Animal Bites

Etiology

In the US, animal bites requiring emergency care predominantly involve domesticated cats and dogs. Bites from other species, such as monkeys, rats, bears, pigs, ferrets, horses, sheep, alligators, crocodiles, guinea pigs, hamsters, prairie dogs, swans, and sharks, are documented primarily through small studies or individual case reports. Infections resulting from animal bites are typically polymicrobial, involving aerobic and anaerobic bacteria. The specific pathogens present often reflect the oral flora of the biting animal, which their diet and environment can influence. The individual's underlying immune status also determines the severity of secondary infection. 

Additionally, bacteria from the victim's skin or the surrounding environment may contaminate the wound. Common aerobic bacteria in dog bites include Pasteurella, Staphylococcus, and Streptococcus species, with Pasteurella canis being the most frequently isolated organism. Anaerobic bacteria are also prevalent in these infections, the most common being Bacteroides species.[11][12][13] Immunocompromised individuals, particularly those who are asplenic, face an increased risk of infection from Capnocytophaga canimorsus, a gram-negative bacterium commonly found in the canine oral cavity.[14]

Cat bites, often resulting in puncture wounds followed by lacerations, harbor aerobic and anaerobic bacteria. These injuries frequently lead to cellulitis and lymphangitis. Pasteurella multocida is the most commonly isolated pathogen, followed by Streptococcus, Staphylococcus, Neisseria, and Moraxella species. Bartonella henselae, transmitted via flea vectors, is classically associated with cat scratches but can also be isolated from cat bites. Anaerobic organisms, including Fusobacterium and Capnocytophaga species, are also present in these wounds.[12][15]

Less common animal bites, such as those from monkeys and rats, pose unique infectious risks. Monkey saliva contains a bacterial spectrum similar to that of humans, including Neisseria, Streptococcus, and Haemophilus species. Viral transmission is also a concern with monkey bites. Rat bites can transmit Streptobacillus moniliformis and Spirillum minus, both gram-negative rods responsible for rat-bite fever, which carries a 10% mortality rate if untreated. Other potential infections from rat bites include leptospirosis, hantavirus, and tetanus.[16]

Rare infectious diseases can also be transmitted through animal bites. Tularemia, caused by Francisella tularensis, can be contracted via bites from infected animals or arthropods. Plague, resulting from Yersinia pestis infection, is another serious condition associated with animal bites. Fungal infections such as sporotrichosis, caused by Sporothrix schenckii, and blastomycosis, due to Blastomyces dermatitidis, have also been linked to animal bites. Additionally, rabies, a fatal viral encephalitis, remains a critical concern following bites from various mammals.[17][18]

Epidemiology

About 4.5 million people sustain animal bites in the US per year, about 750,000 of which require medical attention. Bites are a combination of laceration, crush, avulsion, and puncture injuries and can cause abscess, cellulitis, and lymphangitis.[19] Most dog bite injuries are soft tissue injuries, fractures, sprains, strains, or crush injuries; less than 1% are skull fractures and damage to intrathoracic, abdominal, pelvic, or intracranial cavities. Dog bites account for 60% to 90%, and cat bites 5% to 20% of bites receiving care. Results from a retrospective study of dog and cat bites in California found dog bites were highest among boys younger than 10 and cat bites highest among women older than 80, with both occurring mainly in residences and during the summer.[20]

The incidence of infection after a dog bite ranges from 2% to 25%. Infection risk is increased by comorbidities, location, and bite size, including bites greater than 3 cm, bites on the hand, and time from bite to medical attention.[21] Dog bites are most prevalent among men and children, and the dog is often known to the patient.[20] Children are commonly bitten on the head, face, and neck due to their proportionately larger heads and shorter stature, while adults sustain bites on the hands and arms. Results from a study of pediatric head and neck dog bites over 9 years found approximately 43% were between 2 and 6 years, 77% had injuries to face or mouth, 82% occurred in the home, 61% of the bites were from animals known to the patient, and 33% of the dogs were pitbulls.[22] In a longitudinal study of domestic animal bites in the US, facial injuries were highest among those younger than 5 and lowest among those older than 50. Bites on the head, neck, and face have a high mortality, especially in children, due to hemorrhage and proximity to the brain, increased risk for skull fracture, and brain abscesses. There is a high risk of exsanguination and injury to aerodigestive structures from neck injuries. Chest bites pose a risk for pneumothorax and spinal cord injury.[23]

