Human Bites: Background, Pathophysiology, Etiology (original) (raw)

Overview

Background

A small puncture wound from a human or cat bite can result in significant injury to tendons, joint capsules, or articular cartilage. Human bites often occur from a clenched fist injury, where a tooth injures the metacarpophalangeal joint, typically resulting from a punch to the mouth. The oral flora of humans includes various pathogens such as Eikenella corrodens, staphylococci, streptococci, and anaerobes, which can lead to infections. Patients with clenched fist injuries often delay seeking medical attention, increasing the severity of potential infections. [1, 2]

Animal bites, particularly from cats, introduce multiple pathogens, including Pasteurella multocida, which is especially prevalent in cat bites, as well as staphylococci, streptococci, and anaerobes. Serious complications from both human and animal bites can include infectious arthritis and osteomyelitis. Bites from humans and other mammals, including dogs, cats, squirrels, gerbils, rabbits, guinea pigs, and monkeys, are common and can lead to significant morbidity and disability, particularly when they affect the hands, extremities, and face. While bites from large animals can cause severe tissue trauma, most bites result in relatively minor wounds. In the United States, approximately 10 to 20 fatalities from dog bites occur each year, primarily affecting children. [1, 2]

Human bites have been shown to transmit hepatitis B, hepatitis C, herpes simplex virus (HSV), syphilis, tuberculosis, actinomycosis, and tetanus.

Evidence suggests transmission of human immunodeficiency virus (HIV) through human bites is possible but very unlikely. [1, 2]

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. [3, 4]

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Pathophysiology

Human bite wounds occur as 2 separate entities: clenched-fist injuries and occlusive bites. [2]

Clenched-fist injury

Clenched-fist injuries are the most common and have the greater clinical significance. [2] 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

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. [2]

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.

Disease transmission

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 frequently is 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 for 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 of likely transmission through bites have been documented. [5, 6, 7] 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. [8] Neverthless, in severe wounds it may be reasonable to provide post-exposure prophylaxis to patients if the source of the bite is HIV positive. [9]

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Etiology

The causes of human bite wounds include the following [7] :

Institutionalized patients with poor impulse control create a high-risk environment for human bite wounds.

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Epidemiology

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), whereas 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.

International occurrence

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. [11]

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.

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Prognosis

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. [2]

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:

Occlusive bite injuries among toddlers placed in crowded daycare centers are usually superficial and rarely become infected.

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Patient Education

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 [1, 2] :

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.

For patient education information, see the First Aid and Injuries Center and the Infections Center, as well as Human Bites and Tetanus.

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Author

Coauthor(s)

Don R Revis, Jr, MD Consulting Staff, Department of Surgery, Division of Plastic and Reconstructive Surgery, University of Florida College of Medicine

Don R Revis, Jr, MD is a member of the following medical societies: American College of Surgeons, American Society for Aesthetic Plastic Surgery, American Society of Plastic Surgeons, American Medical Association

Disclosure: Nothing to disclose.

Chief Editor