Mammalian Bites

CHAPTER 144

Mammalian Bites

Presentation

Histories of animal bites are usually volunteered, but the history of a human bite, such as one obtained over the knuckle during a fight, is more likely to be denied or explained only after direct questioning. Mammalian animal bites generally consist of either domesticated animal bites, most commonly dog or cat bites, or wild animal bites, such as those from rodents, lagomorphs (rabbits and hares), skunks, raccoons, and bats. Human bites are either purposeful, occlusional, crushing injuries, or inadvertent clenched-fist injuries, as previously mentioned, which are also known as “fight bites.”

A single bite may contain various types of injury, including abrasions, puncture wounds, avulsions, lacerations, and crush injuries, as well as underlying fractures, foreign bodies, and tendon and nerve injuries, not all of which are immediately apparent.

Patients either will present with a fresh wound soon after the injury or will delay and only seek help after developing painful signs of infection.

What To Do:

All bite wounds require a detailed history that includes the exact mechanism of injury and the approximate time that it took place. General health questions should include discussions regarding underlying illness (diabetes, transplanted organs, asplenia, immunosuppression) or medications (steroids, chemotherapy) that would increase the patient’s risk for infection. The status of previous tetanus immunization should also be determined. Determine the type of animal that bit, whether the attack was provoked (a rabid animal is more likely to make an unprovoked attack), what time the injury occurred, and, when available, the current health status and vaccination record of the animal. Also find out if the animal has been captured and is being held for observation.

Examination should determine the extent and nature of all skin and soft tissue injuries, with special attention given to any possible tendon, nerve, joint, or vascular injury. Fight bite injuries (which are typically over the dorsal metacarpophalangeal joints) should be examined meticulously through a full range of motion with the hand in full flexion and extension to ensure that tendon injuries are not missed. A tendon injury sustained with the fingers flexed will be missed if the hand is only examined in extension. Bony tenderness, pain on range of motion of a joint, swelling, and/or a forceful mechanism (e.g., a large biting animal) may indicate a need for radiographs or special imaging studies to rule out underlying fractures. (Dog bites have caused open depressed skull and facial fractures in small children.) Radiographs of hands injured by human teeth are also recommended.

Report the bite to the police or appropriate local authorities.

Simple abrasions and contusions that do not break through the dermis require only cleansing with 1% povidone-iodine (Betadine) solution or even just soap and water, as well as tetanus prophylaxis when required. (Potential rabies exposure also requires prophylaxis; see later in this chapter.)

Small puncture wounds cannot be successfully irrigated, but large puncture wounds and lacerations should be anesthetized with lidocaine (Xylocaine) 1% and thoroughly cleansed and irrigated with a dilute 1% povidone-iodine solution (10% povidone-iodine solution, diluted 1:10 in normal saline). Puncture wounds can be enlarged with a stab wound from a No. 15 scalpel blade to permit fluid to escape and then slowly irrigated at lower than optimal pressure (to prevent inadvertent soft tissue infiltration with the irrigation solution). This can be accomplished using a large gauge plastic catheter (Angiocath) with a 10-mL syringe. Open lacerations can be cleansed with a high-jet lavage using an irrigation shield (Zerowet or Splashield) with a 10-mL syringe. Devitalized tissue should be sharply débrided and the wound fully explored in a bloodless and well-lit field, looking for foreign bodies or tendon or joint involvement.

Most uninfected facial lacerations should be closed using sutures or tape closures to provide the most effective cosmetic repair. A plastic surgeon or otorhinolaryngologist should be consulted about significant injuries to the special structures of the face and about wounds involving significant tissue loss.

Nonhuman animal bite wounds of the scalp, neck, trunk, and proximal extremities that are clean, uninfected, open lacerations may also be closed using tape closures, staples, or nonabsorbable suture material. Buried sutures should be avoided, because they increase the risk for infection.

Wounds of the hand, foot, or wrist; puncture wounds; wounds with much devitalized tissue or those more than 6 hours old; or wounds that appear to be infected should be left open. Other wounds that should not be closed, except when on the face, include human, cat, monkey, pig, and wild carnivore bites, which result in infection-prone injuries.

Patients who have a high risk for infection, such as those with diabetes, immunosuppressed conditions, and renal failure, should have their wounds left open. These wounds can be loosely packed with saline-soaked fine-mesh gauze for delayed primary closure after approximately 72 hours.

