Animal Bites and Pasturella Infections

It is estimated that 109 million dogs and cats are kept as domestic pets in the United States. The true incidence of mammalian bites is not known because many patients who are bitten by animals do not present to the emergency department. Fortunately, the majority (80%) of the estimated 2 million annual mammalian bites are minor wounds. Still, bite wounds account for approximately 1% of all emergency department visits: 10% of victims require suturing and follow-up visits and 1% to 2% require hospitalization. Mammalian bites account for about $30 million in annual health-care costs. Generally those who present to the emergency department are a self-selected group more likely to have serious wounds or infectious complications.Dog bites account for 80% to 90% of animal bites. Fifty-eight percent of American families own a dog. Most dog bites are by dogs in the family;only 6% of bites are by stray dogs. The risk of being bitten by a dog varies with age; animal bites predominantly involve children. Bites occur most frequently between 2 to 19 years of age (Table 1), with the mean age of dog bite victims being 13.4 years.Overall, dog bite injuries occur most frequently(54% to 85%) in the extremities (upper slightly greater than lower), followed by the head and neck (15% to 27%) and least frequently on the trunk (0 to 10%).Age predicts the location of the bite. Children are bitten predominantly on the face or scalp; adults most often are bitten on the arms and legs. Recently there has been an increased incidence of more serious and fatal dog bites. The reported annual mortality from dog bites is 6.7/100 million population. Children are more likely to be fatal victims of dog bites because their fragile habitus is no match for the adult dog whose jaws can exert a pressure greater than 450 lb per square inch, enough to penetrate light sheet metal.Of note, most fatal dog bites in children are from the family pet that results in exsanguinating hemorrhage from damage to a major vessel in the head or neck. Pit bulls perpetrate the majority of deaths (Table 2). It has been suggested that this breed is more prone to attack without provocation.The teeth of a dog are designed to tear tissue. Lacerations are seen in 31% to 45% of dog bites, puncture wounds in 13% to 34%, and superficial abrasions in 30% to 43%.Cat bites are the second most common type of mammalian bite, accounting for 5% to 15% of animal bites (400,000/year). As with dog bites, the majority of cat bites are from pets of the owner or someone they know.No studies have identified the breed of cats involved in animal bites. Cat bites tend to occur in slightly older patients. The mean age of cat bite victims is 19. 5 years, with 44% of patients being older than 20 years.About 60% of cat bite injuries occur in the arms, forearms, and hands compared with 15% to 20% on the head and neck, 10%to 13% on lower extremities, and 0 to 5% on the trunk (Table 3). The teeth of a cat are slender, extremely sharp, and can penetrate bones and joints easily. Therefore, most cat bites result in punctures, although scratches also are frequent.The infection rate associated with cat bites is nearly double that seen with dog bites.The majority of studies of animal bite wounds focus on Pasturella multocida, ignoring the spectrum of other aerobes and anaerobes also identified frequently in dog and cat bite wounds.Studies of the oral flora of a dog note the presence of multiple organisms, most of which are potential pathogens (Table 4). The most common aerobic isolates have been alpha-hemolytic streptococci(46%) and Staphylococcus aureus (13%). Anaerobic pathogens have been present in 41% of wounds, including Bacteroides species (mostly B melaninogenicus) and Fusobacterium species (19%).Goldstein et al isolated P multocida from only 26% of dog bite wounds. Bailie found S aureus and P multocida in 72% and 60%, respectively, of cultures of the oral and nasal fluids of dogs. The same study noted a carriage rate of 43% for Centers for Disease Control and Prevention (CDC) alpha numeric group M-5,82% for CDC group EF-4, and 24% for C aphocytophaga canimorsus (formerly CDC group DF-2). The types of infecting organisms vary Wgeographically.P multocida is the major pathogen isolated from cat bites. This small, aerobic, facultatively anaerobic, gram-negative coccobacillus is a part of the normal flora in 70% to 90% of cats.Overall, P multocida has been isolated in 50% to 80%of cat bite wound infections and 25% of dog bite infections.Dog bites have the lowest infection rate of all mammalian bites. With good local care, infection occurs in only 2% to 5% of these bites. High-risk areas, such as the hand, are associated with a 30% infection rate. The incidence of infection developing in cat bites is 30% to 50%, almost twice that seen in dog bites. The sharper, slender teeth of cats, higher prevalence of P multocidasubspecies in cat bites, and more frequent involvement of tendons and joint spaces may explain the higher infection rate in cat bites.Clinical infection with P multocida is characterized by the rapid development of an intense inflammatory response, with prominent pain and swelling developing within 24 hours of the initial injury in 70% of cases and by 48 hours in 90% of patients who develop an infection. P multocida infection has resulted in abscess formation, septic arthritis,osteomyelitis, sepsis, meningitis, endocarditis, and pneumonia. Infections usually have localized cellulitis and purulent discharge. Fever, regional adenopathy, and lymphangitis are seen in fewer than 20% of patients.Wounds in proximity to bones or joints may be associated with the complications of septic arthritis, osteomyelitis, and tenosynovitis. If there is edema of the extremity, the risks of sepsis and serious cellulitis are increased.C canimorsus is a rare pathogen in dog bites that can result in a potentially fatal bacterial infection. This gram-negative rod is part of the normal flora in 16% of dogs and 18% of cats. Splenectomy,alcoholism, and chronic pulmonary disease are major risk factors for C canimorsus infections. The case fatality rate is 23%to 28%; 20% to 30% of systemic disease caused by C canimorsus occurs in patients younger than 40 years of age. Clinical manifestations and complications observed include disseminated intravascular coagulation, purpura, symmetric peripheral gangrene, and Waterhouse-Friderichsen syndrome.P multocida, CDC group M-5, CDC group EF-4, and C canimorsus usually are sensitive to penicillin. Amoxicillin/clavulanic acid may provide better coverage for other potentially