Each year in the USA, millions of animal-bite wounds are sustained. The vast majority are inflicted by pet dogs and cats, which number >1.2 million; the annual incidence of dog and cat bites has been reported as 300 - 400 bites per 100,000 population. Other bite wounds are a consequence of encounters with animals in the wild or in occupational settings. While many of these wounds require minimal or no therapy, a significant number result in infection, which may be lifethreatening. The microbiology of bite-wound infections in general reflects the oropharyngeal flora of the biting animal, although organisms from the soil, the skin of the animal and victim, and the animal’s feces may also be involved.
EPIDEMIOLOGY & DEMOGRAPHICS
DOG BITES In the USA, dogs bite > 5 million people each year and are responsible for 82% of all animal-bite wounds, an estimated 15-25% of which become infected. Each year, 800,000 Americans seek medical attention for dog bites; of those injured, 386,000 require treatment in an emergency department, with >1000 emergency department visits each day and about a dozen deaths per year. Most dog bites are provoked and are inflicted by the victim’s pet or by a dog known to the victim. These bites frequently occur during efforts to break up a dogfight. Children are more likely than adults to sustain canine bites, with the highest incidence of 6 bites per 1000 population among boys 5 - 10 years old. Victims are more often male than female, and bites most often involve an upper extremity. Among children < 4 years old, twothirds of all these injuries involve the head or neck. Infection typically manifests 8-30 hours after the bite as pain at the site of injury with cellulitis accompanied by purulent, sometimes foul-smelling discharge. Septic arthritis and osteomyelitis may develop if a canine tooth penetrates synovium or bone.
CAT BITES Although less common than dog bites, cat bites and scratches result in infection in more than half of all cases. Because the narrow, sharp feline incisors penetrate deeply into tissue, cat bites are more likely than dog bites to cause septic arthritis and osteomyelitis; the development of these conditions is particularly likely when punctures are located over or near a joint, especially in the hand. Women sustain cat bites more frequently than do men. These bites most often involve the hands and arms. Both bites and scratches from cats are prone to infection from organisms in the cat’s oropharynx. Pasteurella multocida, a normal component of the feline oral flora, is a small gram-negative coccobacillus implicated in the majority of cat-bite wound infections. Like that of dog-bite wound infections, however, the microflora of cat-bite wound infections is usually mixed. Other microorganisms causing infection after cat bites are similar to those causing dog-bite wound infections.
OTHER ANIMAL BITES Infections have been attributed to bites from many animal species. Often these bites are sustained as a consequence of occupational exposure (farmers, laboratory workers, veterinarians) or recreational exposure (hunters and trappers, wilderness campers, owners of exotic pets). Generally, the microflora of bite wounds reflects the oral flora of the biting animal.
Most members of the cat family, including feral cats, harbor P. multocida. Bite wounds from aquatic animals such as alligators or piranhas may contain Aeromonas hydrophila. Venomous snakebites result in severe inflammatory responses and tissue necrosis-conditions that render these injuries prone to infection. The snake’s oral flora includes many species of aerobes and anaerobes, such as P. aeruginosa, Proteus spp., Staphylococcus epidermidis, Bacteroides fragilis, and Clostridium spp. Bites from nonhuman primates are highly susceptible to infection with pathogens similar to those isolated from human bites. Bites from Old World monkeys (Macaca) may also result in the transmission of B virus (Herpesvirus simiae, cercopithecine herpesvirus), a cause of serious infection of the human central nervous system. Bites of seals, walruses, and polar bears may cause a chronic suppurative infection known as seal finger, which is probably due to one or more species of Mycoplasma colonizing these animals.
