Chapter 46. Bite Wound Infections

This chapter focuses on the clinical evaluation and management of wounds caused by bites from a variety of species, predominantly mammals. Treatment of infected bite wounds, as well as prophylaxis of selected uninfected bite wounds, will be covered.

Animal bites occur frequently in the United States, accounting for at least 1% of all visits to hospital emergency departments each year.1,2 Approximately, 80% of all bite wounds are minor in nature and do not require medical attention.3 Five to ten percent of bite wounds warrant suturing, and 2% require hospital admission. The overall morbidity of bite wounds, however, includes infectious complications, cosmetic complications, disability, psychological trauma, and medical expenses. Annual health care expenditure for the treatment of bite wounds is estimated to be between $30 million and $100 million.2–4

It is widely accepted that bite wounds are more common in children, especially those of age 5–9 years, and are more common in boys. School-aged children comprise 30–50% of all mammalian bite victims, while accounting for only 15% of the population.5 The number of reported bite wounds grossly underestimates the actual number of bites that occur each year. A survey of children aged 4–18 years revealed that 45% had been bitten by a dog during their lifetime, despite annual reported bite rates of 0.5% for children 5–14 years of age.6

The vast majority of mammalian bite wounds (80–90%) are inflicted by dogs. In 1994, an estimated 4.7 million dog bites occurred in the United States, necessitating close to 800,000 medical visits.7 Cat bites are second in frequency (5–10%) with an estimated 400,000 per year,8 followed by human bites (2–3% of mammalian bites). More than 70% of bites are caused by the victim's own pet or an animal known to them.2

Wounds caused by bites, or through accidental contact with teeth or fangs, can cause different types of tissue injury, most commonly abrasions and lacerations. Bite wounds that result in medical attention also include punctures, avulsion of soft tissue, crushing of tissue, fractures, and violation of normally sterile sites, such as joint spaces. The amount of pressure generated by dog bites is likely to produce localized crush injuries, with devitalized tissue that is prone to infection. Sixty percent of dog bite wounds are punctures, 10% are lacerations, and 30% are a combination of both. In contrast, 85% of cat bite wounds are punctures, 3% are lacerations, and 12% are a combination of both.9 Puncture wounds are harder to cleanse and irrigate, and thus have a high infection rate. Human bites are equally likely to produce lacerations, punctures, or a combination of both.10

Most mammalian bite wounds occur on the extremities, especially the hands and arms.1,7 In children younger than 5 years, a larger proportion (50–70%) of bite injuries occur to the face and head, especially those caused by dogs.1,11 This is because of the child's small stature, disproportionate head size relative to the body size, willingness to bring the face close to the animal, and a lack of motor skills to protect themselves.12 The middle third of the face, comprising the lips, nose, and cheeks, has been referred to as the “central target area.”13 Human bite wounds occur predominantly on the hands (86%). Approximately half of these are clenched-fist injuries, caused by the patient's hand forcefully striking another individual in the mouth. Fourteen percent of human bite wounds involve deep structures (e.g., tendons, muscles, and nerves), likely accounted for by the predominance of wounds to the hands.10

Sequelae of bite wounds include scarring, disability, and infection. Ten to twenty deaths per year are caused by dog bites.14 The potential for untoward sequelae directly correlates with delay in appropriate medical attention and improper wound care.15,16 Infection complicates approximately 16% of mammalian bite wounds, with individual studies reporting rates as low as 3% and as high as 67%.2,4,11,17–32 The incidence of infection is highest for cat bites (∼50%), followed by human bites (15–50%), and dog bites (3–18%).7,9,13 Infection rates for wounds exposed to seawater are higher than those for terrestrial wounds.33 Facial and head wounds, even when closed primarily, have the lowest infection rates.34 Hand wounds have the highest infection rate2,16,19,21,23,24,29,30 (Table 46–1). A retrospective review of 322 pediatric patients with human bite wounds showed an infection rate of 9.3%. Bite wounds resulting in abrasions rather than lacerations did not become infected.22