Prophylactic antibiotics are indicated for bites of the hand, wrist, or foot or for a bite over a joint. They are also advised for punctures that are difficult or impossible to irrigate adequately or where there is significant tissue crushing that cannot be débrided. Antibiotics should also be prescribed for patients who are older than 50 years or for those who are asplenic, alcoholic, or diabetic; with altered immune status or peripheral vascular insufficiency; or who have a prosthetic or diseased cardiac valve or a prosthetic or seriously diseased joint.

Human, cat, pig, wild carnivore, and monkey bites that are other than abrasions and superficial split-thickness lacerations also require prophylactic antibiotics. Cats’ teeth are slender, very sharp, and can easily penetrate the soft tissues and joints. Domestic pigs can also inflict deep injuries.

Face, scalp, ear, and mouth injuries do not require prophylactic antibiotics. Rodent and lagomorph bites also do not need antibiotic treatment.

When a prophylactic antibiotic is indicated, prescribe amoxicillin/clavulanic acid (Augmentin), 875/125 mg × 5 days for adults. For children, 45 mg amoxicillin/kg/day, divided bid (80 to 90 mg/kg/day if drug-resistant S. pneumoniae is suspected). Oral solutions 200, 400, 600 mg amoxicillin/5 mL. Initiate the first dose as soon as possible.

If penicillin allergic, prescribe the following:

Dog Bite

Clindamycin (Cleocin), 300 mg qid, + levofloxacin (Levaquin), 500 mg qd × 5 days, for adults, or

Clindamycin, 10 mg/kg tid (oral solution: 75 mg/5 mL) + trimethoprim-sulfamethoxazole (TMP/SMX) (Bactrim, Septra), 8 to 12 mg TMP/kg/day divided bid (oral solution: 40 mg TMP/5 mL) for children

Cat Bite

Cefuroxime axetil (Ceftin), 500 mg bid × 5 days (for children, 15 to 30 mg/kg/day divided bid; oral solution 125 or 250 mg/5 mL) or

Doxycycline (Vibramycin), 100 mg bid × 5 days

Raccoon or Skunk Bite

Doxycycline (Vibramycin), 100 mg bid × 5 days

Human Bite

Clindamycin (Cleocin), 300 mg qid, + ciprofloxacin (Cipro), 500 mg bid (or TMP/SMX [Bactrim, Septra] DS bid) × 5 days

With early signs of infection, the same antibiotic coverage can be used, but it should be continued for a full 10 to 14 days. Before antibiotics are started, obtain aerobic and anaerobic cultures from deep within the wound and then irrigate and débride as described.

Hand infections, joint infections, and moderate to severe soft tissue infections require specialty consultation and consideration for hospitalization, IV antibiotics, and possible surgical intervention. Other indications for admission or specialty consultation after a bite injury include injury or probable injury to deep structures (bones, joints, tendons, arteries, or nerves). A cat bite over a joint may require IV antibiotics. Always consult the appropriate specialist in a timely manner when confronted with any situation that you are uncomfortable treating yourself.

All bite wounds require appropriate tetanus prophylaxis (see Appendix H).

Rabies postexposure prophylaxis (PEP) is required for all bite wounds from animals suspected of being rabid as well as for situations involving contact of mucous membranes with rabid saliva or a scratch from a potentially rabid animal. With potential human exposures involving bats, prophylaxis might be appropriate even if a bite, scratch, or mucous membrane exposure is not apparent when there is reasonable probability that such exposure might have occurred. This would include persons who were in the same room as the bat and who might be unaware that a bite or direct contact had occurred (e.g., a sleeping person awakens to find a bat in the room, or an adult witnesses a bat in a room with a previously unattended child, mentally disabled person, or intoxicated person) and rabies cannot be ruled out by testing the bat (see Appendix F).

Because of local variations in animal vectors and endemics, consultation with a state or local health department is prudent before a decision is made to initiate antirabies postexposure prophylaxis (PEP).

In the United States, urban dogs and cats, domestic ferrets, small rodents (e.g., squirrels, hamsters, guinea pigs, gerbils, chipmunks, rats, and mice) and lagomorphs (including rabbits and hares) are at low risk for being rabies carriers. The animal’s behavior is sometimes helpful and is easily evaluated in wild animals, because most tend to shun humans. The urban appearance of a skunk, fox, or bat in broad daylight showing no fear of humans is abnormal and should greatly raise one’s index of suspicion. An unprovoked bite from such an animal would be considered a bite with a high risk for rabies and requires PEP.

