Infectious Complications of Bites¶

Chapter 146 | Part 5 – Infectious Diseases: Bacterial · Part 5 – Infectious Diseases: Bacterial · Chapter 146

Key Clinical Points¶

Pasteurella multocida is the primary pathogen in cat bites, causing rapid infection within hours.
Capnocytophaga canimorsus causes fulminant sepsis, DIC, and renal failure, especially in asplenic or immunocompromised hosts.
Eikenella corrodens is common in 'fight bites' (clenched-fist injuries) and is resistant to penicillinase-resistant penicillins.
Dog bites are more frequent (>80% of animal bites), while cat bites have a higher infection rate (50–60%) and risk for septic arthritis/osteomyelitis.
Human bites require coverage for Eikenella corrodens, MRSA, and anaerobes.
Aquatic animal bites require coverage for Aeromonas hydrophila and Vibrio species; high risk of necrotizing infection.
Rat-bite fever is caused by Streptobacillus moniliformis (US) or Spirillum minus (Asia).
Septic venous sinus thrombosis requires 6 weeks of antibiotics or until radiographic resolution.
Wounds >12h (arm/leg) or >24h (face) should not be closed primarily.
Clenched-fist injuries require careful tendon assessment due to deep space inoculation.

1. DEFINITION & OVERVIEW¶

• Skin Barrier: The skin serves as a critical nonspecific immune barrier; bites and scratches breach this barrier, allowing microbial inoculation into deeper tissues. • Dog Bites: >4.7 million occur annually in the US; 80% of animal-bite wounds are attributed to dogs (15–20% infection rate). • Cat Bites: Lower frequency but higher infection rates (50–60%) due to deep punctures from sharp teeth. • Human Bites: 10–15% infection risk, often involving mixed aerobic and anaerobic flora. • Seal Finger: Infection caused by Mycoplasma phocacerebrale; responds to doxycycline.

1.1 CNS Complications (Septic Thrombosis)¶

• Superior Sagittal Sinus: Presents with headache, fever, confusion, seizures, and focal neurological deficits. • Cavernous Sinus: Manifests as ptosis, proptosis, chemosis, diplopia, and cranial nerve palsies (III, IV, VI). • Diagnosis: Confirmed by MRI/CT venography showing absent flow void in affected sinuses.

2. EPIDEMIOLOGY¶

• Dog Bites: 80% of animal-bite wounds; 15–20% infection rate. • Cat Bites: 50–60% infection rate; higher risk for septic arthritis/osteomyelitis. • Human Bites: 10–15% infection rate; common pathogens include Eikenella corrodens and anaerobes. • Pediatric Cases: Children <4 years: 2/3 of injuries involve head/neck; peak incidence 6 bites/1000 population in boys 5–9 years. • Occupational Risk: Farmers, veterinarians, laboratory workers (exposed to exotic pets). • Rat-bite Fever: Streptobacillus moniliformis (US) or Spirillum minus (Asia); incubation periods of 3–10 days (S. moniliformis) and 1–4 weeks (S. minus).

2.1 Demographics & Risk Factors¶

• Gender/Location: Male predominance in dog bites; female predominance in cat bites. Upper extremities most commonly involved. • High-Risk Hosts: Immunocompromised hosts (splenectomy, hepatic dysfunction, glucocorticoid use) at higher risk for Capnocytophaga canimorsus sepsis.

3. ETIOLOGY & PATHOPHYSIOLOGY¶

• Dog-bite Flora: Pasteurella species, β-hemolytic streptococci, Staphylococcus (including MRSA), Neisseria weaveri, Eikenella corrodens, Capnocytophaga canimorsus, and anaerobes. • Cat-bite Flora: Pasteurella multocida (primary pathogen) with mixed aerobic/anaerobic organisms. • Human-bite Flora: Viridans streptococci, S. aureus, Eikenella corrodens, Haemophilus influenzae, and 50% involve anaerobes (Fusobacterium, Prevotella). • Aquatic Animal Flora: Aeromonas hydrophila, Vibrio vulnificus. • Snake-bite Flora: Pseudomonas, Morganella, E. coli, group D streptococci, Salmonella, Bacteroides fragilis. • Seal/Walrus/Polar Bear: Mycoplasma phocacerebrale (seal finger). • Monkey Bites: B virus (Herpesvirus simiae).

3.1 Microbiology by Bite Type¶

Table 146-1 summarizes the microbial flora and management: • Dog/Cat: Pasteurella, S. aureus, anaerobes, Capnocytophaga canimorsus. • Human: Viridans strep, S. aureus, H. influenzae, E. corrodense. • Rat: Streptobacillus moniliformis, Spirillum minus. • Aquatic: Aeromonas hydrophila, Vibrio vulnificus.

4. CLINICAL FEATURES¶

• Local Infection: Pain, cellulitis, purulent discharge 8–24h post-bite. • Deep Tissue Involvement: Septic arthritis/osteomyelitis risk with joint proximity (especially hand). • Systemic Manifestations: Fever, lymphadenopathy, lymphangitis. • Clenched-fist Injuries: Deep space inoculation of bones, joints, tendons; often requires delayed tendon assessment. • Neurological Complications: • Septic sinus thrombosis → Stupor/coma, Babinski signs, hemiparesis. • Cavernous sinus thrombosis → Ptosis, proprotsus, diplopia, chemosis. • Gradenigo's syndrome → Retroorbital pain + sixth nerve palsy.

4.1 Systemic Manifestations¶

• Capnocytophaga canimorsus: Sepsis in asplenic/immunocompromised hosts → DIC, renal failure. • Pasteurella: Bacteremia or pneumonia via respiratory droplets. • Rat-bite fever: Maculopapular rash on palms/soles, migratory arthralgias, endocarditis.