Most bite wound infections are polymicrobial. The median number of microorganisms isolated from infected mammalian bite wounds is four to five. Pasteurella spp. are the most common isolates from both dog (50%) and cat (75%) bites.9Streptococcus spp., Staphylococcus spp., Moraxella spp., and Neisseria spp. are common aerobic pathogens in dog and cat bite infections. Anaerobes are found in 30–40% of dog bite wound infections.35 Anaerobic organisms most frequently isolated from dog and cat bite infections are Bacteroides spp., Fusobacterium spp., Peptostreptococcus spp., Prevotella spp., Porphyromonas gingivalis, and Capnocytophaga canimorsus35 (Table 46–2). Infected human bite wounds are most likely to contain Streptococcus spp. and Staphylococcus spp. Eikenella corrodens, a facultative anaerobe, is the third most frequent isolate. Common anaerobic pathogens in human bite wound infections include Prevotella spp., Fusobacterium spp., Veillonella spp., and Peptostreptococcus spp.10 (Table 46–3).

Aquatic environments harbor large numbers of bacteria. Vibrio spp. are the most prevalent. Other microorganisms include Aeromonas hydrophila, Plesiomonas spp., Mycobacterium marinum, and Erysipelothrix rhusiopathiae. The most common pathogens in bite wound infections occurring in the aquatic environment, however, remain Staphylococcus aureus and Streptococcus pyogenes, representing the individual's skin flora.

All bite wounds should be considered contaminated with bacteria. In general, the microorganisms that inoculate bite wounds are the bacteria present either in the oral cavity of the biting animal, on the skin of the victim, or in the environment where the bite occurred (e.g., aquatic vs. terrestrial environment). Pathogens in or on the wound may cause local infection via direct inoculation.

Other potential complications include osteomyelitis, septic arthritis, tenosynovitis, local abscesses, and more rarely, endocarditis, meningitis, brain abscess, and sepsis. Asplenic patients are particularly susceptible to septic complications caused by C. canimorsus infection following dog bites.36 Reported cases of intracranial abscesses have followed animal bites, including bird pecking, to the head.

The risk of infection is greatest for crush injuries, puncture wounds, and wounds to the hand8 (Table 46–4). Crush injuries cause tissue devitalization, hindering wound healing. Puncture wounds prove to be problematic because they are difficult to adequately cleanse, irrigate, and debride. Puncture wounds have been shown to become infected 1.5–3 times as often as lacerations.30 The increased propensity for cats to cause puncture wounds likely accounts for the increased rate of infection of cat bites compared to other mammalian bites. The hand's closed anatomic compartments and close proximity of bones and joint cavities to the skin surface cause decreased resistance to infection. Dog bites to the hand are approximately three times more likely to become infected than those elsewhere in the body.37 The increased infection rate reported for human bites is at least partially accounted for by the increased likelihood for human bites to occur on the hands.

Wound infections associated with dog and cat bites are abscesses in 16%, purulent open wounds in 48%, and nonpurulent cellulitis, lymphangitis, or both in 36%.9 Wound infections associated with human bites are abscesses in 48%, purulent wounds in 46%, and nonpurulent cellulitis, lymphangitis, or both in 6%10 (Figure 46–1).

Clinical Presentation

Victims of bite wounds can be divided into two categories: those that present within 8–12 hours of the injury, and those that seek attention more than 12 hours after the incident. The former group usually does not have evidence of infection, but are seeking medical care secondary to concerns for rabies and other infections or disfigurement. The latter group more often presents with signs and symptoms of infection.13

It is important to distinguish a contaminated but uninfected bite wound from a bite wound infection. Uninfected bite wounds may show a normal inflammatory response of swelling and mild redness, without the frank erythema and warmth of infection. One small series showed that wound site swelling without other signs of infection was present 2–3 days after the bite in 9% of patients.28 The radius of hyperemia surrounding an uninfected wound should not exceed 2 mm.4 Infected bite wounds may present several ways. Wounds may show cellulitis (erythema, warmth, pain), swelling, induration, lymphangitis, adenopathy, frank abscess formation, or purulent discharge (Figure 46–2). Fever is present in less than 10% of patients.37

The latency period between injury and first symptoms of infection varies depending on the source of the bite. The median time to first symptoms is shorter for cat bites (12 hours) than for human bites (22 hours) or dog bites (24 hours).9,10Pasteurella is known for causing a rapidly developing infection.

