Human and animal bites are the most common hand injuries, and most animal bites are from dogs or cats. A number of factors, including characteristics of the bite, age of the victim, bodily location of injury, time to presentation, severity of wound, and virulence of any inciting bacterium, must be considered when deciding on a treatment strategy. Bites to the hand are particularly deleterious owing to the close proximity of tendons, joints, cartilage and bone. Prompt treatment is always appropriate because, a delayed treatment may result in cellulitis, abscess, infectious tenosynovitis, septic arthritis and even sepsis. Primary concerns include the extent of injury and structures involved, animal type and vaccination status, and the patient’s overall health and tolerance to the injury.
Pathophysiology
- Pasturella multocida is the most prevalent bacterium in cat and dog bite wounds; however, other coexisting pathogens and/or patient comorbidities can increase the likelihood of complications, and in extreme cases, death.
- The bacteriology of dog and cat bite wounds is quite different, with that of dogs being more complex than that of cats.
- In early studies, 32 aerobic and anaerobic organisms were isolated from dog bite wounds. By contrast, 11 were isolated from cat bite wounds.
- Other pathogenic organisms include Staphylococcus aureus, Streptococcal species, and Capnocytophaga canimorsus.
Wound Classification and Characteristics
- Wounds are graded I to III with Grade I wounds being more superficial and Grades II and III requiring hospitalization, surgery and follow-up: 1,2
- Grade I: Superficial lesion, torn or scratched skin, bite canal, crushing injury
- Grade II: Wound extending from the skin to the tendon, muscle or cartilage and joint
- Grade III: Wound with tissue necrosis or tissue loss
- Needle teeth of cats penetrate beneath skin into muscle and fascia; damage to the joint capsule, periosteum, and bone also have been reported.1,2
- Dog bites will usually leave a more serrated wound. Dogs have the ability to both tear and crush tissue and bone combined with a bite force greater than 300 pounds.2
- Pasturella species are the most common in dogs (50%) and cats.1-4
- Differentiate high-risk wounds from low risk wounds:
- High risk: cat bite wounds to the hand, particularly to the wrist; wounds involving joints, bone, and/ or neurovasculature of hand; patient having compromised health; displaying fever, lymphangnitis, erythema and swelling.5
- Low-risk wounds: superficial wounds without signs of infection and/or involvement of the vital deeper hand anatomy in a patient with overall good health.1
- If bacteria are present, infection will be apparent within 12-24 hours of the bite. Infection will frequently be accompanied by rapid swelling, intense pain, and purulent discharge from the puncture site.
- Cat bites may appear innocuous at first; however, a larger proportion of cat bite wounds become infected than do dog bite wounds.2
- Hospitalization is recommended in cases presenting with erythema, swelling and lymphangnitis.5
Related Anatomy
- Bodily structures involved will vary depending on the extent and location of the injury.
- Any bite involving the joints, tendons, ligaments, bones, nerves and/or vasculature of the hand becomes more problematic and consultation with hand surgeon is paramount.
Incidence and Related Conditions
- In the United States, the incidence of animal bites is 200 per 100,000 persons per year, corresponding to 1–2 million dog bites and 0.4 million cat bites.
- More than 50% of the population will sustain an animal bite during their lifetime; of these bites, >90% are from domestic animals, including the family pet or a pet known to the patient.
- Most animal bites occur during childhood (ages 6–17 y).
- Most girls/women are bitten by cats; most boys/men are bitten by dogs.
- Bites from guinea pigs, rats, bats, monkeys and other non-domestic animals are less common.
- Rate of infection from cat bite wounds is double that observed from dog bites.
- 70–80% of animal bites are directed to the limbs.
Differential Diagnosis
- Spider bite (eg, brown recluse)
- Snake bite
- Rabies
- Tularemia (rat bite)
- Nail puncture or impaled on foreign body
- Mechanical injury involving machinery
- Previous injury resulting in infection
Patient Evaluation
- Assess tissue damage
- Evaluate wound for involvement of bones, joints, tendons and/or neurovasculature.
- Consider patient health, antibiotic history, and presence of any prosthetic joints or hardware where bacteria could seed.2 (Kennedy et al, 2015)
- Test hand mechanics2:
- For bites to the dorsum of the proximal phalanx or PIP joint, perform the Elson test to ascertain potential disruption to extensor tendon mechanism
- For bites to the dorsum of the hand or MP joint area, do an extensor lag test and general extensor examination to determine integrity of sagittal band
- Imaging
- Baseline radiographs and monitor for osteomyelitis1,3
- High-resolution musculoskeletal sonography; ultrasound-guided aspiration1-3
- Doppler ultrasound to determine vascular damage3
- Culture for aerobes and anaerobes using tissue specimens or aspirates in cases of infection2
- Gram stain3
- Blood work in conjunction with other exam findings: erythrocyte sedimentation rate (ESR), C-reactive protein (CRP), and white cell count (WBC)with differential
- Photograph the wound
- Obtain vaccination history of the animal1,4,7,8
- Rabies: dog bites are the most common source of rabies worldwide;2,6 however, bats, raccoons, skunks and foxes are the most common carriers of rabies in the United States.2,4 Stray cats are more likely to carry rabies in the United States than dogs.
- Rabies immunoprophylaxis in cases of rabid animal bite1,4
- According to the CDC, no individual has been infected with rabies from a quarantined dog or cat which has been observed to stay healthy during the 10 day quarantine. In this situation, the patient with the bite should not be vaccinated unless the animal develops signs of rabies. If the dog or cat is rapid or suspected, then immediate vaccination is recommended. If the bite source is unknown, the attending physician should consult with a public health officer. For wild animal bites, i.e. bat or raccoon, consider vaccination. Domestic animals like horses, rabbits or chipmunks do not usually carry rabies.
- Immediately and thoroughly irrigate wound with 1% benzalkonium chloride or povidone iodine;2 follow with copious saline irrigation4