Cat bites have an infection rate of about 30% due to the inoculation depth of pathogens. In children, cat bites are most common on the upper limbs and, in the very young, on the head and neck. Those bitten by cats often present after the bite becomes infected.[4][24] Most cat bites affect only soft tissue, with less than 1% impacting deeper musculoskeletal structures and 0.02% involving internal elements.[20] Cat scratch fever is more common in children.[20] Cases of tularemia transmission from cats have been reported to be higher in central and western US and parts of Europe, Asia, and Africa, and the incidence of tularemia in animals in the US is increasing.[20][25][26] Rat bites account for less than 1% of emergency department visits in the US, with a stated 10% risk of infection. The mortality of rat bite fever is reported to be 13%, primarily due to sepsis, pneumonia, and cardiac dysfunction.[16][27] While monkey bites are more prevalent globally, they are rare in the US and can pose significant risks of exposure to numerous pathogens. Monkeys may carry rabies and other viruses that can be fatal to humans, including herpes B from Cercopithecine herpesvirus.[28]

Pathophysiology

The initial injury from an animal bite is the mechanical trauma of teeth puncturing, tearing, or avulsing soft tissue, and high pressure causing crush injuries or even bony fractures, depending on the force of the bite. Occasionally, bites can invade internal compartments and pose an immediate threat to life. Dog bites are more commonly macerated due to the ripping and tearing forces involved.

Cat bites are narrow and deep and may appear innocuous until infection is evident; pathogens are transmitted through a break in the skin's defensive barrier. Cat bites have the highest infection rate and often trigger significant localized or systemic inflammatory responses, leading to osteomyelitis, sepsis, meningitis, endocarditis, and pneumonia. Microbes travel in the blood and along nerves, bind, and replicate, causing infection and inflammation. For example, the rabies virus can be injected into deep tissues, where it replicates, binds to receptors at the neuromuscular junction, and travels within nerves, where it further replicates and infects the brain.[29][30]  

History and Physical

A comprehensive history and physical examination are essential when evaluating a patient who has sustained an animal bite. This will guide appropriate management and prevent complications.

History

Key elements to elicit during the patient's history include:

• Incident details
  • Ascertain the time, location, and circumstances of the bite. Determine if the bite was provoked or unprovoked, as unprovoked attacks may raise concerns about rabies risk.
• Animal information
  • Identify the species involved and, if possible, its health status, vaccination history, behavior at the time of the incident, and current whereabouts.
• Wound characteristics
  • Note the number, location, and type of wounds (eg, punctures, lacerations, scratches).
• Prehospital treatment
  • Inquire about any first aid measures taken before seeking medical care.
• Medical history
  • Review the patient's medical background, focusing on allergies; immunosuppressive conditions; chronic diseases such as diabetes mellitus, human immunodeficiency virus, or sickle cell disease; and tetanus and rabies vaccination status.

Physical Examination

A thorough physical examination should assess:

• Wound assessment
  • Evaluate the size, depth, and type of injury. Dog bites often result in crush injuries due to their strong jaws, leading to a combination of tearing, puncture wounds, and avulsions. In contrast, cat bites typically cause deep puncture wounds that can penetrate bones and joints, increasing the risk of septic arthritis and osteomyelitis.
• Signs of infection
  • Look for redness, swelling, warmth, pus, or lymphangitis, which may indicate an infection.
• Neurovascular status
  • Assess for any nerve or vascular injury by evaluating sensation, motor function, and distal pulses.
• Range of motion
  • Test the movement of affected and adjacent joints to detect potential tendon or joint involvement.
• Foreign bodies
  • Inspect for embedded teeth or debris within the wound.
• Regional lymphadenopathy
  • Examine nearby lymph nodes for swelling or tenderness, which may suggest systemic involvement.

Infection Presentations

Infections resulting from animal bites can manifest with various clinical presentations:

• Cat bites
  • Infections typically present within 12 to 18 hours after injury, characterized by erythema, induration, pain, and purulent drainage.
  • Cat scratch disease, caused by Bartonella henselae, usually starts as a blister at the bite site followed by lymphadenopathy, myalgias, fever, and, less commonly, meningoencephalitis, endocarditis, or orbital cellulitis.
  • Tularemia may present with fever, headache, and weakness.[1]
• Rabies
  • Initial symptoms include myalgias, fever, and pain, progressing to paresthesias, confusion, vomiting, salivation, hallucinations, and coma.[31] Rabies contracted from bats may differ in presentation from that contracted from dogs, often exhibiting motor-sensory dysfunction and tremors, usually with more rapid progression and high mortality.[7][32][33]
• Rat bites
  • Rat-bite fever can cause fever, headache, nausea, vomiting, rigors, lymphadenopathy, arthritis with pain and erythema, maculopapular rash, and petechiae of the extremities. Additional manifestations may include pericarditis, myocarditis, endocarditis, vasculitis, meningitis, hepatitis, and pneumonia. Leptospirosis can present severely with pulmonary hemorrhage, jaundice, and renal failure.[16]

Evaluation

When evaluating a patient with an animal bite, a thorough assessment involving laboratory tests, imaging studies, and other diagnostic procedures is crucial to determining the extent of injury, identifying potential infections, and guiding appropriate treatment.

