Mammalian Bites

Background and Importance

Of the nearly 4.5 million Americans bitten by dogs every year, approximately half are children. Dog bites more often occur during summer months, with the dog being known to the victim in the majority of cases. The arm and hand are most often bitten. Compared with adults, children are more likely to be bitten on the head and neck. More than 30 dog bite fatalities occur in the United States each year, with American pit bull terriers responsible for the majority of these attacks. Dog bites to the head and neck are of cosmetic concern and are at greater risk of life-threatening injury. The lips, cheek, and nose are the facial structures most likely to be bitten. Young children are at greatest risk for mortality from a dog bite, with exsanguination after carotid trauma as the major cause of death. Craniofacial fractures and penetrating skull injuries have been reported in young children as a result of dog bites. ^,

Dog bite infection rates range from 1% to 30%, with hand bites at highest risk for infection. ^, Bites to the face and scalp are at lower risk for infection due to their rich blood supply. The type of wound appears to influence infection rates, with puncture wounds at higher risk for infection than superficial wounds, lacerations, or wounds limited to skin and soft tissue defects. Cellulitis is the most common type of infection occurring after a dog bite, with tenosynovitis and septic joints more common after hand bites. ^,

Anatomy, Physiology, and Pathophysiology

Dog biting pressures exert forces estimated to be between 300 and 450 pounds per square inch, and dogs often shake the victim vigorously. This causes a “hole and tear” effect resulting in complex lacerations and avulsions.

Clinical Features

Dog bite wounds may cause contusions or ecchymosis without a break in the skin, but more commonly are puncture wounds, lacerations, abrasions, and avulsions. Most dog bite injuries are superficial, and the majority of patients are treated with medications, dressings, or suturing. Larger breeds cause more severe crush injuries due to the higher pressures exerted by their jaws and pose a greater risk of major organ or vessel injury than smaller breeds. Bites from pit bulls and dogs such as German shepherds used in law enforcement are at increased risk for associated orthopedic injuries when compared with other dog bites.

Dog bites to the hand are often occlusive, with the crush injury component increasing the risk of infection. Hands are the most common bite site to develop infection and long-term morbidity due to the number of bones and joints adjacent to the skin surface, small enclosed compartments and fascial planes, and small nerves that are present. Tenosynovitis, septic arthritis, abscess formation, and traumatic digit amputation from dog bites have been reported.

Dog bite wound infections are usually polymicrobial, with an average of five bacterial isolates per wound culture. The responsible bacteria are a mixture of canine flora, environmental organisms, and the victim’s skin flora. Approximately half of dog bite-related infections contain a mixture of aerobic and anaerobic bacteria, with anaerobes more likely to be present in abscesses and purulent wounds. Pasteurella species are the predominant organisms in infected dog bites, present in approximately 50% of cases. Other common anaerobic bacteria include Fusobacterium, Bacteroides, Porphyromonas, Prevotella, and Propionibacterium species. Common aerobic isolates from infected dog bites include streptococci, staphylococci, Neisseria species, Corynebacterium species, and Moraxella species.

Capnocytophaga canimorsus

C. canimorsus was first identified as a cause of systemic infection following dog bites. It is a slow-growing gram-negative rod found in the normal oral flora of both cats and dogs. C. canimorsus is a rare cause of systemic illness. Transmission is related to a bite in the majority of cases, although it has been documented from licks, scratches, and close animal contact. Most cases involve interaction with a dog, with only a small percentage occurring after contact with a cat. In a small percentage of cases no animal source of infection is identified. The disease is more common in individuals over 50 years of age and affects men more than women. Approximately 50% of patients are immunocompromised by splenectomy, alcoholism, malignancy, or other condition.

C. canimorsus may be associated with localized wound infection in a minority of cases but can cause systemic illness, sepsis, and disseminated intravascular coagulopathy (DIC) in immunocompromised individuals. Symptoms of systemic illness typically begin within 10 days of the exposure with a mortality rate as high as 26%.