Bites that occur in the aquatic environment have characteristic appearances, depending upon the infecting agent. Wounds infected by Vibrio spp. present as vesicles with surrounding edema and erythema that rapidly develop into hemorrhagic bullae. Wounds infected with Aeromonas present similarly to staphylococcal cellulitis, but can develop bullae with purulent discharge. Fever and regional lymphadenopathy are associated signs. The lesions are very painful. If inadequately treated, gas within the soft tissues, necrotizing fasciitis, or osteomyelitis can develop. E. rhusiopathiae, seen often in fish-handlers, causes erysipeloid, a self-limited illness that presents as pain or pruritis at the wound site, and a characteristic ring-shaped lesion with a sharply demarcated purplish–red border.

Differential Diagnosis

Differentiating an infected wound from an uninfected wound is an important first step. The practitioner should then evaluate the extent of injury diligently, including a thorough examination of the affected area, testing of nerve and tendon function, roentograms and laboratory studies as indicated, and consultation with a specialist when warranted. For hand wounds, if there is pain with active or passive motion, a more serious infection of deep structures should be suspected.

Rapid onset of symptoms from the time of bite (<12 hours) is associated with Pasteurella multocida infection. Pasteurella spp. are common in both nonpurulent infections and abscesses. Staphylococci and streptococci have been isolated more frequently from nonpurulent infections, although both are seen commonly in abscesses.9,10 Anaerobic organisms are more commonly associated with abscesses than other types of infections in mammalian bites. E. corrodens is also more frequently associated with abscesses. Abscesses associated with human bite wounds are likely to be associated with Streptococcus (83%), Staphylococcus (54%), Eikenella (42%), Fusobacterium (42%), and Prevotella (42%).10

Diagnosis

Physical examination should determine the status of a bite wound as infected or uninfected. Diagnostic criteria are shown in Table 46–5. A wound should be considered infected if it meets one of three major criteria or four of five minor criteria.9,10

Uninfected bite wounds should not be cultured, as Gram stain and culture have not been shown to reliably predict which wounds will become infected and which organisms will be pathologically responsible.29,38 If infected wounds are cultured, a Gram stain and cultures for aerobic and anaerobic bacteria should be sent. The recommended method for obtaining wound culture is closed-needle aspiration for abscesses and standard swabs of the base of open lacerations. Puncture wounds large enough for insertion of a swab may be cultured after cleansing the edges of the wound with a povidone–iodine solution. Practitioners should be mindful that most routine laboratories will not employ techniques adequate for isolation of all pathogenic organisms from bite wounds. For this reason, negative cultures do not exclude infection being present in a wound. Cultures instead should be used to expand coverage for organisms recovered in culture that are not adequately covered by empiric antibiotic therapy. Some authors recommend reserving cultures for wounds showing mixed organisms on Gram stain or in cases not responding to previous antibiotic treatment.29

Management

Adequate cleansing of the wound, along with prevention of tetanus and rabies, are universally accepted measures for the management of uninfected bite wounds. The use of prophylactic antibiotics and wound closure are controversial. An algorithm for the management of bite wounds is presented in Figure 46–3.

