Overview
Human & Animal Bites
Topic Overview
Summary
Bite wounds carry high risk of polymicrobial infection due to the diverse oral flora of humans and animals. Cat bites have particularly high infection rates (~50%) due to deep puncture mechanisms. Human bites, especially "fight bites" over MCP joints, can cause devastating hand infections requiring surgical intervention. Management includes thorough wound care, appropriate antibiotic prophylaxis/treatment, tetanus assessment, and consideration of rabies exposure.
Key Facts
- Dog bites: Most common; Pasteurella, Capnocytophaga (fatal sepsis risk in asplenic patients)
- Cat bites: Deep punctures, 50% infection rate, Pasteurella multocida, tenosynovitis
- Human bites: Eikenella, oral streptococci; "fight bite" = surgical emergency
- Antibiotics: Co-amoxiclav first-line (covers Pasteurella + anaerobes)
- Tetanus: Ensure immunisation up to date
- Rabies: Consider in endemic area exposures
Clinical Pearls
Cat bites look innocuous but have the highest infection rate (~50%) — ALWAYS give antibiotics
"Fight bite" over MCP joint — assume joint penetration until proven otherwise; needs surgical washout
Capnocytophaga canimorsus from dog bites causes fulminant sepsis in asplenic/immunocompromised patients
Why This Matters Clinically
Bite wounds are extremely common in emergency medicine. Underestimating the risk — particularly with cat bites and fight bites — leads to devastating complications including septic arthritis, osteomyelitis, and tenosynovitis. Appropriate wound care and antibiotic prophylaxis prevent most complications.
Visual Summary
Visual assets to be added:
- Bite wound management algorithm
- Hand anatomy with "fight bite" location diagram
- Wound care technique illustration
- Pasteurella multocida microscopy
Epidemiology
Incidence & Prevalence
- Dog bites: 80-90% of mammalian bites; 150-300/100,000/year in UK
- Cat bites: 5-15% of mammalian bites
- Human bites: 2-3% of bite wounds presenting to ED
- ED presentations: ~250,000 bite wounds/year in UK
Demographics
- Dog bites: Children most commonly affected; face and neck in young children
- Cat bites: Predominantly adult women (cat owners)
- Human bites: Young adult males (often alcohol-related "fight bites")
Infection Rates
| Animal | Infection Rate |
|---|---|
| Dog | 5-15% |
| Cat | 30-50% |
| Human | 10-20% (higher for "fight bites") |
Pathophysiology
Mechanism of Injury
| Animal | Bite Characteristics |
|---|---|
| Dog | Crush/tear injuries; devitalisation of tissue |
| Cat | Sharp canines → deep puncture wounds |
| Human | Variable; occlusional or clenched fist |
Microbiology
Dog Bites:
- Pasteurella canis/multocida (50%)
- Staphylococcus aureus
- Streptococcus species
- Capnocytophaga canimorsus (rare but serious)
- Anaerobes (Fusobacterium, Bacteroides)
Cat Bites:
- Pasteurella multocida (75%) — rapid onset infection within 24h
- Staphylococci
- Streptococci
- Bartonella henselae (cat scratch disease)
Human Bites:
- Eikenella corrodens
- Oral streptococci (viridans group)
- Staphylococcus aureus
- Anaerobes (Prevotella, Fusobacterium)
- Hepatitis B/C, HIV (rare, theoretical risk)
"Fight Bite" Pathophysiology
- Clenched fist strikes teeth
- Puncture over MCP joint
- Wound is small but penetrates to joint capsule/tendon
- When fist opens, wound moves away from joint → foreign material/bacteria sealed in
- Result: Septic arthritis, tenosynovitis, osteomyelitis
Clinical Presentation
Fresh Wound (Pre-Infection)
Established Infection (12-48 hours post-bite)
Red Flags — Concerning Features
| Feature | Suggests |
|---|---|
| Rapid onset (<24h) | Pasteurella infection (cat bite) |
| Wound over MCP joint | Fight bite — joint penetration |
| Sausage finger | Tenosynovitis |
| Limited finger flexion | Tendon involvement |
| Systemically unwell | Bacteraemia/sepsis |
| Immunocompromised | Higher risk of severe infection |
History of bite (may be concealed in assault)
Common presentation.
