Bites including human and animal

OVERVIEW: What every practitioner needs to know

Are you sure your patient has a bite? What should you expect to find?

  • What kind of animal caused the injury?

  • How did the animal bite occur? For example, location, situation, the animal’s aggression, ownership of the animal.

  • When did the injury occur?

  • A thorough past medical history including all medical conditions and medications.

  • Allergies

  • Tetanus immunization status

Physical exam
  • Depth of injury

  • Vascular supply of the site

  • Evaluate joints close to the animal bite

  • Nerve function

  • Tendon function with extension and flexion

Signs of infection from the animal bite
  • Fever, erythema, swelling, tenderness, purulent drainage, foul odor, and lymphangitis are key findings that suggest a wound is infected.

  • Inflamed, warm, tender joints when a bite is close to a joint are concerning for septic arthritis.

  • Clenched fist injury (sustained during fist fights) presents with small wounds around the metacarpophalangeal (MCP) joints and will have erythema, purulent drainage and tenderness.

  • On physical exam for clenched fist injury or any animal bite that is close to a MCP joint, the wound should be examined on both extension and flexion of the joint to ensure the tendon sheath has not been compromised.

  • Pasteurella multocida infection can occur after dog or cat bites. These infections develop rapidly and can start showing signs of infection such as erythema, edema and tenderness 12-24 hours after a bite.

  • Signs of infection from a cat bite usually occur 12 hours after the bite.

  • Signs of infection from dog bites usually occur 24 hours after the bite.

  • Capnocytophaga canimorsus can cause a systemic illness after a dog bite. Systemic signs such as fevers, petechial rash, maculopapular rash, cellulitis, hypotension, and sepsis will be seen in these patients.

  • Cat scratch disease is a syndrome characterized by regional lymphadenopathy that is painful. There can also be low-grade fever, malaise, fatigue, headache, nausea, and anorexia.

  • Rat-bite fever can occur after a bite from rats, mice, squirrels, or gerbils. It can also occur through exposure to rat feces or urine. Symptoms start 3-7 days after exposure and include rigors, fever, petechial or purpuric rash.

  • Rabies should always be a consideration with animal bites. In the US most rabies cases are transmitted from bats or non-domesticated animals instead of domestic animals. Prodrome symptoms include fever, malaise, anorexia, vomiting, paresthesia, pruritis, or pain. Encephalopathic symptoms such as hallucinations, bizarre behavior, agitation, and hydrophobia can also occur. Paralysis followed by coma and death are likely.

How did the patient develop disease from the bite? What was the primary source from which the infection spread?

  • The source of the infection usually comes from the oral flora of the animal’s mouth. It can also be from the feces of the animal, soil, or the natural flora of the skin of the victim.

  • Cat bites are usually puncture wounds. The long, slender teeth can cause deeper wounds which are at more of a risk for osteomyelitis and septic arthritis.

  • Dog and cat scratches can introduce infection as well.

  • Dog bites most often are on extremities. For children younger than 4 years old, the injuries are more likely to be on the neck and face.

  • Human bites are frequently related to aggression. Clenched fist injury are the most prevalent human bites. They can be severe due to the close proximity of teeth to the MCP joints. With extension of the hand, bacteria can be inoculated into the joints.

Which individuals are of greater risk of bites?

Risk factors for animal bites
  • Pets are the most common offending animal. Most patients who are bitten are familiar with the animal that bit them.

  • Human bites become infected more frequently.

  • Most mammalian bites are caused by dogs.

  • Dog bites occur most frequently in the age group of 2-19.

  • German shepherds, pit bull terriers and rottweilers account for the majority of dog bites.

  • Dog bites occur more often in males.

  • Cat bites occur more often in females.

Risk Factors for infection
  • Location on hand, foot, major joint, scalp, or face

  • Puncture wounds

  • Crush injuries

  • Treatment delay greater than 12 hours

  • Age greater than 50 years old

  • Immunosuppressive agents

  • Corticosteroids

  • Splenectomy

  • Alcohol abuse

  • Diabetes Mellitus

  • Systemic Lupus Erythematosus

  • Vascular disease

  • Pre-existing edema of affected extremity

What laboratory studies should you order and what should you expect to find?