All wounds, even if apparently minor, deserve careful exploration to evaluate for underlying fractures, lacerated tendons or nerves, or penetration of joint spaces. All of these deeper injuries may likely require hospitalization or surgical repair. Hand bites in particular require very careful exploration. Any wound overlying a joint capsule or tendon, with suspected violation of either, necessitates consultation with a hand surgeon. Roentograms may be helpful in delineating the extent of injury and excluding the presence of a foreign body. (Figure 46–4) Consider ordering an X-ray when a fracture is possible, when a foreign body may be present, when the bone or joint may have been penetrated, or when infection is documented in proximity to a bone or joint. Once serious injury is ruled out, the practitioner should evaluate the wound for possible functional and cosmetic consequences, and the likelihood of local or systemic infectious complications. Documentation should include a diagram of the location and extent of injuries, the presence or absence of swelling or devitalized tissue, range of motion, status of nerve and tendon function, and presence or absence of signs of infection, including regional adenopathy.

The history should include the circumstances of injury (whether the animal was provoked or not), as well as ownership of the animal. It is important to note the patient's allergies, current medications, and immunization status. Underlying medical conditions, such as diabetes, steroid use, or other immune-modulating disorders should be noted. Consultation with local public health authorities may be necessary for rabies prophylaxis recommendations and reporting of animal bites.

Wound irrigation and careful debridement significantly reduces the incidence of infection.29 All lacerations should be irrigated using high pressure, by using a 20-mL syringe attached to an 18- or 20-gauge intravenous catheter. A minimum of 250 mL of sterile saline should be used for irrigation of the wound, but an increased volume is often necessary. Puncture wounds are more difficult to irrigate, and should be done with careful consideration to prevent subcutaneous tissue damage caused by high-pressure irrigation. Eschars should be removed in order to eliminate all contamination from the wound. Necrotic skin tags and devitalized tissue should be sharply debrided, with careful consideration of the resultant wound edges and their approximation. Puncture wounds should not be debrided. Any foreign material should be removed from the wound during irrigation and debridement.8 In the pediatric patient, the practitioner should strongly consider sedation, anxiolysis, and analgesia, in consultation with a specialist if warranted, to facilitate complete irrigation, debridement, and potential wound repair.

Bite wounds should be immobilized, especially if located in an area of significant movement, like the hand. Patients should be instructed to keep the wounded extremity elevated. Hand and wrist wounds should be treated with a splint and sling. Lower extremity wounds should be treated with bed rest. Immobilization and elevation should be continued for several days.8

Patients who have not completed the primary series of tetanus immunization (< three doses) should receive tetanus toxoid and tetanus immunoglobulin. Patients who have completed the primary series, but have not had a booster within the last 5 years should receive tetanus toxoid39 (Table 46–6). It is recommended that adolescents and adults receive tetanus toxoid as the newer Tdap vaccine. Rabies prophylaxis should be given to patients who are bitten by dogs, cats, or ferrets known or suspected to be rabid; and to patients bitten by bats, skunks, raccoons, foxes, and woodchucks caused by the high incidence of rabies in these species. Public health officials should be consulted for bites caused by livestock, rodents, lagomorphs (rabbits, hares, pikas), and escaped dogs, cats, and ferrets40 (Table 46–7).

Wound closure is especially important for improved cosmetic outcomes, especially in large gaping lacerations or bites to prominent sites, such as the face (Figure 46–5). A prospective evaluation of 145 patients with mammalian bite wounds presenting to an emergency department within 0–7 hours of injury, all treated with skin sutures, showed an infection rate of 5.5%. In comparison, reported infection rates for sutured nonbite wounds range from 3% to 7%.34 A prospective study of 759 dog bite wounds, exclusive of hand, foot, and puncture wounds, found no statistically significant difference in the infection rate between wounds that were sutured and those that were not.14 A smaller study of dog bite wounds, comparing those treated with sutures to those left to heal by secondary intention, showed an overall infection rate of 7.7%, with no significant difference between the treatment groups. None of these patients received prophylactic antibiotics, and a significant proportion of infected wounds were on the hands.21

Bite wounds that pose a risk for poor cosmesis should be sutured after thorough irrigation and debridement. Wounds seen more than 24 hours after injury should not be sutured. Because foreign bodies may serve as an additional risk for infection, deep layer sutures should be avoided. Hand wounds should be evaluated by a specialist before deciding to close with skin sutures. In the absence of an available specialist, hand wounds are best left to heal by secondary intention.