If a biting animal with normal behavior has been captured, it should be quarantined with a veterinarian or reliable owner for 10 days. If the animal displays no signs of illness during this time, PEP is not indicated.

If a person is bitten by an animal that cannot be observed or tested for rabies (e.g., one that has escaped), the decision to initiate PEP is based on the local epidemiology of rabies.

When the decision has been made to provide postexposure prophylaxis (note that the 2010 CDC guidelines have reduced the total number of human diploid cell [rabies] vaccine [HDCV] vaccine to four), give:

20 U/kg of human rabies immune globulin (Hyperab, Imogam, RIG) injected into or around the bite. If unable to give the full dose in this manner, inject the remainder IM in the gluteal muscle.

One mL of vaccine, human diploid cell culture (HDCV, Imovax), or 1 mL of either of the two other available rabies vaccines given IM in the deltoid area in adults, or into the lateral thigh in young children. Repeat doses of the vaccine are given on days 3, 7 and 14.

For patients previously immunized (usually veterinarians or animal handlers), rabies immune globulin is not given, and only 1 mL of the vaccine is given IM with a single repeat dose on day 3.

For human bites, while the presence of HIV inhibitors in saliva renders the virus noninfective in most cases (the risk of infection is one twentieth the risk for transmission through a needle stick), there are case reports of human immunodeficiency virus (HIV) transmission by human bites. Centers for Disease Control and Prevention (CDC) guidelines recommend postexposure prophylaxis for both the bite victim and the bite source if either party is known to be HIV positive or at high risk and if any blood exposure has occurred. When prophylaxis is indicated, offer zidovudine (Retrovir), 200 mg tid, + lamivudine (3TC), 150 mg bid, +/− indinavir (Crixivan), 800 mg qd, all PO, × 4 weeks. These patients require follow-up testing and counseling. The chance for hepatitis B transmission from a bite appears to be greater than that for HIV. Unlike HIV prophylaxis, it is reasonable to delay treatment of potential hepatitis B exposure for 48 to 72 hours pending serologic results from bite victim and source. Provide hepatitis prophylaxis for patients who have been bitten by known carriers of hepatitis B. Administer hepatitis B immune globulin, 0.06 mL/kg IM, at the time of injury, and schedule a second dose in 30 days.

For hand injuries or crushing injuries and contusions, apply an immobilizing splint with a mild compressive dressing and have the patient keep the extremity elevated above the level of the heart.

After 24 hours, the patient should begin cleansing the wound once daily with gentle soap and water, followed by reapplication of a new dressing. Hand injuries should remain immobilized for 2 to 3 days until edema and pain have mostly resolved.

Have the patient return for a wound check in 2 days, or 1 day for cat bites and bites of the hand and any time there is any sign of infection. Explain the potential for a serious complication, such as septic arthritis, osteomyelitis, and tenosynovitis, which will require specialty consultation. Preparing patients for the worst while initiating aggressive treatment is the best defense against any potential future litigation. Always provide patients with clear, specific, written discharge instructions.

What Not To Do:

Do not overlook a puncture wound.

Do not infiltrate irrigant solution into tissue planes in puncture wounds.

Do not suture debris, nonviable tissue, or a bacterial inoculum into a wound.

Do not use buried absorbable sutures, which act as a foreign body and a nidus for infection.

Do not attempt to treat bite wounds using monotherapy with penicillin, clarithromycin, amoxicillin, or a first-generation cephalosporin. These antibiotics will not provide the coverage necessary for the mixed aerobic and anaerobic floras that are commonly cultured from these wounds.

Do not waste time and money obtaining cultures and Gram stains of fresh wounds. The results of these tests do not correlate well with the organisms that subsequently cause infection.

Do not provide rabies prophylaxis for incidental contact, such as petting a rabid animal or contact with blood, urine, or feces (e.g., guano) of a rabid animal. These experiences do not constitute an exposure, according to the Centers for Disease Control and Prevention.

Discussion

Animal bites are often brought promptly to the attention of medical personnel, if only because of a legal requirement to report the bite or because of fear of rabies. Bite wounds account for 1% of all emergency department visits in the United States, most caused by dogs and cats. Most dog bites are from household pets rather than strays. Dog bites account for 80% to 90% of all animal bites requiring medical care. A disproportionate number of these dog bites are from German Shepherd dogs.