Results consistent with the diagnosis

  • Complete blood count

  • Gram stain from wound

  • Culture from wound

  • Blood cultures, aerobic and anaerobic

  • ESR and CRP to assess for osteomyelitis or septic arthritis

  • Peripheral blood smears should be used to look for eschariform lesions when concerned about Capnocytophaga canimorsus infection.

Results that confirm the diagnosis

  • Cultures may not come back positive even when wound is infected

  • Leukocytosis suggests infection

  • Elevated C-reactive protein (CRP) and erythrocyte sedimentation rate (ESR) can be seen with osteomyelitis

What imaging studies will be helpful in making or excluding the diagnosis of bites?

  • Radiographs can help identify osteomyelitis or fracture.

  • Ultrasound can also be used to evaluate for abscess.

  • Computed tomography (CT) scan can be used for evaluation of head and neck wounds.

What consult service or services would be helpful for making the diagnosis and assisting with treatment?

If you decide the patient has a bite, what therapies should you initiate immediately?

Surgical consult should be considered when a wound is deep and has potentially penetrated bones, tendons or joints. A surgical consult should also be considered in patients with facial wounds and possible vascular or nerve injury.

Key principles of therapy:

  • Establishment of last tetanus vaccination is imperative. If the patient has had a tetanus shot in the past ten years, then a tetanus toxiod booster should be given. Tetanus immune globulin should be given if the patient has not received a tetanus vaccination in the past ten years or it cannot be confirmed.

  • Empiric antibiotic therapy should be started on all wounds inflicted from an animal bite unless they have presented after 72 hours and have no sign of infection.

  • Rabies vaccine should be given in patients who have been bitten by an animal that is high risk for rabies. If it is deemed high risk then the patient should be given rabies immune globulin and the rabies vaccine. If the patient has already been immunized for rabies then the patient will not need the rabies immune globulin. It is important to start these things as early as possible.

  • Patients who have been bitten by a human need to be evaluated for hepatitis B. If the patient has not been vaccinated or is negative for anti-hepatitis B antibodies and is bitten by someone who is hepatitis B positive, should be given hepatitis B immune globulin and the hepatitis B vaccine. If this information is unavailable then the hepatitis B vaccine series should be started.

  • HIV post-exposure prophylaxis should be discussed with all patients who have been exposed to a human bite. The risk of infection with HIV is small after a bite but is possible.

1. Anti-infective agents

If I am not sure what pathogen is causing the infection what anti-infective should I order?

Empiric Therapy:

  • For dog and cat bites, Staphylococcus aureus, Pasteurella species, Capnocytophaga canimorsus, Streptococci and oral anaerobes should be covered.

  • For human bites, Staphylococcus aureus, H. influenzae, and beta-lactamase positive anaerobes should be covered.

  • Extended spectrum beta-lactamase antibiotics should be used for coverage of animal and human bites.

  • Penicillin allergic patients can be given clindamycin with trimethoprim/sulfamethoxazole or a fluoroquinolone.

Table I presents common bacteria from bite wounds.

Table I.
Dog Bites Cat Bites Human Bites
Aerobic and FacultativeAnaerobic Bacteria

Aeromonas hydrophilic

Acinetobacter sp.

Actinobacillus sp.

Bacillis sp.

Brucella canis

Capnocytophaga canimorsus

Corynebacterium sp.

Eikenella corrodens

Enterobacter cloacae

Enterococcus sp.

Escheria coli

Hemophilus aprophilus

Klebsiella sp.

Moraxella sp.

Neisseria sp.

Pastuerella multocida

Pseudomonas sp.

Staphylococcus aureginosa

Staphylococcus epidermidis

Staphylococcus sp.

Streptococci sp.

Acinetobacter sp.

Corynebacterium sp.

Enterobacter cloacae

Neisseria sp.