Several studies have attempted to elucidate the role of prophylactic antibiotics, but have suffered from small numbers of enrolled patients and a low incidence of infection. In a recent Cochrane Collaboration review of antibiotic prophylaxis for mammalian bites, analysis of eight randomized controlled trials showed no statistically significant reduction in the infection rate of mammalian bite wounds after administration of prophylactic antibiotics.2 These results are to be interpreted cautiously, because of the small number of patients and heterogeneity of studies. When human bites and hand bites were analyzed independently, infection rates for patients treated with prophylactic antibiotics were significantly lower than the infection rate in the control groups (0% vs. 47% in human bite wounds, and 2% vs. 28% for hand bites).2 Several researchers have found a significant reduction in infection for bite wounds to the hands treated with prophylactic antibiotics.19,24

Bite wounds can be stratified in terms of risk for infection. Patients with low risk wounds do not require prophylactic antibiotics, while those with high-risk wounds should be treated prophylactically with antibiotics. All hand and wrist wounds are high risk, including those that are less than 24 hours old and those that do not penetrate a joint capsule or tendon, and should be treated prophylactically with antibiotics. Other high-risk bite wounds are listed in Table 46–8.8 Low-risk human bites include those that occur at sites other than the hands or feet, are not over cartilaginous structures, present within 24 hours of injury, and are not puncture wounds.4 Patients who present >24 hours following injury, with no signs of wound infection, may not require prophylactic antibiotics, as most wounds that are going to become infected will have done so by this time. Prophylactic antibiotics are not recommended in the routine treatment of patients with snakebites who show no signs of tissue necrosis.

Certain patients should always receive prophylactic antibiotics, regardless of type or site of bite. Any patient with immune compromise, such as patients with hematologic malignancies or diabetes, patients on corticosteroid therapy, and patients with prior splenectomy or functional asplenia, should receive wound prophylaxis.

Patients whose bite wounds are infected at initial presentation should also receive tetanus and rabies prophylaxis, as indicated, as well as empiric antibiotics while awaiting wound cultures. Abscesses should be incised and drained using standard techniques. Most infected wounds can be treated in the outpatient setting. Inpatient treatment of bite wound infections is required in several situations. Severe cellulitis, the presence of fever and/or chills, or rapidly spreading infection should be managed in an inpatient setting. Any infection that has advanced past one joint or involves bone, joint, tendon, or nerve should be treated with parenteral antibiotics. Finally, any patient who has failed outpatient therapy or who may not reliably complete outpatient therapy should be hospitalized.

Patients with bite wounds should be seen in follow-up within 24 hours of initial treatment. Any increase in pain, swelling, or cellulitis should prompt hospitalization.

Treatment

Empiric and prophylactic antibiotic therapy should be directed against the microorganisms found most frequently in bite wound infections. The most common bacteria vary according to the biting animal, the environment of the bite, and the type of wound infection, if present. Empiric therapy for dog and cat bite wounds should be directed against Pasteurella, Streptococci, Staphylococci, and anaerobes, while empiric therapy for human bite wounds should include coverage against Streptococci, Staphylococci, E.corrodens, and anaerobes41 (Table 46–9).

Bite wounds that occur in the aquatic environment require additional empiric antibiotics to cover microorganisms present in the water, regardless of the type of biting animal. The choice of antibiotic used for aquatic bite wounds is determined by the type of water in which the bite occurred. Injuries that occur in saltwater should receive antibiotic coverage for Vibrio spp. such as ceftazidime or cefotaxime plus doxycycline or ciprofloxacin. Freshwater wounds should be treated with antibiotics that cover A. hydrophila and Pseudomonas aeruginosa; ceftazidime, imipenem, or ciprofloxacin are reasonable options.

