Animal Bite-associated Infections

Microbiology and Treatment

Nicole Thomas; Itzhak Brook

Disclosures

Expert Rev Anti Infect Ther. 2011;9(2):215-226. 

In This Article

Management

Most of the management guidelines for the treatment of bite infections are based on expert opinion. The goals of therapy are to prevent or treat infection, as well as minimize soft tissue damage. In general, a number of therapies may be undertaken for all bite wounds.

Smaller wounds should be flushed with large amounts of cool clean water. A mild soap may be gently used to cleanse the area, and may have some prophylactic effects against rabies.[15] The WHO recommends thorough flushing and washing of bite wounds for a minimum of 15 min with soap and water, detergent or povidone iodine.[205] Soaking the injury is not recommended as this may introduce infection into the wound.[95] Alcohol or peroxide is also not recommended because of the potential for further tissue injury.[15] Application of an ice pack may aid in pain relief and decrease swelling.

Professional medical care should start with obtaining the history of the injury to include timing, and details about the biter (i.e., human vs animal, domestic vs wild, vaccination status of the biter and availability of animal to be quarantined). Rabies risk should be determined with an inquiry into unusual behavior of the animal, and whether or not the attack was provoked. History of the patient's health should include medications and allergies, past medical history and risk factors for infection.

Examination should determine the type of wound (i.e., laceration, puncture, contusion or crush-avulsion), depth of wound and possible involvement of underlying structures (e.g., tendons, joints or bones). Range of motion of joints near and involved with the injury should be evaluated, as well as examination for signs of infection.

Radiological studies are often obtained after any 'mauling' injuries, any extensive wounds, as well as on closed-fist injuries and other bites to the hand. In addition to identifying fractures, radiographs may reveal air in a joint, or may be used to determine if a piece of broken tooth or a foreign body is in the wound. Radiographs are also used to look for evidence of osteomyelitis in a chronic wound infection. If there is evidence of purulence in the wound, aerobic and anaerobic cultures should be obtained.

Meticulous cleaning with saline irrigation should occur, as the wound should always be considered contaminated. Saline lavage with a 30-ml syringe and 18-gauge intravenous catheter is recommended, with volumes up to 150 ml for wounds less than 3 cm.[95] When necessary, foreign body removal and debridement of dirt and nonviable tissue can occur after irrigation is complete.

Aside from bite wounds to the face, there is little evidence and agreement as to whether or not to close a bite wound. Primary closure of neck and face wounds (even cat and human bites) with avoidance of buried sutures, and usage of prophylactic antibiotics, is reasonable because of the excellent vascular supply.[96] Most authors do not recommend wound closure, although those who propose that wounds may be primarily closed, indicate that the decision depends on the species of the biter, size and location of the wound and the time between injury and presentation for medical attention. In general, if one chooses to suture a wound, it must be done within 8 h of the bite.[95.97] Garbutt and Jenner performed a review that looked at 74 papers on management of bite wounds, with the specific question of whether primary closure of animal bites increases wound infection rates.[98] They found one study that they felt adequately addressed the question. The study completed by Maimaris and Quitoc looked at 96 patients with 169 dog bite lacerations, who were randomized to primary closure, or leaving the wound open (puncture wounds were excluded).[97] the study concluded that there was no significant difference in infection rates between the two groups except in those wounds occurring to the hands. It was then proposed that dog bite wounds to the hand should be left open, but that nonpuncture wounds elsewhere may be safely treated by primary closure after thorough cleaning.

Cat bites are generally not sutured unless on the face or scalp. Nonsutured wounds should be bandaged with bulky dressings. Facial bites usually heal well and have adequate cosmesis if adequate irrigation and debridement occur prior to primary repair.[99,18]

Tetanus and rabies prophylaxis are very important considerations when managing a bite wound. Every patient with a bite injury that seeks medical care should have their tetanus status checked, with appropriate immunization as needed. Tetanus booster should be given if the victim received a primary series, but no booster within the last 5 years. If tetanus status is unknown, or if the vaccine series was not completed, then both tetanus vaccine and tetanus immune globulin (in the arm opposite the vaccine) should be administered.[100]

