Chapter 132: Human and Animal Bites

• Wound infection is the greatest potential complication of bites.

• Pasteurella species are common in dog and cat bite wound infections.

• Eikenella corrodens is a common pathogen in human bite wounds.

• Rabies and tetanus prophylaxis should be considered in all animal bite wounds.

• Cat scratch disease due to Bartonella henselae may develop 7–12 days after a cat bite or scratch.

• Bites wounds that are treated on an outpatient basis should be reevaluated in 48 hours.

• Due to the risk of infection, tissue adhesives should not be used for wound closure.

Mammalian bites are a common injury in the United States. According to the Centers for Disease Control and Prevention, there are 4.7 million people bitten each year with 800,000 seeking medical care, and approximately half are children.¹ It is estimated that direct healthcare cost are as high as $165 million a year.² The majority of bites are caused by dogs (80–90%), followed by cats (5–10%), humans (2–3%), and the remainder by other domesticated or wild animals.³ There are 10–20 fatal cases annually that mainly involve young children and infants.³

Domesticated house dogs may exert a force of 200–400 pounds per square inch and police and fighting dogs up to 2000 pounds per square inch,⁴ enough force to penetrate sheet metal.⁵ Most bites are superficial crush injuries that cause ecchymoses and hematomas, without skin penetration,⁵ but may also cause lacerations, puncture wounds, severe crush injuries, tissue avulsions, fractures, dislocations, and neurovascular injuries.⁴ The upper and lower extremities are the most common site of injury for older children.³ Children less than 5 years old are commonly bitten on the head and neck due to their face-to-face contact with the offending dog.^3,7 Most of the head and neck injuries involve the nose, lips, and cheeks.^3,5 On rare occasions, especially in children less than 2 years of age, the bite may penetrate the cranium and lead to depressed skull fractures, intracranial lesions, and meningitis.^3,5 Infection rates are generally low for dog bites. However, the site of injury plays a major role. Dog bites to the face have a lower risk of infection (1–5%), in contrast bites to the hands have a high infection rate (12–30%).⁵

Cat bites are common in the upper extremities, mainly presenting with a puncture wound rather than a laceration or contusion.³ The feline characteristic long sharp teeth introduces bacteria into deeper tissues and may involve tendons, bones, or joints.⁵ This causes a wound that may appear small and unremarkable but have an infection rate as high as 80%.⁶ Cat scratch disease due to Bartonella henselae may develop 7–12 days after a cat bite or scratch. The syndrome consists of regional lymphadenitis, constitutional symptoms, and infrequently organ involvement such as encephalopathy and atypical pneumonia. The disease is usually self-limiting and resolves in 2–5 months without antibiotics, but precautions should be taken for those who are immunocompromised, due to the risk of bacteremia and severe infection. A 7–10-day course of trimethoprim–sulfamethoxazole or ciprofloxacin may be used in this high-risk population.⁹

Human bites in children are generally caused by other children.⁷ Common sites include the face, trunk, and upper extremities and present as superficial abrasions.⁷ There should be a high index of suspicion for child abuse for bites where the intercanine distance is greater than 3 cm, this indicates the biter had permanent teeth.⁷ Human bites in older children and adolescents commonly occur during an altercation when an individual strikes another in the mouth with a clenched fist, frequently referred to as a fight bite. This serious injury often occurs over the metacarpal–phalangeal joint and presents with high infection rates, which may range from cellulitis to osteomyelitis.³ HIV and Hepatitis B transmission through human bites, and post-exposure prophylaxis are not routinely indicated for all human bites.^7,8 However, if the assailant is positive for either virus and the bite is deep with possible blood transfer, post-exposure prophylaxis should be started.⁸