Many microorganisms isolated from infected dog and cat bite wounds are beta-lactamase producers. Optimal empiric therapy should include either a combination of a beta-lactam antibiotic and a beta-lactamase inhibitor, a second-generation cephalosporin with anaerobic activity (e.g., cefoxitin), or combination therapy with either penicillin and a first-generation cephalosporin or clindamycin and a fluoroquinolone. Antimicrobial agents of choice for dog bite wound infections are amoxicillin/clavulanic acid and ampicillin/sulbactam. Metronidazole plus ampicillin offers another good choice.35 One study has evaluated the efficacy of prophylactic trimethoprim–sulfamethoxazole for dog bites, and demonstrated a nearly significant reduction in the incidence of hand infections.23 The role of clindamycin, increasingly used for coverage of community-acquired methicillin-resistant S. aureus, has not been examined in bite wound infections.

Recent surveillance data of Pasteurella strains isolated from infected bite wounds in humans indicate that amoxicillin, cefotaxime, azithromycin, and clarithromycin are all effective as single agents.42 Trimethoprim–sulfamethoxazole usually offers good coverage for Pasteurella. In addition, doxycycline, minocycline, and several fluoroquinolones are effective, but may not be appropriate drugs for pediatric patients.

Amoxicillin–clavulanic acid and moxifloxacin demonstrate the best activity against the most frequently isolated strains of bacteria from human bite wounds. E. corrodens has a peculiar susceptibility pattern, being susceptible to penicillin, but resistant to penicillinase-resistant penicillins, such as dicloxicillin.43E. corrodens has been found to be uniformly resistant to clindamycin.43 Erythromycin, antistaphylococcal penicillins, first-generation cephalosporins, metronidazole, and most aminoglycosides also have poor activity against E. corrodens. In vitro susceptibility testing of 151 clinical strains of E. corrodens found all isolates to be sensitive to ampicillin/sulbactam, amoxicillin/clavulanic acid, and cefoxitin. Relative resistance to doxycycline was found in 17.8%.43 For penicillin-allergic pediatric patients, a combination of antibiotics may be necessary for empiric coverage of human bite wounds, including clindamycin plus a second- or third-generation cephalosporin.10

Marine environment wounds should be treated with doxycycline and a third- or fourth-generation cephalosporin or a fluoroquinolone. Patients with bite wounds occurring in freshwater should receive a third- or fourth-generation cephalosporin.

Ciprofloxacin, like other fluoroquinolones, is associated with arthropathy and histopathologic changes in weight-bearing joints of juvenile animals. The American Academy of Pediatrics states that the use of fluoroquinolones (e.g., ciprofloxacin, gatifloxacin, levofloxacin, moxifloxacin) in children younger than 18 years of age may be justified. The most recent recommendations for the use of fluoroquinolones in children state two circumstances in which they may be useful: (1) infection is caused by multidrug-resistant pathogens, and (2) parenteral therapy is not feasible and no other effective oral agent is available. The drugs should be used only after careful assessment of the risks and benefits for the individual patient and after these benefits and risks have been explained to the parents or caregivers.44 Doxycycline and minocycline are generally not recommended for use in children younger than 8 years owing to risk for tooth enamel hypoplasia and discoloration.

Course and Prognosis

Patients with infected bite wounds should be reevaluated 24 hours after initial treatment. If the wound appears clinically worse, inpatient therapy is required. If the condition is better, further follow-up can be dictated by the return of signs or symptoms of infection. With initial adequate wound care, most bite wound infections will resolve after 5–7 days of antibiotics. High-risk wounds that are initially left open should be considered for delayed closure in 3–5 days. The results of cultures should guide treatment of refractory infections.

Pearls

1. Wounds evaluated more than 24 hours after the injury should not be sutured, when possible.

2. Rabies prophylaxis should be administered to patients bitten by dogs, cats, or ferrets with known or suspected rabies and to all patients with bites from bats, skunks, raccoons, foxes, or woodchucks.

3. While not all otherwise healthy patients with bite wounds require antibiotic prophylaxis, patients with immunocompromising conditions such as hematologic malignancies, diabetes, splenectomy, or functional asplenia should always receive antibiotic prophylaxis regardless of the type or site of the bite.