Diagnostic Testing

Obtain radiographs if there is a likelihood of damage to underlying bones and joints. Bite penetration of joint capsules may be seen as air in the joint on plain radiographs. Scalp bites in children younger than 2 years old may require computed tomography (CT) imaging because intracranial penetration and injury may occur. Preemptive wound cultures sent from fresh bites are rarely of value and are generally not indicated.

C. canimorsus grows slowly and requires special media and growth conditions. In cases of sepsis without an obvious source following a dog or cat bite, C. canimorsus is a consideration and the laboratory should be notified to arrange for appropriate testing. Polymerase chain reaction (PCR) and detection of characteristic gene coding by molecular testing are the gold standard for detection of C. canimorsus infection. ^,

Differential Diagnoses

The differential diagnosis of dog bites includes bites from other domestic pets such as cats, and bites from wild canines including coyotes and wolves. The differential diagnosis for the patient with symptoms suggestive of C. canimorsus infection is extensive. Given its relative rarity and initial symptoms that are indistinguishable from other bacterial infections, C. canimorsus infections are likely to be misdiagnosed and treated as other more common infections initially. A history that includes mammalian bites may be the only diagnostic clue. Consider Neisseria meningitidis or Streptococcus pneumoniae in patients presenting with fever, headache, hypotension, and evidence of DIC.

Management

Management hinges on several historical elements that can be divided into three main parts: (1) the circumstances of the attack, (2) information about the biting animal, and (3) information about the bite victim. The timing of the bite should be determined. Untreated bites more than 6 hours old are at higher risk for infectious complications. When possible, it should be determined whether the bite was provoked or unprovoked, because this may influence the decision to administer rabies prophylaxis. Information about the biting animal should include ownership and immunization status, as well as the animal’s current location. With dog bites, inquiring about the specific breed is important, because certain breeds (e.g., German shepherds, Rottweilers, and pit bulls) deliver more severe bites with a higher risk of damage to underlying structures. Information about the bite victim should include their medical history, current medications, and tetanus status.

The general principles of wound management apply to bite wound patients (see Chapter 50 ). Adequate analgesia is vital to allow for appropriate examination and wound care. Washing the wound with soap and water, ideally with gentle scrubbing with a fine pore sponge to minimize additional tissue trauma, should be performed. A virucidal agent such as povidone-iodine solution should be used on the skin surface surrounding the bite but can be toxic to exposed tissue beneath the skin. Irrigate wounds under pressure with saline or sterile water as this is the most effective method of reducing bacterial counts (see Chapter 50 ).

Examination of bite wounds should ideally take place in a bloodless field to allow for adequate visualization of deep structures. A blood pressure cuff inflated above the patient’s systolic pressure for up to 20 minutes can be used for this purpose. Examine the wound for damage to tendons and possible joint capsule violations. Remove any retained teeth fragments. It is often helpful to extend the margins of puncture wounds in high-risk areas (e.g., overlying joints and tendons) to allow for better visualization. A neurovascular assessment is required in all extremity bites.

Primary closure of mammalian bites is controversial. Ultimately, the benefit of improved cosmesis of primary closure must be weighed against the increased risk of infection. Studies have found no significant difference in rates of infection between repaired dog bite wounds and those left open. Carefully selected dog bites closed primarily have an overall infection rate of 5%, similar to sutured non-bite wounds. When closing bite wounds, use subcutaneous sutures cautiously as the presence of additional foreign material increases the risk of infection. To optimize cosmetic outcome in those wounds deemed too high risk for infection for primary closure, loosely approximating the wound edges using adhesive strips may be performed. Another option is to reevaluate the wound in 48 to 72 hours and if no evidence of infection is present, perform a delayed primary closure.

Given the available data, we recommend the following guidelines summarized in Table 52.1 . Bite wounds of the face and scalp from any species that are less than 6 hours old may be sutured after appropriate wound preparation. It is probably safe to suture most other uncomplicated dog bite wounds, although bites of the hands and feet are at high risk for infection and should rarely be sutured. Puncture wounds, wounds more than 12 hours old, and wounds infected at presentation should not be sutured.