Laboratory Tests

• Wound cultures
  • If there are signs of infection, such as redness, swelling, warmth, pus, or lymphangitis, obtaining a wound culture before initiating antibiotic therapy can help identify the causative pathogens and guide targeted treatment.
• Blood cultures
  • Blood cultures are warranted in patients exhibiting systemic signs of infection or sepsis to detect bacteremia and guide antibiotic therapy.
• Complete blood count 
  • A complete blood count may be performed to assess for leukocytosis, which can indicate an infectious process.

Radiographic Studies

• X-rays
  • These are recommended in the following scenarios:
    • Fracture detection
      • To identify bone involvement or fractures resulting from the bite
    • Foreign body identification
      • To detect retained foreign bodies, such as teeth fragments or debris
    • Gas in soft tissues
      • To reveal gas formation indicative of anaerobic infections
• Advanced imaging
  • This includes computed tomography and magnetic resonance imaging. These modalities are considered when there is suspicion of deep tissue involvement, abscess formation, or complications not evident on standard x-rays.

Additional Diagnostic Considerations

• Tetanus prophylaxis
  • Assess the patient's immunization status and administer tetanus toxoid or immunoglobulin as needed, especially for deep or contaminated wounds.
• Rabies risk assessment
  • Evaluate the risk based on the type of animal, its behavior, and the prevalence of rabies in the area. If indicated, initiate postexposure prophylaxis.

Special Considerations

• Rat bites may require a biopsy. Skin lesion biopsies may reveal leukocytoclastic vasculitis, a key finding in rat-bite fever. 
• If the biting animal is available, conducting serological tests or a necropsy can provide valuable information regarding potential zoonotic infections.[1][5][34]

Treatment / Management

Timely source control is essential in managing animal bites. Proper wound care, including thorough irrigation, debridement of devitalized tissue, and prompt antibiotic administration when indicated, significantly reduces the risk of infection. Irrigation can be performed with a syringe or a high-pressure system, using 1% to 5% povidone iodine in normal saline or potable water in out-of-hospital settings. Anesthesia may be required for extensive wounds for thorough exploration, while biopsies and cultures may guide treatment.[6][35]   (B3)

Vaccination status should be updated, including tetanus, diphtheria, and pertussis. Tetanus prophylaxis is indicated if more than 5 years have elapsed since the last dose; immunoglobulin and vaccination are required if more than 10 years have elapsed. Rabies post-exposure prophylaxis should be administered in cases of uncertain exposure, particularly in regions with a high prevalence of rabies. Prophylaxis consists of a rabies vaccine with or without intradermal immunoglobulin. According to the World Health Organization’s 2018 guidelines, high-risk individuals should receive rabies vaccination on days 0, 3, 7, and 14, plus immunoglobulin. Immediate rabies prophylaxis is recommended for bites near the brain.[1][35] 

Prophylactic antibiotics targeting oral flora and skin microbes are warranted for all bites penetrating the epidermis or deeper.[21] High-risk areas include the hands, feet, face, genitals, and puncture wounds. Immunocompromised individuals, particularly those with chronic diseases like diabetes, face an increased risk of infection.[36] Cat bites are inherently high-risk and typically left open. The infection risk depends on whether to close the wound or leave it open for uncomplicated dog bites. If closed, a 7-day course of amoxicillin-clavulanate is prescribed. The Infectious Disease Society of America advises leaving bite wounds open unless there is a cosmetic concern, such as on the face. Deep puncture wounds and those involving joint spaces should not be closed. Consultation with surgical specialists, including plastic, orthopedic, or general surgeons, may be necessary. Uncomplicated wounds can be safely closed after extensive cleaning and debridement if presenting within 8 hours of injury.[37][36] (A1)

