Medical Consequences of Animal Bites

Primary Bite Trauma and Severity

Most (est. 80%) animal bites cause no or only minor injury, which can be adequately managed in the home with cleansing and first aid [Cornelissen 2010; Goldstein 1992]. However, of the nearly five million dog bites sustained in the U.S. each year, approximately 800,000 dog bite victims seek medical attention for their injuries [Sacks 1996a]. From 2006 to 2009, 144,926 emergency department visits for animal bites were recorded in California. The median charge for an emergency department visit for a dog bite in a child was estimated at $300; estimated medical charges increased to $3600 for 23-hour observations and $5900 for hospitalizations [Daniels 2009]. Approximately one-third of bite-related medical visits were covered by private insurance.

Several factors can contribute to the severity of the bite wound(s) and need for medical attention:

Species of animal

Of nearly 145,000 bite wounds treated at emergency departments in California between 2006 and 2009, 80 percent were attributed to dogs. Canine teeth create cutting and crushing damage to tissues, in contrast to the needle-like dentition of cats that typically leave discrete puncture wounds. Bites to the head and face are more frequently sustained from dogs and ferrets [Constantine 1986, Ferrant 2008]; of 6300 emergency department visits for bite-related wounds to the head or face in North Carolina, 94.6 percent were associated with dog bites [Rhea 2014]. In one survey of ferret bite incidents, which occasionally represented dozens to hundreds of laceration wounds, 28 percent of infant victims required plastic and reconstructive surgery [CDHS1988].

Bites from aquatic animals can also cause significant tissue damage. Bites of a barracuda, a fish with sharp teeth, lead to shearing injuries, whereas eel bites cause severe lacerations and deep puncture wounds. While bites from sharks are often sensationalized, most result in fairly minor injuries such as puncture wounds and linear lacerations. Rarely, shark bites can cause large (>20 cm) lacerations, significant tissue damage, loss of limbs, or death—the last most commonly from exsanguinating hemorrhage [Woolgar 2001]. Ninety-five percent of shark attacks recorded in California waters between 1950 and 2011 were attributed to great white sharks [Florida Museum of Natural History], which typically inflict long, deep lacerations [Woolgar2001].

Bite force

Size, skull shape, dentition, and jaw musculature all contribute to the force with which an animal can bite. Bite force has been directly measured in only a few species. Mean bite force in dogs ranged from 442 N for dogs weighing more than 34 kg, to 52 N for dogs weighing less than 11 kg [Linder 1995]. In comparison, the bite force of a 29 domestic cat was estimated at 56N [Wroe 2005]. Among terrestrial mammals studied, estimated bite forces was greatest for tigers (1525N) and African lions (1768N). The greatest bite force calculated among all animals is nearly 6000N by an adult bull shark [Habegger 2012].

Age/size of victim

Children are more likely than teenagers and adults to sustain bite injuries that require medical attention. In 2001, approximately 370,000 emergency department visits were recorded in the U.S. for dog bite- related injuries; 42 percent of these patients were children <14 years old [CDC 2003]. Similarly, children<14 years old comprised 30 percent of bite-related emergency department visits in North Carolina from 2008 to 2010 [Rhea 2014]. In 1992-94, the incidence of emergency department visits for dog bites was highest for boys aged 5 to 9 years; dog bite injuries represented 3.6% of all emergencydepartment injuries for this age/sex group [Weiss 1998]. Also, because of their smaller stature, children are more likely to suffer bites to the head and face [Lang 2005; Harris 1974; Daniels 2009]. Injuries to the head and face represented nearly two-thirds of bite injuries among children aged <4 years in one study [CDC 2003] and nearly three-quarters of bite injuries among children aged 0 to 9 years in another [Weiss 1998].

Number of animals

Most animal bites victims are bitten by only a single animal. Unusual circumstances such as persons attempting to intervene in a fight between two or more dogs might increase their vulnerability to bites from multiple animals. Furthermore, because the communication cues operational between dogs during normal circumstances are often ignored during a fight, animals may persist in frenzied biting behavior and inflict more bites per animal than would occur in an altercation between a single dog and the human victim. Of 238 dog bite-related fatalities identified between 1979 and 1998, 78 (33%) involved 2 or more dogs [Sacks 2000].

Behavior

Failure to recognize and appropriately respond to early signs of fear or aggression in a dog can lead to both the initial and sustained attack. A fearful or aggressive dog is unlikely to back down if its initial aggressive threats are challenged with reflected aggression. A person who responds to canineaggression in a dominant, violent, and loud manner, rather than assuming a calm and submissive posture, is more likely to sustain multiple wounds as the dog persists in itsattack.

