Bites

Animal Bites

Bite wounds are common injuries caused by a wide variety of domestic and wild animals, as well as humans. The American Pet Products Association 2017–2018 survey reported the US pet dog and cat ownership number at approximately 184 million. Bites occur in 4.7 million Americans each year and account for 800,000 medical visits, including approximately 1% of all emergency department visits. There were 1610 animal-related fatalities in the United States during the period of 2008 to 2015. The majority of human fatalities were from farm animals (e.g., cattle and horses), insects (hornets, wasps, and bees), and dogs. Most dog bites (85%) are provoked attacks by either the victim's own pet or a dog known to the victim and occur during the warm weather months. Bite wounds are often to the extremities, especially the dominant hand. Facial bites are more frequent in children younger than 10 years of age and lead to 5 to 10 deaths per year, often because of exsanguination. Larger dogs can exert more than 450 lb/in ³ of pressure with their jaws, which can lead to extensive crush injuries.

The bacteria associated with bite infections may come from the environment, the victim's skin flora, or most frequently, the oral flora of the biter, which can also be influenced by the microbiome of the biter's ingested prey and other food ( Table 315.1 ). Patients who present early after an incident often do not have an established infection and are usually concerned about crush injuries, care of disfiguring wounds, or the need for rabies or tetanus immunization. These uninfected wounds are frequently contaminated with multiple strains of aerobic and anaerobic bacteria, similar to the spectrum found in documented bite infections. Between 2% and 30% of wounds will become infected and, rarely, may require hospitalization. Patients presenting late, often longer than 8 hours after injury, usually have established infection. Infection is manifested as localized cellulitis or abscess with gray and malodorous purulent discharge. Fever, regional adenopathy, and lymphangitis occur in the minority of patients. Bites involving bones and joints may result in tenosynovitis, septic arthritis, and osteomyelitis. Chronic pain in a joint with limited range of motion may be suggestive of infection within a joint or bony structure.

Rarely, overwhelming sepsis, endocarditis, meningitis, or brain abscesses may develop after a bite injury. Fatal infections caused by Capnocytophaga canimorsus in association with asplenia or liver disease have been noted. This organism may be difficult to isolate and identify and may require up to 14 days of incubation to grow on blood cultures. In addition, it has the potential capacity to escape the host immune system by both passive and active mechanisms. It is generally susceptible to penicillin, cephalosporins, and fluoroquinolones but variably resistant to aztreonam and aminoglycosides ( Table 315.2 ).

Individuals with immunocompromising conditions, including chronic corticosteroid use, and those with preexisting edema of an extremity are more prone to severe infections and complications.

Dog and cat bite wound infections are considered to be predominantly related to the microbiology of their oral flora. Although most attention has focused on Pasteurella multocida, the spectrum of organisms associated with dog and cat bite wound infections is much greater. Holst and colleagues noted the following distribution of 159 Pasteurella strains isolated over a period of 3 years from human infections, mostly from bite wounds: Pasteurella multocida subsp. multocida (60%), which was the isolate in all bacteremia cases; Pasteurella multocida subsp. septica (13%), which has a greater prevalence in cats than in dogs and may have a preferential affinity for the central nervous system; Pasteurella canis biotype 1 (18%), which was isolated exclusively from dog bite wound infections; Pasteurella stomatis (6%); and Pasteurella dagmatis (3%), which may cause systemic infections. A study of 107 infected dog and cat bite wound infections showed that 75% of cat bites grew Pasteurella species on culturing (P. multocida subsp. multocida, 54%), as did 50% of dog bites ( P. canis, 26%; P. multocida subsp. multocida, 12%). In this study, other common aerobic organisms isolated from infected dog and cat bite wounds included Streptococcus, Staphylococcus species (including Staphylococcus aureus ), and Neisseria species. Anaerobic organisms, when present, were almost always in mixed infections with aerobic organisms and commonly included Fusobacterium, Porphyromonas, and Prevotella species in dog bites and Fusobacterium, Porphyromonas, and Bacteroides species in cat bites.

Table 315.1 lists common pathogens found in dog and cat bite wound infections. Staphylococcus intermedius is coagulase positive, can be mistaken for S. aureus, and is fourfold more common in canine flora but possesses β-galactosidase activity, which differentiates it from S. aureus. It may masquerade as methicillin-resistant S. aureus (MRSA) owing to false-positive rapid penicillin-binding protein 2a latex tests, although an increasing number (approximately 30%) of isolates may be resistant to oxacillin. MRSA has been cultured from a variety of companion animals, including cats, and has been documented to be transmitted from a healthy pet cat to humans, and the human strains and feline strains are indistinguishable. MRSA may be a potential causative secondary invader, especially in patients who are not responding to initially administered antibiotics that often do not exhibit activity against MRSA and in those known to be colonized with MRSA.

Capnocytophaga canimorsus is difficult to grow on most routine solid media but can grow on chocolate agar and heart infusion agar with 5% rabbit blood when incubated in CO ₂ and a variety of liquid media, including BACTEC aerobic medium (Becton, Dickinson and Company, Franklin Lakes, NJ). This species can be differentiated from other Capnocytophaga species by the presence of positive oxidase and catalase reactions. Centers for Disease Control and Prevention (CDC) group DF-2–like strains have been classified as Capnocytophaga cynodegmi. CDC group M-5 has been classified as Neisseria weaveri and has been associated with dog bites. CDC group EF-4a is now called Neisseria animaloris, and EF-4b is Neisseria zoodegmatis. Haemophilus felis, initially identified as Aggregatibacter (Haemophilus) paraphrophilus, requires factor V and CO ₂ for growth and is common in cat nasopharyngeal flora.