Disposition

Indications for admission after a dog bite are listed in Box 52.1 . Reevaluate patients deemed appropriate for outpatient therapy in 24 to 48 hours.

Background and Importance

Females are twice as likely as males to be the victim of a cat bite, with a peak age incidence in the third decade. Approximately two thirds of cat bites are to the upper extremity, typically involving the hand and fingers. More than 80% of cat bites are caused by a domestic pet that is owned by the victim. Most cat bites are perceived as minor injuries and often go unreported, with the majority of patients seeking medical care only after complications have developed. ^,

Anatomy, Physiology, and Pathophysiology

Cats possess narrow, sharp teeth that easily penetrate skin and the underlying soft tissues like a needle. This mechanism creates a small break in the skin that heals quickly, trapping the bacteria in the deeper structures, often resulting in invasive infection. Because of this, cat bites are more likely to become infected than dog bites, with a reported infection rate of more than 50%. This increased rate of infection is multifactorial: the higher incidence of puncture wounds, a higher proportion of hand bites, an older average age of cat bite victims, and a higher likelihood of Pasteurella species in the feline flora.

Clinical Features

The majority of infected cat bites (90%) will manifest symptoms within 48 hours of the bite. Although not as likely to cause damage by crush injury as dog bites, cat bites are more likely to cause infection in deeper structures, including osteomyelitis and soft tissue abscesses. Of infected cat bite wounds, over 50% present with cellulitis, 20% with tenosynovitis, 15% with osteomyelitis or septic arthritis, and 10% with an abscess. Mixed aerobic and anaerobic bacteria infections are the rule, being present in more than 60% of cases. One-third of cat bite infections are due to only aerobic bacteria. Pasteurella species are the most common pathogens, present in 70% to 75% of infected cat bites, with Bacteroides, Fusobacterium, Porphyromonas, Propionibacterium , and Prevotella species also common anaerobic pathogens. Common aerobes include streptococci, staphylococci, and Moraxella species.

Pasteurella multocida

An important factor contributing to the risk of infection after cat bites is the presence of P. multocida, a highly virulent, facultative anaerobic, gram-negative rod found in the normal oral flora of up to 90% of cats. It can also be found in the oral flora of the majority of dogs and several wild animals. Most human infections with P. multocida are caused by cat bites or scratches, dog bites, or open wounds that have been licked by a dog or cat.

The most common initial manifestation of P. multocida infection is a rapidly spreading cellulitis, usually presenting within 12 to 24 hours of the exposure. A low-grade fever and serosanguineous or purulent discharge at the site may be present. Regional lymphadenopathy is often present. Local infectious complications may occur if left untreated, most commonly subcutaneous abscesses and tenosynovitis. Less often, septic arthritis and osteomyelitis may occur. Systemic illnesses from P. multocida including bacteremia, pneumonia, endocarditis, and meningitis have been described in small case series. Up to 11% of patients with Pasteurella infections will have an associated bacteremia, which carries with it a 30% mortality. The majority of patients with systemic illness have an underlying medical condition, with liver disease, malignancy, and chronic obstructive pulmonary disease being the most frequent comorbidities.

Differential Diagnoses

P. multocida cellulitis has a more rapid onset and progression than cellulitis caused by more common pathogens. Cat scratch disease, an infection caused by Bartonella henselae associated with a cat scratch or bite, presents with regional lymphadenopathy often with an associated fever. A preceding cat or dog bite may be the only initial clue that indicates P. multocida as the cause of a systemic illness, such as bacteremia.

Diagnostic Testing

As with dog bites, obtain radiographs if there is a likelihood of a retained tooth or damage to underlying bones and joints. Bite penetration of joint capsules may be seen as air in the joint on plain radiographs. Preemptive wound cultures sent from fresh bites are rarely of any value and are generally not indicated.