Supported by an unrestricted educational grant from Wyeth-Ayerst Laboratories

Volume II, Issue 5 - March 1998

Animal Bites: A Practical Approach to Management

More than 100 million cats and dogs are kept as pets in the United States. An estimated 1 to 2 million animal bites occur yearly, accounting for in excess of 1% of emergency room visits during summer months and more than 10,000 hospital admissions. Injuries related to dog bites claim at least 15 lives annually, the majority in children <10 years of age. The highest incidence of animal bite wounds is among children 5 to 14 years of age. Boys sustain dog bite wounds twice as frequently as girls but girls have a higher frequency of injuries associated with cat bites.

Epidemiology

Only a fraction (perhaps 20%) of victims of animal bites are estimated to seek or require medical attention. The majority of bite wounds cause minor injury for which self care suffices. Among severe wounds warranting medical attention, dogs account for 80% to 90% of injuries and cats 10% to 20%. A small fraction of injuries, about 1% to 2% of the total, are sustained through contact with other animals kept as pets (for example, ferrets) or nondomestic animals. Bites tend to occur more often in summer than in winter months, in the late afternoon or early evening hours, and in familiar surroundings. More than 70% of dog bites are inflicted by the victim's pet or by an animal known to the child. When circumstances are known, most bite wounds are provoked.

Microbiology

An estimated 15% to 20% of dog bites and 50% of cat bites, for which medical intervention is sought, become infected. The mouths of cats and dogs have been called microbial incubators because of the myriad of microorganisms that inhabit them and which theoretically can cause wound-related infections. At the practical level, however, only a handful of microbial species need be considered as potential pathogens when one is considering empiric treatment (Table 1).

Pasteurella multocida is the most common cause of wound infections associated with cat bites, accounting for more than 50% of cases. It also is implicated in 20% to 30% of dog bite infections. Infection caused by P. multocida is characterized by a rapid onset of intense cellulitis, often within hours or a day after the wound, and with serosanguineous or seropurulent discharge. P. multocida may be confused morphologically with Haemophilus influenzae and the laboratory should be notified as to the culture source to provide proper conditions for isolation.

Infections associated with animal bite wounds often are polymicrobial and, when conditions are optimal for isolation, several species of aerobes and anaerobes may be isolated from wound drainage. Alpha hemolytic streptococci are the pathogens most frequently isolated from infected dog bites, but Staphylococcus aureus is isolated from 20% to 30% of these wounds. Staphylococcus intermedius is a newly recognized pathogen. It may be coagulase positive, requires differentiation from S. aureus, and is penicillin-susceptible. Eikenella corrodens is a slowly growing, facultative, anaerobic gram-negative bacillus that has been implicated in a number of sinopulmonary infectious processes. E. corrodens also has been recognized as an important pathogen in both human and dog bite wound infections. Capnocytophaga canimorsus is a gram-negative bacillus that has caused serious and fatal infection, characterized by shock and disseminated intravascular coagulopathy in individuals with functional or anatomic asplenia, hepatic dysfunction, or immunosuppression. A number of anaerobic species may contribute to bite wound infection, usually as isolates in mixed aerobic-anaerobic infections. A number of bacteroides species have been implicated, as well as fusobacterium and peptostreptococcus, among others. Isolates from the mouths of cats or dogs rarely produce b-lactamases.

Management

The acronym HELICOPTER is helpful for recalling the principles of bite wound management (Table 2). Historical information should include the circumstances in which the bite was sustained; the size, species, and current location of the biting animal; whether the bite was provoked or apparently unprovoked; the time elapsed in hours since the wound was inflicted; and the immune status and current medications of the child sustaining the bite. A complete physical examination should be performed, particularly for young infants and children, because there may be multiple injury sites. The possibility that bone or joint penetration has occurred should be noted. Radiographs should be obtained for possible fracture, foreign body, or penetration of a bone or joint. All wounds should be cleansed liberally with saline and, with the exception of puncture wounds, irrigated with saline using high-pressure irrigation. This may be accomplished using an 18 gauge needle and 20 mL or 50 mL syringe. The use of antimicrobial-containing or antiinfective solutions in place of saline may aggravate tissue injury. Puncture wounds should be cleansed but not irrigated because this may compound tissue injury. Small tags of devitalized tissue may be débrided cautiously. The use of local anesthesia can facilitate these cleansing and irrigation procedures.

The issue of closure is controversial since prospective trials that have been conducted have not adequately controlled for variables that confound bite wounds. As a general guideline, fresh, noninfected wounds may be sutured after thorough wound cleansing and irrigation. Some clinicians prefer wound edge approximation with sterile adhesive strips in place of suturing for smaller wounds. Hand wounds have a high rate of infectious complications and should be managed with the assistance of a plastic or hand surgeon. Although facial wounds uncommonly become infected, the concern for cosmetic outcome may warrant consultation with a plastic surgeon. Infected wounds and wounds seen 24 hours after injury should be left open.