Children are especially prone to animal bites, especially of the face. Bites occur most commonly among children who disturb the animals while they are sleeping or feeding, separate them during a fight, try to hug or kiss an unfamiliar animal, or accidentally frighten an animal. Women are more often bitten by cats, and young men are commonly bitten by dogs. Dog bites tend to be avulsion injuries with a component of crush. Cat bites more commonly are puncture wounds. Because most cat bites are inflicted by the patient’s own animal, cat bite victims tend to delay care until signs of infection develop. Malpractice claims and other civil lawsuits often follow bite injuries.

Although these wounds may look innocuous initially, they frequently lead to serious infection with a potential for serious complications. A single bite may contain various types of injury, including abrasions, contusions, avulsions, lacerations, crush injuries, or puncture wounds. Less readily apparent are injuries to deeper tissues (including vascular structures, tendons, nerves, and bone), as well as potential foreign bodies. Both dog and cat bites show high rates of infection with Staphylococcus and Streptococcus species, as well as Pasteurella multocida and many different gram-negative and anaerobic bacteria. Infecting organisms generally result from the aerobic and anaerobic microbial flora of the oral cavity of the biting animal, rather than the victim’s own skin flora. In addition to these organisms, 10% to 30% of all human bites are infected with Eikenella corrodens, which sometimes shows resistance to the semisynthetic penicillins but sensitivity to penicillin. Most infections are polymicrobial. A number of risk factors that identify the likelihood of wound infection and define the patient likely to develop this complication have been identified. An important risk factor is delay of more than 24 hours in seeking treatment. Puncture wounds are much more likely than other types to become infected. Facial wounds show an infection rate of only 4%, regardless of treatment, whereas hand wounds have an infection rate of 28%. Septicemia mainly occurs in compromised hosts.

Human bites generally are more severe than animal bites, particularly in clenched-fist injuries. The teeth may cause a deep laceration that implants oral organisms into the joint capsules or dorsal tendons, causing devastating complications that include cellulitis, septic arthritis, tenosynovitis, and osteomyelitis.

Adequate débridement and irrigation are clearly more effective than prophylactic antibiotics and are often all that is required to prevent animal bite infections. Not all bites cause infection. Approximately 2% to 5% of all typical dog bite wounds seen in emergency departments become infected. This figure includes, however, many trivial surface abrasions. Wounds that have fully penetrated the skin have an infection rate of 6% to 13%, depending on location. In comparison, the infection rate of clean lacerations of all types repaired in the emergency department is approximately 3% to 5%.

Less than 0.1% of all animal bites in the United States result in rabies. The incubation period for humans depends on the distance from the bite to the brain, with an average of 1 to 3 months. Given this relatively long incubation period, postexposure prophylaxis for rabies is considered a medical urgency, not a medical emergency. For questions on local rabies risk, local public health services may be available and provide valuable support. In the United States, only Hawaii remains consistently rabies free. Since 1980, a total of 21 (50%) of the 36 human cases of rabies diagnosed in the United States have been associated with bat variants.

Signs of rabies among all forms of wildlife cannot be interpreted reliably; therefore any wild animal that bites and is captured should be euthanized at once (without unnecessary damage to the head) and the brain submitted for rabies testing. If the results of testing are negative by immunofluorescence testing, the saliva can be assumed to contain no virus, and the person bitten does not require postexposure prophylaxis; if it has been started, it can be discontinued.

In the continental United States, rabies among dogs is reported most commonly along the United States–Mexico border and sporadically in areas with enzootic wildlife rabies. During most of the 1990s, more cats than dogs were reported to be rabid in the United States. Most of these cases were associated with the epizootic of rabies among raccoons in the eastern United States.

When consultation is necessary for managing a case of potential rabies exposure and local or state health departments are not available, help can be obtained through the Division of Viral and Rickettsial Diseases of the Centers for Disease Control and Prevention (CDCP). During work hours, phone (404) 639-1050; after hours and on weekends and holidays, call (770) 488-7100. Help is also available at http://www.rabies.com.

In most other countries—including most of Asia, Africa, and Latin America—dogs remain the major species with rabies and the most common source of rabies among humans.

Monkey bites must be given special consideration. In addition to being highly prone to severe infection, they may cause an inoculum of the herpes B virus and require antiviral therapy with acyclovir, valacyclovir, or famciclovir.