Pasteurella multocida

Staphylococcus aureus

Staphylococcus epidermidis

Streptococcus sp.

Acinetobacter sp.

Corynebacterium sp.

Eikenella corrodens

Enterobacter cloacae

Enterobacter sp.

Escherichia coli

Haumophilus sp.

Klebsiella pneumonia

Microccus sp.

Moraxella sp.

Neisseria sp.

Nocardia sp.

Proteus mirabilis

Pseudomonas aeruginosa

Serratia marcescens

Staphylococcus aureus

Staphylococcus epidermidis

Staphylococcus saprophyticus

Alpha and beta-hemolytic streptococcus

Anaerobic Bacteria Actinomyces sp.

Bacteroides fragilis

Fusobacterium sp.

Leptotrichia bacillus

Petococcus sp.

Peptostreptococcus sp.


Prevotella sp.

Veillonella parvula

Weeksella zoohelicum

Bacteroides sp.

Bartonella henslae

Fusobacterium sp.



Actinomyces sp.

Bacteroides sp.

Clostridium sp.

Eubacterium sp.

Fusobacterium sp.

Peptostreptococcus sp.

Prevotella sp.

Veillonella sp.

Table II presents oral antibiotic therapy for bite wounds.

Table II.
Antibiotic Dose Alternative

Broad Spectrum for

Animal and Human Bites

Amoxicillin/Clavulanic acid
875/125 mg every 12 hours
500 mg every 12 hours
1 gram daily
Doxycycline + Metronidazole or Clindamycin
Dog Bites Amoxicillin/Clavulanic acid
875/125 mg every 12 hours Doxycycline + Metronidazole or Clindamycin
Cat Bites Amoxicillin/Clavulanic acid
Penicillin and/or dicloxacillin
875/125 mg every 12 hours Doxycycline + Metronidazole or Clindamycin
Human Bites Amoxicillin/Clavulanic acid
Penicillin and/or dicloxacillin
875/125 mg every 12 hours Doxycycline + Metronidazole or Clindamycin
Empiric intravenous antibiotic therapy for animal and human bites
  • Ampicillin/sulbactam 3 grams every 6 hours, Piperacillin/tazobactam 4.5 grams every 8 hours or ticarcillin/clavulanate 3.1 grams every 4 hours

  • Alternative therapy includes a fluoroquinolone with metronidazole.

More antibiotic information
  • Amoxicillin/Clavulanic acid has good coverage for both human and animal bites.

  • In children, doxycycline and fluoroquinolones should be avoided and the dosing will be different.

  • 40% of bite wounds have beta-lactamase producing organisms.

  • First generation cephalosporins are not as effective as amoxicillin/clavulanic acid due to resistance of anaerobic bacteria and E. corrodens.

  • Clindamycin and penicillinase-resistant penicillins should not be administered without penicillin due to poorer activity against P. multocida.

  • Moxifloxacin and ciprofloxacin also offer broad spectrum coverage for animal and human bites, however, this cannot be used in children.

  • E. corrodens is susceptible to penicillin, ampicillin and quinolones but resistant to oxacillin, methicillin, nafcillin and clindamycin.

  • Treatment course is usually a 7-14 day course for animal and human bites.

  • For joint or bone infections a minimum 21 day course should be given.

2. Next list other key therapeutic modalities.

  • What other therapies are helpful for reducing complications?

    The wound should be irrigated with normal saline using pressure irrigation.

    The wound should be explored for any damage to the nerves, vasculature, tendon sheath, joints or bones.

    Debridement of wound, removal of devitalized tissue, and all foreign bodies should be removed.

    Puncture wounds should not be sutured.

    Elevation of the extremity should be done if there is swelling.

  • Controversial therapies:

    Suturing of wound is not always recommended. Wounds that are deep puncture wounds should not be sutured. Wounds with anaerobic bacteria should also not be sutured. Primary closure of wounds that present within 24 hours and has no signs of infection is controversial.