Secondary Complications and Adverse Health Events

In addition to the acute trauma inflicted by an animal bite, the compromised and damagedtissue can lead to localized or systemic infection, disability, and disfigurement. Even apparently minor bites that directly cause negligible tissue damage can develop serious, possibly fatal, infection. Potentially pathogenic bacteria have been recovered from approximately 85% of fresh bite wounds; up to 30% of bite wounds progress to frank infection [Goldstein 1989; Goldstein 1992]. According to the American Society of Plastic Surgeons statistical data, nearly 30,000 reconstructive procedures were performed for dogbite injuries in 2011. Moreover, the long-term psychological impact that follows disfiguring bite injuries, particularly to the face, is difficult to quantify.

A small percentage of bite incidents result directly or indirectly in the death of the bite victim. An average of 12 people die each year in the U.S. from dog biterelated injuries AVMA. Factors associated with bite wound complications include:

Species of animal

Wound infections are more frequently observed in bites from cats (28% to 80%) than from dogs (3% to 18%) [Douglas 1975; Rhea 2014]. The deep puncture wounds resulting from cat bites are superficially less severe than the typical crushing wounds from dog bites, but are also less amenable to thorough cleansing, irrigation, and debridement, rendering them more vulnerable to infection. The period from infliction of the bite to the first symptoms of infection is shorter (7-18 hours) for cat bites compared to dog bites (12 -48 hours) [Talan 1999]. Cat bites are also more frequently associated with severe systemicsequelae, such as meningitis, osteomyelitis, endocarditis, septic arthritis, and septic shock. In one case review, 30 percent of patients bitten on the hand by a cat were hospitalized [Babovic 2014].

Besides dogs and cats, reptile bites can lead to complications. Native to California are six species of rattlesnake and one species of lizard (Gila monster) that produce hemotoxic, neurotoxic, or proteolytic venom. In addition to profound pain and disability, these venoms can cause extensive tissue damage and, in extreme cases, necrosis and loss of digits or limbs.

Tissue trauma

Dog bites that create dead space—whether from lacerations or puncture wounds-- were nearly three times as likely to be infected (32%) as similar wounds that did not create dead space (11%) [Myers 2008].

Anatomic location

Bites to the hands are particularly susceptible to infection due to the proximity to the skin of underlying bones, joints, and tendons [Smith 2000, Brook 1989, Thomas 2011]. In one study of patients bitten bya cat on the hand or wrist, patients who were bitten over joints or tendon sheaths were nearly three times as likely to be hospitalized as those bitten on soft tissue [Babovic 2014]. Bites over or near a joint can lead to osteomyelitis and septic arthritis. Bites to the cranium may result in infections or abscesses in the brain or supporting structures. Wounds and resultant scars to the face are more likely to be considered "disfiguring" than similar wounds to the trunk or extremities.

Health of bite victim

As with nearly any acute health incident, animal bites are more likely to precipitate 31 significant complications and sequelae in victims who have pre-existing conditions that compromise their ability to respond to injury. Persons who are elderly, immunosuppressed, have sub-optimal hepatic or splenic function, or suffer from chronic conditions such as diabetes or cardiovascular disease may be more susceptible to infection and other secondary complications of animal bites. Furthermore, these pre- existing conditions may complicate the therapeutic options available to treat thesequelae.

Timeliness and appropriateness of medical attention

Immediate wound care can significantly reduce the possibility for secondary infection, loss of devitalized tissue, and irreparable disfigurement. Bite victims who have grossly severe and serious wounds are more likely to seek emergency medical attention (< 12 hours after incident). Persons who delay seeking medical attention until later (>12 hours after incident) are more likely to already be experiencing symptoms and signs of infection or neuromuscular damage, often from grossly less significant wounds.

Infection

The chief medical concern of animal bites is infection. Bite wounds compromise the normal barrier between the outer protective epidermis and the deeper, normally sterile subcutaneous tissues. This entryway for microbial inoculation can persist for days to weeks following the bite incident. Studies have identified contamination with potentially pathogenic bacteria in >85 percent of fresh bite wounds, however only 15-20 percent of bite wounds develop frank infection [Goldstein 1992].

Bacterial infection

A wide range of bacteria from the biting animal's mouth, the victim's skin, and the surrounding environment can infect a bite wound. The number of different bacteria isolated from dog bite wounds has been reported to range from 1 to 4 in nonpurulent wounds, and up to 16 in purulent and abscessed wounds [Abrahamian 2011].

Over 130 species of bacteria have been recovered from dog and cat bite wounds [Talan 1999]. Pasteurella spp. are the most common microbial isolates from infected bites, occurring in more than 75 percent of cat bites (P. multocida ssp multocida and ssp septica) and approximately 50 percent of dog bites (P. canis) [Talan 1999]. Symptoms of infection—typically cellulitis--develop rapidly, often within 24 hours. Abscess formation, osteomyelitis, septic arthritis, endocarditis, meningitis, and sepsis are possible [Goldstein 1989; Luchansky 2003].