Surface cultures need not be performed in fresh (<8 hours) wounds that do not appear to be infected. Samples for aerobic and anaerobic bacterial cultures should be collected from wounds evaluated more than 8 hours after injury. Wounds more than 24 hours old with no evidence of infection need not be cultured.

Cleansing and closure in the operating room under general anesthesia should be considered in several circumstances. These include wounds in which there is extensive devitalized tissue, larger wounds, wounds involving the cranium in infants or toddlers, wounds involving the metacarpophalangeal joint (clenched fist injuries) and bites in uncooperative or very young children where cosmetic outcome is an important consideration.

The use of antimicrobial agents for prophylaxis of apparently uninfected wounds is another "unsettled" aspect of animal bite management. There are no prospective, controlled, and randomized trials of sufficient magnitude to determine their value. The use of a short (2- to 3-day) course of an antimicrobial agent for apparently noninfected wounds evaluated within the first 24 hours after injury is associated with nominal side effects and may reduce the number of overt infections management (Table 3)This listing should exclude minimal injuries that barely abrade the skin. Antimicrobial therapy also should be employed for wounds that appear to be infected.

A child's tetanus immunization status should be ascertained. Children with animal bites who have an unknown history or who have received less than three previous injections of tetanus toxoid should receive 250 units of intramuscular tetanus immune globulin and an additional dose of tetanus toxoid, which may be administered as diphtheria and tetanus toxoid or diphtheria and tetanus toxoids with acellular pertussis. A booster dose of tetanus toxoid also should be given when more than 5 years has elapsed since the last dose.

Wound elevation is an important component of therapy. Hand injuries should be elevated with the assistance of a sling and bedrest recommended for persons with leg wounds to minimize edema formation.

Rabies risk should be assessed. Rabies associated with domestic dog bites is exceedingly uncommon but, if concern exists in a particular locale, the local health department should be consulted. Rabies prophylaxis may be indicated for bites caused by nondomestic animals, particularly raccoons, skunks, foxes, or bats. All of the 3 rabies vaccines currently employed are associated with a low risk of adverse effects. The vaccines should be administered as recommended by the manufacturers. Rabies immune globulin is administered in conjunction with the first dose. Full instruction for the use of rabies prophylaxis in children is summarized in the 1997 Red Book of the American Academy of Pediatrics.

Antimicrobial Treatment

Amoxicillin/clavulanate potassium is an ideal antimicrobial agent for the outpatient treatment of infections due to animal bite wounds (Table 4). Ampicillin sodium/sulbactam sodium is the parenteral drug of choice for bite wound management. If not available on the hospital formulary, ticarcillin disodium/clavulanate potassium (200 mg/kg_d of the ticarcillin component) may be substituted. These provide excellent in vitro activity for essentially all species of bacteria commonly isolated from infections treatment (Table 1). Therapy may be altered, based on the susceptibility patterns of specific bacteria isolated from cultures.

Treatment options for the penicillin-allergic child are problematic. A monotherapeutic alternative for satisfactory empiric treatment is not available. Two potential options are presented in Table 4. The first, trimethoprim-sulfamethoxazole plus clindamycin phosphate may be administered by mouth or intravenously. The spectrum of trimethoprim-sulfamethoxazole includes S. aureus, E. corrodens, and P. multocida whereas that of clindamycin includes anaerobes, C. canimorsus, streptococci, and S. intermedius, as well as S. aureus. The combination is required because neither antimicrobial agent alone has a sufficient spectrum for all potential pathogens. For example, trimethoprim-sulfamethoxazole is not active against anaerobes and clindamycin is not active against P. multocida. An alternative regimen for a penicillin-allergic child able to tolerate cephalosporins is cefuroxime sodium or cefuroxime axetil plus metronidazole. In this regimen, the cefuroxime is provided for coverage of aerobic pathogens and metronidazole for anaerobes. However, occasional strains of P. multocida or E. corrodens may not be susceptible to cefuroxime.

Other monotherapeutic considerations for outpatient therapy are inadequate. For example, first-generation cephalosporins are inactive against most strains of P. multocida and E. corrodens. Similarly, many strains of P. multocida are resistant to erythromycin. Third-generation cephalosporins have an inadequate staphylococcal spectrum of activity and are a poor choice for anaerobes.

Followup

Children in whom bite wound infections are managed on an outpatient basis should be reevaluated within 24 to 48 hours, or sooner if necessary, to assess progression of signs of infection. Usually, an antimicrobial treatment course of 7 to 14 days is required to resolve documented infection.

Prevention

The likelihood of bites inflicted by family pets can be minimized by choosing more docile breeds. For example, pit-bull attacks are more likely to be unprovoked than those of other breeds of dogs. Children should be educated in behavior that will lessen the risk for provoking bites, including not approaching animals while they are feeding or caring for their young. While a bite event is occurring, an adult should seek to thwart further aggressive behavior of the animal while instructing the child involved to remain still. These types of interventions may prevent bite wounds or minimize the extent of injury when a bite wound does occur.

Morven S. Edwards, M.D.BR Department of Pediatrics
Baylor College of Medicine
Houston, Texas

Animal Bites: A Practical Approach to Management

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