PHYSICAL FINDINGS & CLINICAL PRESENTATION
1. The appearance of the bite wound is variable (for example: puncture wound, tear, avulsion).
2. Cellulitis, lymphangitis, and focal adenopathy may be present in infected bite wounds.
3. Patient may experience fever and chills.
1. Increased risk of infection: human and cat bites, closed fist injuries, wounds involving joints, puncture wounds, face and lip bites, bites with skull penetration, bites in immunocompromised hosts
2. Most frequent infecting organisms:
a). Pasteurella spp.: responsible for majority of infections within 24 hours of dog (P. canis) and cat (P. multocida, P. septica) bites
b). Capnocytophaga canimorsus (formerly DF-2 bacillus): a gram-negative organism responsible for late infection, usually following dog bites
c). Gram-negative organisms (Pseudomonas, Haemophilus): often found in human bites
d). Streptococcus spp., Staphylococcus aureus
e). Eikenella corrodens in human bites
1. Bite from a rabid animal (often the attack is unprovoked)
2. Factitious injury
1. Determination of the time elapsed since the patient was bitten, status of rabies immunization of the animal, and underlying medical conditions that might predispose the patient to infection (for example DM, immunodeficiency)
2. Documentation of bite site, notification of appropriate authorities (for example: police department, animal officer)
1. Generally not necessary
2. Hct if there has been significant blood loss
3. Wound cultures (aerobic and anaerobic) if there is evidence of sepsis or victim is immunocompromised patient; cultures should be obtained before irrigation of the wound but after superficial cleaning
X-rays are indicated when bony penetration is suspected or if there is suspicion of fracture or significant trauma; x-rays are also useful for detecting presence of foreign bodies (when suspected).
Established Infection Antibiotics should be administered in all established bite-wound infections and should be chosen in light of the most likely potential pathogens, as indicated by the biting species and by Gram’s stain and culture results (Table 15). For dog and cat bites, antibiotics should be effective against S. aureus , Pasteurella spp., C. canimorsus , streptococci, and oral anaerobes. For human bites, agents with activity against S. aureus , H. influenzae , and b -lactamasepositive oral anaerobes should be used. The combination of an extendedspectrum penicillin with a b -lactamase inhibitor (amoxicillin/clavulanic acid, ticarcillin/clavulanic acid, ampicillin/sulbactam) appears to offer the most reliable coverage for these pathogens. Second-generation cephalosporins (cefuroxime, cefoxitin) also offer substantial coverage. The choice of antibiotics for penicillin-allergic patients (particularly those in whom immediate- type hypersensitivity makes the use of cephalosporins hazardous) is more difficult and is based primarily on in vitro sensitivity since data on clinical efficacy are inadequate. The combination of an antibiotic active against gram-positive cocci and anaerobes (such as clindamycin) with trimethoprim- sulfamethoxazole or a fluoroquinolone, which is active against many of the other potential pathogens, would appear reasonable. In vitro data suggest that azithromycin alone provides coverage against most commonly isolated bite-wound pathogens.
1. Local care with debridement, vigorous cleansing, and saline irrigation of the wound; debridement of devitalized tissue
2. High-pressure irrigation to clean bite wound and ensure removal of contaminants (for example: use saline solution with a 35 - to 40 - ml syringe equipped with a 20-gauge needle or catheter with tip of syringe placed 1 to 4 cm above the wound)
3. Avoid blunt probing of wounds (increased risk of infection)
ACUTE GENERAL Rx
1. Avoid suturing of hand wounds and any wounds that appear infected.
2. Puncture wounds should be left open
3. Give antirabies therapy and tetanus immune globulin and toxoid as needed.
4. Use empiric antibiotic therapy in high-risk wounds (for example: cat bite, hand bites, face bites, genital area bites, bites with joint or bone penetration, human bites, immunocompromised host): amoxicillin-clavulanate (Augmentin) 500 to 875 mg bid for 10 days or cefuroxime (Ceftin) 250 to 50 mg bid for 10 days.
5. In hospitalized patients, IV antibiotics of choice are cefoxitin 1 to 2 g q6h, ampicillin-sulbactam 1.5 to 3 g q6h, ticarcillin-clavulanate 3 g q6h, or ceftriaxone 1.5 to 3 g q24h.
6. Prophylactic therapy for persons bitten by others with HIV and hepatitis B
Prognosis is favorable with proper treatment.