Bergeyella (previously designated as Weeksella ) zoohelcum has been associated with bite cellulitis, sepsis, and meningitis. Other new aerobic species include Neisseria canis from a cat bite, Flavobacterium group IIb–like isolates from a pig bite, Actinobacillus lignieresii and Actinobacillus equi –like bacterium from horse bites, and CDC group NO1, a nonoxidative gram-negative rod different from Acinetobacter species associated with dog and cat bites. Orf virus infection has been transmitted by a sheep bite. When appropriate culturing techniques are used, anaerobes are isolated in up to 70% of animal bite wounds, almost always in mixed culture. Approximately 50% to 60% of cat and dog bite wounds contain Bacteroides tectus, Prevotella heparinolytica, Prevotella zoogleoformans, Prevotella bivia, Porphyromonas gingivalis, Porphyromonas canoris, Fusobacterium nucleatum, and Peptostreptococcus anaerobius. Fusobacterium canifelinum is an intrinsically fluoroquinolone-resistant species isolated from dog and cat bites.

Little difference has been noted in the types of bacteria isolated from noninfected wounds seen early and infected wounds seen later. All moderate-to-severe bite wounds, except those not clinically infected and more than a few days old, should be considered contaminated with potential pathogens.

Wounds inflicted by cats are frequently scratches or tiny but somewhat deep punctures located on the extremities and are at higher risk of becoming infected. Deep puncture wounds over or near a joint, especially on the hands, may result in osteomyelitis and septic arthritis. Pasteurella multocida has been isolated from 50% to 70% of healthy cats and is a frequent pathogen in cat-associated wounds. Erysipelothrix rhusiopathiae has also been isolated from cat bite wounds. Cougar, tiger, and other feline bites also yield P. multocida. Tularemia has likewise been transmitted by cat bites. People are also bitten by a variety of other animals, including unusual domestic pets, farm animals, wild animals, aquatic animals, and laboratory animals. Monkey bites cause more swelling and infection than do many other animal bites. Old World monkeys may transmit a potentially lethal subtype B virus (herpesvirus simiae; see Chapter 141 ). Case series or reports of various animal bites—including terrestrial mammals (e.g., monkeys, bears, pigs, ferrets, horses, sheep, Tasmanian devils); reptiles (e.g., snakes, Komodo dragons, lizards, iguanas, alligators, crocodiles); rodents (e.g., rats, guinea pigs, hamsters, prairie dogs); swans; and sharks with unusual isolates—have been reported.

Management of Animal Bites

Table 315.3 notes the elements for the treatment of animal bite wounds. The most problematic elements of the management of wounds that are seen early include the following:

• 1

  The use of preemptive antibiotics in wounds that are seen early but are not infected. As a routine, antimicrobial therapy is not advocated in every uninfected animal bite injury. However, because 85% of such wounds harbor potential pathogens and one cannot reliably predict which wounds will become infected, selected wounds are best treated with oral antimicrobial therapy with agents active against common bite pathogens (see Table 315.2 ) for 3 to 5 days. Recommendations about patient selection vary and are based on limited evidence. In general, 3 to 5 days of preemptive antimicrobial therapy is advocated for acute moderate-to-severe injuries, especially those to the hand or face, in the presence of preexisting edema, and in immunocompromised hosts. Longer courses may be considered for injuries that are severe or those penetrating joints or bony structures.

• 2

  The decision to suture the wound. The role of primary wound closure in animal bites remains controversial. The literature on this topic has a number of limitations such as the time from injury to presentation, the extent and location of injuries, wound closure methods, and administration of antibiotics. For cosmetic and functional reasons, facial and neck wounds and extensive large wounds, especially those overlying the joints, are difficult to leave open and are usually sutured after irrigation coupled with antimicrobial prophylaxis. Limited studies are available to determine whether the risk of infection is increased with primary wound closure in animal bites. In one study of 345 dog bite wounds, puncture wounds and wounds that were closed were more likely to become infected. In a randomized controlled trial of primary closure versus nonclosure of dog bite wounds, the infection rate between the two groups was similar but primary suturing improved cosmetic appearance. It is our experience and recommendation that, if cosmetically or functionally reasonable, small bite wounds can be left open and allowed to close by secondary intention. Larger wounds, especially those overlying the face, neck, and joints, may be sutured or loosely approximated and closed by delayed primary closure.

The most common causes of therapeutic failure are the following:

• 1

  Failure to elevate the affected edematous extremity.

• 2

  Inadequate antimicrobial therapy (see Table 315.2 ). Most fastidious animal pathogens are susceptible to penicillin or amoxicillin. Because of resistance of certain bacteria, including P. multocida, first-generation cephalosporins, dicloxacillin, erythromycin, clindamycin, and metronidazole should be avoided as a sole antimicrobial agent; however, they can be used in combination with antibiotics that have activity against Pasteurella species. In vitro data suggest that some fluoroquinolones (ciprofloxacin, levofloxacin, and moxifloxacin), trimethoprim-sulfamethoxazole, ertapenem, and second-generation oral cephalosporins (e.g., cefuroxime) are active against many bite isolates. Ceftaroline, an anti-MRSA cephalosporin, has activity against Pasteurella species and other aerobic gram-positive isolates recovered from animal bite wounds. Empirical regimens for marine- and freshwater-acquired infection should cover Vibrio and Aeromonas species, respectively, with agents such as third-generation cephalosporins (e.g., cefotaxime) and fluoroquinolones.

• 3

  Failure to recognize osteomyelitis and septic arthritis. Long-standing pain or continuous diminished range of motion of a joint near a bite wound may be an indication of complications such as osteomyelitis or septic arthritis. Pain out of proportion to the severity of the wound may be an indicator of potential periosteal or joint penetration.