pathogenic aerobic and anaerobic bacteria present in the wound and introduced by the dog’s mouth. Tetracycline is an option for the child allergic to penicillin.Cat bites rarely are associated with tularemia, plague, sporotrichosis, blastomycosis, and rabies.The risk of infection is determined by local wound care, host factors, and wound location. As noted previously, the majority of animal bites do not become infected. When an infection does occur, though, it will progress quite rapidly, usually becoming apparent within 8 to 24 hours. Puncture wounds have a twofold greater risk for infection compared with lacerations and account for 40% of animal bite infections.Patients presenting within 8 hours of injury usually have either a more serious wound,concerns about rabies, or questions about tetanus prophylaxis. Patients presenting after 12 hours usually have clinical evidence of infection.Wound management begins with a focused history to identify the patient at increased risk for infectious complications (asplenia, diabetes, and immunocompromised state).Wounds are evaluated for evidence of injuries to deeper structures such as tendons, joint spaces, blood vessels, nerves,and bone.Radiographs should be taken if there is considerable edema and tenderness around the wound and if bony penetration or foreign bodies are suspected.Subcutaneous emphysema may represent gas from necrotizing infections, air introduced during wound manipulation, or air introduced into the wound by the animal grunting or snorting.Wounds seen less than 8 to 24 hours after injury that have no signs of infection need not be cultured. Similarly, wounds without signs of infection examined 24 hours after a bite do not need to be cultured. In both of these cases, empiric antimicrobials should be administered based on local susceptibility patterns.If infection is present, aerobic and anaerobic cultures should be obtained. The crust of the wound should be removed prior to culturing, and cultures should be taken prior to debridement or irrigation.The most important step to decrease the probability of wound infection is proper wound cleansing with large amounts of sterile saline solution by employing a 20- or 30-cc syringe attached to an 18- to 19-gauge catheter. This arrangement generates high pressures that are sufficient to debride the wound and reduce the bacterial inoculum but will not result in undue tissue injury.Animal data support the use of diluted povidone-iodine solutions to decrease the transmission rate of rabies. However, some feel that these solutions may be harmful to viable tissue.Obviously necrotic, devitalized tissue should be debrided sharply. Debridement of puncture wounds is not advised because it may lead to a defect that cannot be closed.There is ongoing controversy about whether animal bite wounds should be closed primarily (Table 5). There are data to support primary closure of dog bite wounds in low-risk locations after proper local wound care. However, reliable prospective data are lacking, and the proper approach remains controversial.Deep puncture wounds, wounds examined more than 24 hours after injury, clinically infected wounds, and bites of the hand should not be closed primarily. Low-risk wounds seen within 24 hours after injury may be sutured; uninfected high-risk wounds seen 72 hours after initial injury may undergo delayed primary closure.Bites to the face and head have a good outcome and may be closed primarily.When to administer prophylactic antibiotics also is controversial. Prophylactic antibiotics are suggested for moderate and severe wounds; for all cat bites; and for patients who have diabetes mellitus, are immunocompromised, have face and hand involvement,and have deep puncture wounds. The recommended course is 3 to 5 days. The antimicrobial agents recommended for prophylaxis include: penicillin V potassium, amoxicillin, a first-generation cephalosporin in penicillin-allergic patients, and erythromycin in patients allergic to penicillin and cephalosporin.The treatment of obvious infection should be directed at aerobic and anaerobic oral flora of the animal as well as the flora of the patient’s skin. When possible, the antibiotic treatment should be guided by the antibiotic sensitivities of the organism isolated from the wound.Elevation of the involved area is critical and should be continued for several days or until the edema has resolved. Failure to do so can contribute to therapeutic failure.An untreated person has a less than 20% chance of contracting rabies from the bite of a rabid animal. However,the 100% mortality rate if rabies is contracted argues for prophylactic treatment.Human rabies is rare in the United States, a consequence, no doubt, of effective animal control, vaccination, easy access to medical care,and rabies prophylaxis. Worldwide, the dog is the major animal reservoir for rabies; in the United States, the principal vectors are wild animals—skunks,raccoons, bats, and foxes. Domestic animal rabies is seen most commonly in cats, followed by cattle and dogs.Local wound care is critical in cases where there is a high risk for rabies. The wound should be irrigated and debrided. Prompt, thorough wound irrigation with soap or iodine solution reduces the development of rabies by up to 90%. When considering the need for vaccination, it must be kept in mind that once clinical signs of rabies have developed, the course is uniformly fatal. The therapy for rabies is based on the administration of passive immunization with human rabies immune globulin (HRIG) and full active immunization with human diploid cell vaccine(HDCV) or the rabies vaccine adsorbed (RVA). If a primary tetanus series has never been given, hyperimmune globulin initial series should be administered.Animal bites should be regarded as tetanus-prone. Recommendations for tetanus prophylaxis depend on the patient’s immunization history (Table 6). Although bite wounds are common injuries in the pediatric population,most are minor. Dog bites are the most common animal bite wound. Children are more likely to have head and face injuries and share a disproportionate number of fatal wound injuries.Cat bites have a twofold higher risk of infection than dog wounds, with the most frequent and important pathogen being P multocida. Both cat and dog bite wounds are associated with a wide range of aerobic and anaerobic pathogens. Principles of wound management include copious irrigation, cautious debridement, appropriate antibiotics,elevation, and immobilization. The patient should have follow-up contact and usually a visit within 24 to 48 hours after the initiation of therapy.