If the bite is from an animal capable of transmitting rabies, it is important to determine the animal's vaccine status. If it the animal has received the rabies vaccine and it can be quarantined for 10 days, then rabies prophylaxis is not necessary. If the status is unknown, then postexposure prophylaxis should be initiated immediately. The regimen consists of one dose of RIG, with up to 50% of the dose being injected around the wound and the remainder being given intramuscularly. In March of 2010, the CDC changed the postexposure rabies vaccine recommendation from five to four doses (first dose in the arm opposite the dose of RIG), with immunization administration on days 0, 3, 7 and 14.[208]

There is much debate on whether or not to use prophylactic antibiotics. In general, if the wound is not infected then there is no need for antibiotic prophylaxis, unless the wound is deemed moderate to severe, or if the bite injury is considered a high risk for infection. These risks include: extremes of age, immunocompromise (especially asplenics), comorbid factors such as chronic disease (i.e., diabetes), usage of certain medications, the type of animal inflicting the bite (i.e., cats or sharks), the number/type of pathogen inoculated in the bite, and the size and location of the wound.[101] Deep puncture wounds beyond the epidermal layer, crush injury, bites to hands, feet, genitalia or over joint surfaces are considered especially high risk.[102] In general, empiric therapy is usually an oral medication covering the most likely pathogens (see Table 1) for 3–5 days.

Empiric choice of antibiotics for an actual wound infection will depend on the predicted inciting organisms. Human bites must cover both anaerobes and aerobes, with special attention to E. corrodens, which is resistant to first-generation cephalosporins. For cat bites, it is important to cover for Pasteurella spp., whereas dog bites require coverage for both Pasteurella and Capnocytophaga spp. Other mammalian bite infections have common oral flora including staphylococcus and streptococcus species. If infection can be managed on an outpatient basis, usage of oral amoxicillin/clavulanate or moxifloxacin (in patients >18 years of age) is effective. Moxifloxacin has been shown to have excellent in vitro activity against Pasteurella and Eikenella spp., as well as other organisms found in bite wounds,[103] including a number of anaerobes.[54,104–106] Although there have not been studies specifically looking at the use of moxifloxacin against bite-associated infections, this once-a-day drug could be considered as a potential addition to the antbiotic regimen armamentarium. Merriam et al. recently showed adequate in vitro activity of azithromycin to 98% of 296 strains of oral anaerobes, including all fusobacteria and β-lactamase-producing strains of Prevotella spp., as well as E. corrodens.[106] Although promising, the use of azithromycin has not yet been established as an alternate oral treatment of human bites.

If parenteral therapy is required, choices include ampicillin/sulbactam, ticarcillin/clavulanic acid, or piperacillin/tazobactam. Appropriate alternative regimens include cefoxitan, cefotetan or a carbapenem such as ertapenem.[107,108] If an extended-spectrum cephalosporin (cefotaxime or ceftriaxone) is chosen, then clindamycin should be added to cover anaerobes. PCN-allergic patients requiring intravenous therapy may be treated with doxycycline or a fluoroquinolone with clindamycin. Of note, tigecycline has an indication for skin and soft tissue infections, and although not formally studied, may be an intravenous alternative for a patient who is highly PCN allergic.

For mammalian bites, PCN-allergic patients should receive doxycycline (children >8 years of age), moxifloxacin (if >18 years of age), or a choice of metronidazole or clindamycin (for anaerobes), plus either trimethoprim/sulfamethoxazole or the other flouroquinolones such as ciprofloxaxin. Of note, antibiotics used for other skin and soft tissue infections, such as antistaphylococcal PCNs, the first-generation cephalosporins and erythromycin are not active against Pasteurella and Eikenella species.[109]

Reptiles, especially alligators, must include coverage for Aeromonas spp. Third-generation cephalosporins are very effective against these organisms,[110] whereas with marine animals, coverage for Vibrio species with a tetracycline or fluoroquinolone is necessary.

The decision to use parenteral versus oral antibiotics as well as length of therapy will depend on location and extent of infection. Soft tissue infections usually only require 7–14 days of therapy after appropriate wound care is accomplished, whereas septic arthritis or osteomyelitis may require longer treatment regimens, with courses from 3 to 6 weeks. Ultimately, duration and route of antibiotic should be individualized, based on the infected site, culture and sensitivity results, and response to therapy.

Finally, adequate follow-up and education are crucial. Patients should be taught wound care as well as the signs of infection. A follow-up wound check should take place in 24–48 h (sooner if there are signs of infection). Also, if indicated, the bite should be reported to local health or law enforcement agencies.

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