to minimize possible soft tissue deformity and to prevent or appropriately treat infection. Recognition of the high risk of infectious complications and early aggressive treatment are mandatory to prevent serious wound infection and its associated complications.[1, 2] (See Prognosis, Treatment, and Medication.)
Human bite wounds occur as 2 separate entities: clenched-fist injuries and occlusive bites.
Clenched-fist injuries are the most common and have the greater clinical significance. They occur as the closed fist strikes the teeth of another individual with sufficient force to create a small wound, usually 3-8 mm in length. The injury usually occurs over the dorsal surface of the third and fourth metacarpophalangeal (MCP) or proximal interphalangeal joints of the dominant hand. Because of the thinness of the skin in these areas, potential injuries include joint penetration, metacarpal fracture, and extensor tendon laceration. Injury to the digital nerve or artery is rare.
As the fingers extend following injury, the bacterial inoculum may be carried proximally with the extensor tendons. This makes adequate irrigation of the wound more difficult. These are the most serious human bite wounds, and they require the most aggressive treatment.
Occlusive bites occur when there is sufficient force to break the skin. Such injuries to the hand have a higher infection rate than similar bites to other parts of the body because of the thinness of the skin in this area.
When a finger is bitten, such as in a chomping-type injury, tendons and their overlying sheaths are in close proximity to the skin. The wound may appear to be a minor abrasion-type injury, but careful inspection is required to rule out deep injury.
Occlusive human bite wounds of the head and neck result in avulsion, laceration, and crushing of the tissues. Even so, when a tooth strikes the head, even a deep puncture wound may appear innocuous. However, deep, subgaleal, bacterial contamination is possible. This is especially true in young children who have relatively thin, soft scalp and forehead tissue.
Regardless of the mechanism and anatomic location of the bite wound, the composition of the bacterial inoculum is the same. Cultures of human bite wounds are commonly polymicrobial in nature, and aerobes and anaerobes are represented almost equally. Beta-lactamase production occurs frequently. Commonly isolated aerobes include Eikenella corrodens and Staphylococcus, Streptococcus, and Corynebacterium species. Staphylococcus aureus is isolated in up to 30% of infected human bite wounds and is associated with some of the most severe infections.
E corrodens is a slow-growing, facultative, anaerobic, gram-negative bacillus. It is frequently associated with chronic infection and abscess formation. This pathogen is isolated in 30% of human bite wounds. Other commonly isolated anaerobes include Bacteroides, Fusobacteria, Prevotella, and Peptostreptococcus species.
In addition to the acute risk of localized infection, human bites pose the potential for the transmission of systemic infections, which can be life threatening. Hepatitis B transmission via human bites is well documented. In approximately 75% of patients with hepatitis B, the antigen is detectable in their saliva, and it is approximately 100 times more infectious than HIV.
Less likely is the transmission of HIV, although several cases in the literature suggest this as a mode of transmission.[3, 4] HIV is found in the saliva of affected patients, although at lower levels than in the blood. In addition, salivary inhibitors render the virus noninfective in most cases. As a result, the risk of transmission of HIV via human bites is exceedingly low.
The causes of human bite wounds include the following:
Aggressive behavior, often in combination with alcohol (the cause of most clenched-fist injuries)
Rough sexual play or sexual assault
Occupational injury to dental personnel
Seizure-related tongue lacerations
Nose biting (punishment for adultery in several cultures  )
Accidents during sporting events
Aggressive play of children in daycare centers
Self-inflicted wounds in persons who are emotionally disturbed or mentally handicapped - Lesch-Nyhan syndrome is an uncommon disorder that includes self-mutilation through biting
Institutionalized patients with poor impulse control create a high-risk environment for human bite wounds.
Occurrence in the United States
Human bites are ranked as the third leading cause of all bites seen in hospital emergency departments (after dog and cat bites), accounting for 3.6-23% of bite wounds. However, the true frequency is difficult to estimate because most human bites are probably unreported or patients fail to seek medical attention. Of those reported, approximately 60% occur in an upper extremity (most frequently the dominant one), while 15% occur in the head and neck region, most commonly the ears, nose, or lips. The remainder occur on the breasts, genitals, thighs, and other areas.
In a 4-year retrospective review in the United Kingdom, 421 (13%) human bites were identified out of 3136 case notes. The majority of those bitten were young males, with 44% of the males aged 16-25 years. The male-to-female ratio was 3:1.
Sex- and age-related demographics
Clenched-fist infections are predominantly found in men, presumably owing to their more aggressive behavior. Occlusive bite wounds occur with equal frequency in males and females.
The peak incidence of human bites, including occlusive bites and clenched-fist injuries, occurs in individuals aged 10-34 years.
The prognosis is excellent in patients who promptly seek medical attention following injury. However, patients frequently present days to weeks after injury, when the infectious process is well established. Conversely, the severity of a human bite injury may initially be underestimated, especially by an inexperienced observer, resulting in a significant delay of appropriate therapy.
Morbidity of human bites is primarily related to the degree of permanent function and/or cosmetic impairment.
Bite infections of poorly vascularized structures, such as ear cartilage, are particularly difficult to cure. In particular, in ear infections, plastic surgery is often needed to achieve an acceptable cosmetic result.
The morbidity of human bites is also related to infection and its sequelae. Prior to the era of antibiotics, up to 20% of hand bites required amputation of a finger. While amputation is seldom required today, residual scarring may result in permanent functional and/or cosmetic impairment; complications include the following:
Cosmetic deformity resulting from wound contraction
Permanent hand disability secondary to stiffness and/or chronic pain
Transmission of disease (eg, hepatitis B or C, HIV) 
Occlusive bite injuries among toddlers placed in crowded daycare centers are usually superficial and rarely become infected.
Patients must clearly understand the signs and symptoms of wound infection that signal a need to return for immediate reevaluation. These include, but are not limited to, the following:
Patients must also clearly understand the importance of early and regular follow-up care for this seemingly minor injury, as well as the rationale for providing antibiotics and the importance of compliance with this recommendation.
Moreover, patients need to be informed of potential complications that may develop even with complete compliance with the care plan, and they should understand that wound revision for cosmetic or functional purposes may be desirable at a later date.
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