Staphylococcus, Streptococcus, Enterococcus, and Neisseria are the bacteria most commonly isolated from horse, pig, and primate bite wounds [Brook 2009]. Bacterial pathogens commonly found in the oral cavity of terrestrial reptiles include Pseudomonas aeruginosa, Proteus spp., coagulase-negative staphylococci, Salmonella groups IIa and IIIb, and Clostridium spp. Bites from fish and aquatic reptiles are infected most frequently with Vibrio and Aeromonas spp. Infection with Mycoplasma phocacerebrale following bites or other contact with seals/sea lions can 32 cause severe, painful swelling without abscess or other signs of infection [White 2009].

Anaerobic bacteria, most commonly gram-negative bacilli, are recovered from up to 75% of infected dog and cat bite wounds [Brook 1987; Talan 1999]. The isolated strains are most frequently Porphyromonas spp., and less frequently Fusobacterium, Bacteroides, and Prevotella [Brook 2009].

Bartonella henselae is the causative agent of cat scratch disease, which can follow a bite or scratch from a cat. Cat scratch disease most commonly manifests as regional lymphadenitis, but systemic infections, including osteomyelitis and encephalopathy, can occur, particularly in immunocompromised individuals. B. henselae does not cause obvious illness in cats, and up to half of domestic cats carry the organism at some point in their lives, usually as kittens.

Tetanus, caused by Clostridium tetani, is a concern for contamination of any wound, including animal bites. Patients who have had a primary series of three previous immunizations with tetanus toxoid (Td) or tetanus-diphtheria-acellular pertussis (Tdap) vaccine, with the last dose or the last booster within the last five years, are protected against tetanus and do not require a tetanus toxoid-containing vaccine. Patients who have completed the three-dose series but the last vaccine or booster was >5 years ago should receive a booster dose. Patients whose primary tetanus immunization history is unknown or incomplete should receive the full threedose primary tetanus vaccination series. Finally, patients whose primary tetanus immunization history is unknown or incomplete, and whose wound is particularly large, penetrates into muscle, is dirty, or results in visible devitalized tissue, should receive tetanus immune globulin (TIG), in addition to the three-dose primary series [CDC 2006].

Capnocytophaga canimorsus is part of the normal canine oral flora. C. canimorsus is rarely isolated directly from bite wounds—likely partly a consequence of its fastidious growth requirements--but can contribute to severe systemic infections, including sepsis, septic arthritis, meningitis,endocarditis, renal failure, and disseminated intravascular coagulopathy [Lion et al 1996]. Cutaneous manifestations are common, including maculopapular, petechial, or ecchymotic rashes. Cellulitis, necrotizing eschar, and gangrene can require amputation of digits or limbs. Up to a third of infections may be fatal despite the organism's susceptibility to penicillins, fluoroquinolones, and cephalosporins. Liver disease, asplenism, immunocompromising disease or pharmacotherapy, and advanced age are apparent risk factors for C. canimorsus systemic infection. Of 19 patients from whom isolates of C. canimorsus were forwarded to the CDPH Microbial Disease Laboratory between 2002 and 2011, the median age was 62 years (range, 23 to 85 years) and four reported a history of dog bite.

Rodent bites can also lead to bacterial infection. It is estimated that 20,000 persons are bitten by rodents each year in the U.S. [Abrahamian 2011]. Streptobacillus moniliformis (less commonly Spirillum minus) is the cause of rat bite 33 fever--an acute illness characterized by fever, chills, myalgia, recurrent arthralgia/arthritis, and maculopapular rash. Severe manifestations include endocarditis, meningitis, sepsis, and death in up to 10 percent of untreated patients. Rat bite fever is rare and most commonly associated with bites from laboratory or pet rats. Feline, canine, and other carnivore predators of rodents may be transiently infected and transmit the organism through bites.

Viral infection

Compared to bacterial infections, viral complications of bite wounds are rare. Rabies is the chief viral pathogen of concern in bites from a mammal, but other viruses can be transmitted in bites from selected species.

Lymphocytic choriomeningitis virus is found in numerous rodents, most commonly house mice, and transmitted to humans through direct contact, infected aerosols, or bites. Infected rodents are asymptomatic, but in humans the virus causes fever, headache, myalgia, and in rare instances meningitis or meningoencephalitis.

Cercopithecine herpesvirus 1, also known as B virus or Herpesvirus simiae, is a herpes virus enzootic in Old World monkeys including Rhesus, Cynomolgus, and other Asiatic macaques. The prevalence of infection is low among immature macaques, but approaches 90 percent or higher among sexually active adults [Holmes 1995].