Most animal bites are polymicrobial, but common bacterial isolates include Staphylococcus aureus, Pasteurella multocida, streptococci, coagulase-negative staphylococci, and enteric bacteria.^3,5 Human bites also tend to be polymicrobial with Streptococcus anginosus, Staphylococcus aureus, and Eikenella corrodens being the most common pathogens.⁸ Signs and symptoms include erythema, tenderness, swelling, and purulent drainage around the site of the bite⁷ but may also present with systemic symptoms such as fever, chills, swollen lymph nodes, and racing heart. Signs of infection may present within 3 days, but as early as 24 hours in P. multocida infections.⁷ A rare systemic infection caused by Capnocytophaga canimorsus is seen 2–3 days after a dog bite and mainly occurs in the immunocompromised. The infection usually manifest as hypotension, disseminated intravascular coagulation, and renal failure. Mortality is as high as 30% with a majority being those who are immunocompromised. A 5-day course of amoxicillin/clavulanate will suffice for prophylaxis in those who sustained a dog bite and are immunocompromised.⁵

Careful wound exploration and cleaning is critical to prevent infection. High-pressure irrigation with large amounts of saline is essential in decreasing the bacterial load. This may be accomplished with the use of an 18 gauge catheter attached to a large syringe irrigating roughly 50–100 mL of normal saline per centimeter of wound. Solution such as hydrogen peroxide, ethyl alcohol, or povodone–iodine may cause tissue damage and should be avoided.⁶ The wound should be explored in a bloodless field to identify involvement of deep structures, removal of foreign bodies, and debridement of devitalized tissues. Plain film radiographs may be useful to identify retained foreign bodies, such as teeth. Surgical irrigation and drainage in the operating room may be necessary for wounds that are deep and extensive.⁹

Primary closure of some bite laceration may be safely accomplished for cosmetic reasons. For example, lacerations to the face have a general low chance for infection but have a high cosmetic value. Indication for primary wound closure includes simple face or scalp laceration and injury within 6 hours. Sutures should be somewhat loosely approximated to allow for drainage in the event that a wound infection ensues. Wounds that present with signs of infections, puncture wounds, wounds to the hands or feet, wounds greater than 6 hours old, and patients who are immunocompromised should have the wounds heal by secondary intention with reevaluation in 24–48 hours^6,9,10 Due to the risk of infection after bite injuries, tissue adhesives should not be used to close these wounds.

The use of prophylactic antibiotics is controversial.³ Common practice is the use of antibiotics for high-risk wounds, those that are being sutured, and wounds presenting with infection. The type of wound, location, and underlying risk factors all factor in to determine if prophylactic antibiotics should be used.⁷ (Table 132-1). The most effective antibiotic choice for dog, cat, and human bites is a 7-day therapy of amoxicillin/clavulanate (20 mg/kg PO BID; based on the amoxicillin component) for children with no penicillin allergies or clindamycin (10 mg/kg PO q6h) plus trimethroprim/sulfamethoxazole (5 mg/kg PO BID; based on the trimethroprim component) for children with penicillin allergy.⁶ Children presenting with infected bite wounds should receive aggressive wound care, wound cultures, and antibiotic therapy. Treatment antibiotic choices and duration are similar to the prophylactic antibiotic therapy (Table 132-2). A child should be admitted to the hospital and started on parenteral antibiotics if they have systemic symptoms, extensive wounds requiring surgery, or involvement of bone, joint, or tendon.¹⁰

Tetanus immunization status should be assessed for every wound, and standard use of immunization and immunoglobulin should be used. If the wound is minor and the child has received the three previous doses of the tetanus toxoid, a booster is indicated if the last dose was given greater than 10 years ago. If the wound is dirty, and the patient received three previous doses of the tetanus toxoid, a booster is indicated if the last dose was given greater than 5 years ago. If the patients immunization status is unknown or less than three doses, both the tetanus booster and the immunoglobulin are used.

Rabies prophylaxis is not routinely indicated for bites from healthy cats and dogs who can be observed for 10–14 days. However, certain exposures to certain terrestrial animals have been known to transmit rabies through bites, these include raccoons, bats, skunks, and foxes. Post-exposure treatment includes administering the human rabies immune globulin (20 IU/kg), if feasible the full dose is infiltrated around the wound and the remaining is given in the deltoid or lateral thigh opposite of the vaccine site. A course of the human diploid vaccine is administered IM on day 0, 3, 7, and 14.¹¹ All possible exposures should be reported to the health department.