Amoxicillin-clavulanate is the preferred prophylactic antibiotic, with extended-spectrum cephalosporins recommended for patients who are allergic to penicillin. Alternative regimens include doxycycline or trimethoprim-sulfamethoxazole (TMP-SMX) plus metronidazole or clindamycin.[1][37] Systemic infections or those involving joints or intracavitary spaces require broad-spectrum intravenous antibiotics, such as ampicillin-sulbactam, and hospital admission for further care. In pediatrics, the first-line treatment is amoxicillin-clavulanate (50 mg/kg), with clindamycin plus TMP-SMX or an extended-spectrum cephalosporin used in cases of penicillin allergy. Antibiotic therapy is adjusted based on culture results, with methicillin-resistant Staphylococcus aureus (MRSA) coverage including TMP-SMX, doxycycline, or clindamycin.[38][39][40] In a 2016 prospective study by Jaindl involving over 5000 bite cases at a level 1 trauma center, 14% of dog bites were infected compared with 58% of cat bites.[41] Puncture wounds and wounds >3 cm had a higher risk of bacterial infection.  (A1)

Less common pathogens require targeted treatment based on culture results and established guidelines. Bartonella henselae, the causative agent of cat scratch disease, is treated with azithromycin. Francisella tularensis, responsible for tularemia, is treated with streptomycin or gentamicin for moderate-to-severe cases and with tetracycline or doxycycline for milder infections. Rat bites necessitate thorough irrigation, with fever cases treated using intravenous penicillin G or ceftriaxone. Cardiac involvement following a rat bite requires prolonged therapy with streptomycin and gentamicin. Leptospirosis is managed with supportive care, amoxicillin, and doxycycline.[16][42][43] 

Monkey bites, which carry a high risk for infection due to deep puncture wounds, require irrigation, rabies, and tetanus prophylaxis, and antiviral prophylaxis for herpes B infections.[6][44] Rabies prophylaxis is particularly critical for individuals exposed to bats, including those with suspected or confirmed contact with bat saliva or neural tissue through open wounds or mucous membranes. (B3)

Differential Diagnosis

Several diseases and conditions can mimic the presentation of an animal bite, leading to potential misdiagnosis if a thorough history and examination are not performed. Skin infections such as cellulitis, abscesses, and necrotizing fasciitis may present with erythema, warmth, swelling, and pain similar to an infected bite wound. Insect bites, particularly from spiders, ticks, and fleas, can cause localized inflammation, ulceration, and systemic symptoms resembling zoonotic infections from animal bites. For example, Loxosceles reclusa (brown recluse) spider bites may lead to necrotic ulcers that could be mistaken for infected bite wounds.[45]

Human bites and self-inflicted injuries can also resemble animal bites, especially when dental patterns are unclear. Child abuse, domestic violence, and self-harm behaviors must be considered when the reported history does not align with the clinical findings. Crush injuries, thermal or chemical burns, and pressure ulcers can have irregular wound borders and necrosis, mimicking a poorly healing or infected bite wound. Additionally, atypical infections such as Mycobacterium marinum, Eikenella corrodens, and fungal infections can present with chronic ulcerations and draining wounds similar to those seen in animal bite complications.

Autoimmune diseases like pyoderma gangrenosum can lead to rapidly progressive ulcerations that may be mistaken for infected bites, primarily when associated with systemic inflammatory conditions such as inflammatory bowel disease or rheumatoid arthritis. Vasculitides, including leukocytoclastic vasculitides and granulomatosis with polyangiitis, can present with ulcerating lesions and purpuric changes resembling an infected or necrotic bite wound. In such cases, biopsy and additional laboratory testing, including inflammatory markers and autoantibody panels, may be necessary to differentiate these conditions from animal bites.

Maintaining an index of suspicion for related injuries such as bony fractures, tetanus, and rabies is essential in the setting of a known animal bite.[46] Fractures may result from the crushing force of larger animal bites, necessitating radiographic evaluation. Additionally, tetanus prophylaxis should be considered based on immunization history, and rabies prophylaxis should be administered in cases of suspected exposure, particularly with bites from high-risk animals or those in endemic regions.

Prognosis

The prognosis for most animal bites is excellent with prompt intervention and appropriate infection control measures. However, dog bites result in approximately 30 to 50 fatalities annually.[47][48] The severity of these injuries has been increasing, often involving deeper structures and necessitating more complex interventions, including hospitalization and surgical management.[49][50]

Complications

The complications of animal bites vary based on the biting mechanism and the pathogens involved. Common infectious complications include cellulitis, abscess formation, osteomyelitis, tenosynovitis, endocarditis, and meningitis. MRSA can be transmitted bidirectionally between animals and humans, posing a recurrent risk of infection. In immunocompromised individuals, anaerobic and gram-negative bacteria such as Bacteroides, Fusobacterium, Neisseria, and Prevotella can lead to severe morbidity and mortality. Without appropriate prophylaxis, tetanus and rabies can have fatal consequences. Additional complications include delayed-onset injuries, such as tendon rupture or nerve damage. Psychological effects, including post-traumatic stress disorder, may also develop following an animal bite in both adults and children.[51][52]