Variable wound appearance (puncture vs laceration vs avulsion)
Common presentation.
Minimal initial symptoms
Common presentation.
Clinical Examination
Wound Assessment
- Mechanism: Dog vs cat vs human
- Location: Hand, face, proximity to joints/tendons
- Depth: Superficial vs deep puncture
- Time since injury: Fresh vs delayed presentation
- Wound characteristics: Puncture, laceration, avulsion, tissue loss
Hand Examination (Critical for Hand Bites)
| Assessment | Technique |
|---|---|
| Tendon function | Test each tendon individually (FDP, FDS, extensors) |
| Range of motion | Active and passive |
| Joint stability | Stress testing |
| Neurovascular | Capillary refill, two-point discrimination |
| Signs of infection | Erythema, swelling, warmth, discharge |
Kanavel's Signs (Flexor Tendon Sheath Infection)
- Fusiform (sausage-shaped) swelling of finger
- Finger held in flexion
- Tenderness along tendon sheath
- Pain on passive extension
Fight Bite — Specific Examination
- Examine with hand in clenched fist position
- May reveal deeper penetration when wound aligns with MCP
- Probe wound depth
- XR to look for tooth fragments, joint involvement
Investigations
First-Line
| Investigation | Indication |
|---|---|
| Plain X-ray | All hand bites, fight bites, deep wounds — look for foreign bodies (teeth), fractures, gas |
| Wound swab | If infected — for culture and sensitivity |
| Blood cultures | If systemically unwell or immunocompromised |
Additional If Established Infection
| Investigation | Indication |
|---|---|
| FBC, CRP | Markers of infection |
| U&E | If sepsis |
| Ultrasound | Abscess localisation, tenosynovitis |
| MRI | Osteomyelitis, deep space infection |
In Specific Circumstances
- Hepatitis B/C, HIV serology: Human bites with blood exposure
- Rabies risk assessment: Animal bites in endemic areas
Classification & Staging
Classification by Animal
| Animal | Key Organisms | Special Considerations |
|---|---|---|
| Dog | Pasteurella, Staph, Strep, Capnocytophaga | Crush injuries; Capnocytophaga in asplenic |
| Cat | Pasteurella multocida | Deep punctures; high infection rate; rapid onset |
| Human | Eikenella, oral flora | Fight bites; joint penetration; blood-borne virus risk |
Classification by Severity
| Severity | Features | Management |
|---|---|---|
| Low risk | Superficial, not on hand/face, healthy patient | Wound care, consider antibiotics |
| Moderate risk | Deep, hand/face, cat bite | Wound care + prophylactic antibiotics |
| High risk | Fight bite, joint involvement, immunocompromise, established infection | Antibiotics + likely surgical referral |
Management
Wound Care (Critical First Step)
- Irrigate copiously with normal saline or tap water (250-500ml minimum)
- Debride devitalised tissue
- Do NOT close primarily (most bites — exceptions: face, low-risk wounds)
- Delayed primary closure at 3-5 days if clean
Antibiotic Therapy
| Setting | Regimen | Duration |
|---|---|---|
| Prophylaxis (all cat bites, punctures, hand/face, immunocompromised) | Co-amoxiclav 625mg TDS PO | 5-7 days |
| Established infection (cellulitis) | Co-amoxiclav 625mg TDS PO or 1.2g TDS IV | 7-10 days |
| Severe/systemic infection | Co-amoxiclav IV or Ceftriaxone + Metronidazole IV | Guided by response |
| Penicillin allergy | Doxycycline 100mg BD + Metronidazole 400mg TDS | As above |
Tetanus Prophylaxis
- Check immunisation status