    Prophylactic antibiotic therapy is controversial. Treating an animal bite wound has not been shown to prevent infection. However, in high-risk wounds such as deep puncture wounds, hand wounds or wounds that require surgical intervention, treatment with antibiotics is advised.

What complications could arise as a consequence of bites?

  • Systemic infection, septic arthritis, cellulitis and osteomyelitis could all arise secondary to an animal or human bite.

  • Endocarditis, meningitis, brain abscess and sepsis with disseminated intravascular coagulation are also possible but less frequent complications.

  • Rabies is a possible complication as well.

What should you tell the family about the patient's prognosis?

  • Patients who are immunocompromised, diabetic, cirrhotic or have had a splenectomy are at higher risk for systemic infection.

  • Patients with splenectomies, hepatic dysfunction or those who are immunocompromised are at increased risk for sepsis due to Capnocytophaga carnimorsus from dog bites. Severe infection from Capnoctyophaga can lead to septic shock with renal failure. Treatment is IV penicillin.

  • Prognosis depends on how severely the bite has damaged neurovascular, bone or joint structures.

  • Animal and human bites often do not cause severe infection but this is a potential complication.

Add what-if scenarios here:

  • Patients with osteomyelitis, septic arthritis or sepsis from animal or human bites may need hospitalization for IV antibiotics and more aggressive care.

How do you contract disease from bites and how frequent is this disease?

  • Animal and human bites comprise 1-2% of ER visits.

  • $100 million dollars a year are spent on treating dog bite wounds.

  • 1-2 million animal bite wounds occur each year in the US.

  • Most of these are minor injuries and do not require medical attention.

  • Children are the most common victims for animal bites.

Dog Bites
  • 80-90% of bites wounds are from dogs.

  • 1/3 of dog bites are from family dogs.

  • Infection rate of dog bites is 15-20%.

  • Dog bites are more frequently on extremities with the upper extremities being more common than lower extremities. Bites to the face and neck are the next most common site for dog bites.

  • Dog bites are more likely to occur in warmer months.

  • Dog bites occur more commonly in males.

Cat Bites
  • 2nd most common type of mammalian bite in the US.

  • 5-15% estimated annual incidence.

  • Cat bites have a 30-50% infection rate.

  • Puncture wounds to the hands are the most common.

  • Cat bites occur more commonly in females.

  • Incidence is higher in the spring and summer.

Human Bites
  • 3rd most frequent type of mammalian bite in the US.

  • Human bites are usually due to aggressive behavior such as fighting, sports, school activity or sexual activity.

  • The infection rate of human bites is 10-50%.

  • Human bites commonly involve the hand.

  • Human bites occur most frequently on weekends and have a higher incidence in the spring.

  • Incidence is higher in males than females.

What pathogens are responsible for this disease?

Pathogens isolated from dog, cat and human bites are listed above.

Monkey or simian bites also have aerobic and anaerobic bacteria as the pathogen. The bacteria isolated from these animal bites include Streptococcus, Staphylococcus spp, Neisseria spp, Enterobacteriaceae, E. corrodens, and Fusobacterium. A potential viral pathogen is Herpesvirus simiae or is also known as B virus.

Horse bite wound pathogens include S. aureus, Neiserria spp, E. coli, Actinobacillus ligniersii, Streptococcus spp, Pasteurella species, Bacteroides ureolyticus, B. fragilis, Prevotella melaninogenica, and Prevotella heparinolytica.

Aquatic animal bite infections can have Vibrio and Aeromonas spp.

Rodent bites can cause rat-bite fever through Streptobacillus moniliformis or Spirillum minus/minor.

How do these pathogens cause disease?

The pathogens associated with animal and human bites are transmitted through direct inoculation from the oral flora of the offending animal. The pathogen can also come from the natural skin flora of the victim.

What other clinical manifestations may help me to diagnose and manage bites?

History and physical exam findings are described above.

How can bites be prevented?

  • Neutered and female dogs have been shown to be less aggressive than other dogs and may reduce the risk of dog bites.

  • Avoiding aggressive dog breeds such as rottweilers or pit bulls may reduce the risk for dog bites.