Humans who have direct contact with monkeys can be infected with B virus. Bites and scratches are most common, but other contact with tissues and secretions can effect transmission. Infected persons first experience vesicular lesions and abnormal sensation at the bite site, as well as fever, headache, and fatigue, 1-3 weeks after the incident. More severe systemic symptoms such as lymphadenitis, nausea and vomiting, and abdominal pain can also develop. Spread of the virus to the central nervous system leads to increased sensitivity to stimuli, uncoordinated movements, double vision, agitation, and ascending flaccid paralysis—the last frequently contributing to fatal respiratory paralysis.

Human illness due to B virus is rare. Despite many persons with frequent, close contactwith macaque monkeys in the decades since the virus was first described in 1932, only approximately 30 well- documented cases of human infection have been reported. It is estimated that only 0.04 to 2.0 percentof contacts with macaques have the potential to result in exposure to material contaminated with B virus[Cohen 2002]. Nevertheless, all persons who interact with macaques—in research settings, at zoological parks, in veterinary clinics, or as tourists at interactive venues—are at risk for B virus. Guidelinesfor exposure assessment and prophylaxis have been developed by the B Virus Working Group [Cohen 2002].

Medical Management of Animal Bites

The key priorities in management of bite victims are to address immediate or potential life- threatening sequelae, preserve normal tissue/organ function, prevent and treat localized or systemic infection, and maximize long-term cosmetic 34 appearance. Most bite wounds are superficial, not life- threatening, and do not require hospitalization or intensive care. Nevertheless, a thorough examination is essential for even superficial and overtly minor wounds, especially those involving the hands or face, as they may overlie fractures, involve lacerated tendons, vessels, or nerves, penetrate into body cavities or joint spaces, or damage vital structures such as the eye [Fleisher 1999]. Because of the high risk of infection and functional loss for delicate bones and tendons, it is recommended that bite wounds to the hand be referred to a hand surgeon [Brook 1989]. Similarly, a cosmetic surgeon should be consulted early in the management of severe bite wounds to the face.

Bite wound first aid is an indispensable component of management and arguably more beneficial toward preventing infection than prophylactic antibiotic therapy. Vigorous washing with soap andwater within the first few hours following the bite can substantially reduce the risk of infection. Liberal irrigation with normal saline or lactated Ringers solution further decreases the concentration of bacteria, particularly in deep, extensive, or puncture wounds. Most bite wounds are minor and can be managed by the victim at home. However, victims who incur bites that are severe, entail considerable tissue damage, are to parts of the body more sensitive or susceptible to infection, should seek immediate medical care.

Key steps in medical management of bite wounds

• Wash with soap and water
• Liberal irrigation
• Debridement of deviated tissue
• If signs of infection are present:
  • Swab for culture
  • Antibiotic therapy

Bite wounds that are severe, deep, or are more vulnerable to infection (e.g., were not immediately cleansed) should be monitored for signs of infection. Because most bite wound infections are due tothe direct introduction of oral or skin flora into the wound at the time of the bite, evidence of infection is typically apparent within 24- 72 hours. Common initial signs of infection include localized pain,swelling, and discoloration, cellulitis, a purulent or clear discharge, and regional lymphadenitis [Brook 1989, 2009; Smith 2000].

Swabs should be obtained from obviously infected wounds prior to cleaning and submitted for Gram's stain, culture (both aerobic and anaerobic), and antimicrobial sensitivity. Culture and sensitivity should be reserved for wounds that show clinical signs of infection and need for antimicrobial treatment [Goldstein 1989]. In one case series, bacteria were cultured from 80 percent of bite wounds in patients who lacked other evidence of infection (viz., fever, neutrophilic inflammation, or purulentdischarge) [Myers 2008].

Antimicrobial treatment of clinically infected bite wounds ideally should be based on culture and sensitivity results. In the absence of this information, empirical 35 therapy should be directed against those micro-organisms most likely to be present; for dogs and cats the pathogens of principal concern include Pasteurella, Streptococcus, Staphylococcus, andanaerobes.

The cost-effectiveness of routine antimicrobial prophylaxis for bite wounds has yet to be demonstrated by well-designed randomized clinical trials. Nevertheless, antimicrobial prophylaxiscould be considered for higher risk injuries, including a) moderate to severe wounds less than 8 hours old if edema or crushing of tissue is present; b) puncture wounds with possible bone or joint penetration;c) hand wounds; d) wounds adjacent to a prosthetic joint or in the genital area, and e) wounds in immunocompromised patients [Goldstein 2005]. Bite wounds that present more than 72 hours after the incident with no signs of infection do not merit prophylactic antimicrobial treatment.Prophylactic antimicrobials should be selected based on the micro-organisms most likely present in the biting animal's mouth and on the bite victim's skin. Standard recommendation for prophylactic treatment of animal bites is a broad-spectrum antimicrobial that covers both aerobic and anaerobic organisms, administered orally for 3-5 days.