- Td vaccine if incomplete course or >10 years since booster
- TIG if heavily contaminated wound and incomplete primary course
Rabies Assessment
- UK animal: No PEP needed
- Endemic country: Risk assessment → consider PEP (see local guidelines)
Surgical Referral — Indications
| Indication | Reason |
|---|---|
| Fight bite over MCP | Joint penetration — needs washout |
| Tenosynovitis | Emergency surgical washout |
| Septic arthritis | Emergency washout |
| Abscess | Drainage required |
| Significant tissue loss | Reconstructive surgery |
| Devitalised tissue | Debridement |
Complications
Acute Complications
- Cellulitis: Most common
- Abscess formation
- Tenosynovitis: Pyogenic flexor tenosynovitis (surgical emergency)
- Septic arthritis: Joint sepsis
- Osteomyelitis: Especially with delayed treatment
Rare but Serious
- Sepsis/bacteraemia: Especially Capnocytophaga, Pasteurella
- Capnocytophaga sepsis: Fulminant in asplenic patients (DIC, purpura, gangrene)
- Cat scratch disease: Bartonella henselae lymphadenopathy
- Necrotising fasciitis: Rare
Long-Term Complications
- Chronic osteomyelitis
- Joint destruction/arthritis
- Tendon adhesions
- Functional impairment
- Scarring, disfigurement
Prognosis & Outcomes
With Appropriate Treatment
- Superficial wounds: Excellent outcomes
- Prophylactic antibiotics: Prevent majority of infections in high-risk bites
Delayed/Inadequate Treatment
- Fight bites: Up to 75% complication rate if not surgically explored
- Tenosynovitis: Poor functional outcome if delayed
Prognostic Factors
| Factor | Impact |
|---|---|
| Early presentation | Better |
| Cat bites | Higher infection risk |
| Hand involvement | Higher complication risk |
| Immunocompromise | Higher risk of serious infection |
| Delay to antibiotics | Worse outcomes |
Evidence & Guidelines
Key Guidelines
- NICE CKS: Bites — Animal and Human (2023)
- PHE/UKHSA: Rabies Post-Exposure Treatment Guidelines
- BSSH Guidelines for Human Bites to the Hand
Key Evidence
- Cochrane review: Antibiotic prophylaxis reduces infection for hand bites and high-risk wounds
- Prophylaxis for all cat bites supported by infection rate data
- Fight bites require surgical exploration regardless of wound appearance
Patient & Family Information
What Should I Do If I'm Bitten?
- Wash the wound immediately with soap and running water for several minutes
- Apply pressure to stop bleeding
- See a doctor for advice on antibiotics and tetanus
- Tell the doctor what animal bit you and when
When to Seek Urgent Help
- Bite on the hand, face, or near a joint
- Deep puncture wound (especially cat bites)
- Signs of infection: increasing redness, swelling, pus, fever
- Bite from a wild animal or animal abroad
Do I Need Antibiotics?
You may need antibiotics if:
- Cat bite
- Bite to the hand or face
- Deep wound
- You have diabetes or a weakened immune system
Resources
References
Primary Guidelines
- NICE CKS. Bites — Animal and Human. 2023. cks.nice.org.uk
- PHE. Rabies: Post-Exposure Treatment. 2019. gov.uk
Key Studies
- Medeiros I, Saconato H. Antibiotic prophylaxis for mammalian bites. Cochrane Database Syst Rev. 2001;(2):CD001738. PMID: 11406003
- Kennedy SA, et al. Human and other mammalian bite injuries of the hand: evaluation and management. J Am Acad Orthop Surg. 2015;23(1):47-57. PMID: 25538130
- Benson LS, et al. Dog and cat bites to the hand: treatment and cost assessment. J Hand Surg Am. 2006;31(3):468-473. PMID: 16516740