4.2 Neurological Complications¶

• Septic sinus thrombosis: Stupor/coma, Babinski signs, hemiparesis. • Cavernous sinus thrombosis: Ptosis, proptosis, diplopia, chemosis. • Gradenigo's syndrome: Retroorbital pain + sixth nerve palsy.

5. DIFFERENTIAL DIAGNOSIS¶

• Rat-bite fever vs. acute wound infection: Look for delayed rash (palms/soles) and arthralgias in streptobacillary form. • Septic thrombosis vs. meningitis: Presence of cranial nerve palsies, papilledema, and retinal vein dilation.

5.1 Rat-Bite Fever vs. Other Zoonoses¶

• Streptobacillary form: Incubation 3–10 days; maculopapular rash. • Spirillium form (sodoku): Incubation 1–4 weeks; purple swelling at bite site with eschar formation.

6. INVESTIGATIONS & DIAGNOSIS¶

Imaging: MRI/CT venography for septic sinus thrombosis (look for absent flow void).
Microbiology: • Blood cultures: Note Capnocytophaga canimorsus may require 14-day incubation. • Rat-bite fever: Direct visualization of Spirillum in blood/tissue or animal inoculation.
Decision Logic: • Suspected sinus thrombosis → Headache + fever + cranial nerve deficits → MRI/CT venography. • Suspected Rat-bite fever → Delayed rash + arthralgias → Blood culture for Streptobacillus or Spirillum.

6.1 Diagnostic Criteria & Algorithms¶

• Septic sinus thrombosis: Headache + fever + cranial nerve deficits + MRI/CT venography. • Rat-bite fever: Delayed rash + arthralgias + positive blood culture for Streptobacillus or Spirillum.

7. MANAGEMENT & TREATMENT¶

Wound Management: • Immediate irrigation with copious saline. • Debridement of necrotic tissue. • Tetanus prophylaxis as indicated. • Primary closure: Avoid if wound >12h (arm/leg) or >24h (face).
Antibiotic Therapy (based on Table 146-1): • Cat bites: Amoxicillin/clavulanate (875/125 mg PO q12h) OR ampicillin/sulbactam (3.0 g IV q6h) OR Ceftriaxone 2 g IV daily + metronidazole 500 mg q8h. • Dog bites: Same as cat bites + ceftriaxone for Capnocytophaga canimorsus. • Human bites: Amoxicillin/clavulanate + TMX-SMX (if MRSA coverage needed) OR ampicillin/sulbactam OR ceftriaxone + metronidazole. • Snake bites: Piperacillin/tazobactam 3.375 g IV q6–8h. • Rat-bite fever: Penicillin VK (500 mg PO qid) or ceftriaxone IV; Doxycycline (100 mg PO bid). • Aquatic animal bites: Ceftriaxone 1 g IV q24h + doxycycline 100 mg PO bid.
Specialized Interventions: • Snake bites: Administer antivenin for venomous snakebite; Tetanus prophylaxis. • Septic sinus thrombosis: Ceftriaxone for 6 weeks or until radiographic resolution; early anticoagulation if neurologic deterioration persists.

7.1 Wound Management¶

• Immediate irrigation with copious saline. • Debridement of necrotic tissue. • Avoid primary closure in wounds >12h (arm/leg) or >24h (face). • Tetanus prophylaxis as indicated.

7.2 Antibiotic Therapy¶

Table 146-1 summary of regimens: • Cat bites: Amoxicillin/clavulanate, ampicillin/sulbactam, or ceftriaxone + metronidazole. • Dog bites: Same as cat + ceftriaxone for Capnocytophaga. • Human bites: Amoxicillin/clavulanate + TMX-SMX (if MRSA), ampicillin/sulbactam, or ceftriaxone + metronidazole. • Snake bites: Piperacillin/tazobactam 3.375 g IV q6–8h. • Rat-bite fever: Penicillin VK (500 mg PO qid) or ceftriaxone IV; Doxycycline (100 mg PO bid). • Aquatic animal bites: Ceftriaxone 1 g IV q24h + doxycycline 100 mg PO bid.

8. PROGNOSIS & COMPLICATIONS¶

• Capnocytophaga canimorsus: Mortality 10–25% in asplenic hosts; leads to DIC and renal failure. • Untreated infections: Osteomyelitis, joint destruction from septic arthritis. • Neurological: Permanent cranial nerve palsies, visual loss from cavernous sinus thrombosis. • Rat-bite fever: Endocarditis, meningitis, and multiorgan abscesses.

8.1 Systemic Complications¶

• Endocarditis, meningitis, and multiorgan abscesses in rat-bite fever. • DIC and renal failure in Capnocytophaga canimorsus sepsis.

9. SPECIAL CONSIDERATIONS¶

• HIV patients: Low risk of transmission unless blood in saliva with severe trauma. • Seal finger: Mycoplasma phocacerebrale → Doxycycline effective despite chronic suppurative infection.

10. KEY PEARLS & CLINICAL TRAPS¶

• Clenched-fist injuries: Require tendon assessment even without overt infection. • Capnocytophaga culture: May be missed on initial blood cultures (requires 14-day incubation). • Septic sinus thrombosis: Early anticoagulation indicated if neurologic deterioration persists; 6 weeks of antibiotics or until radiographic resolution. • Aquatic bites: High risk for necrotizing infection; obtain prompt surgical consultation.

Reference Tables¶

TABLE 146-1 Management of Wound Infections Following Animal and Human Bites¶

Harrison's 22e, p.1145

BITING SPECIES	COMMONLY ISOLATED PATHOGENS	PREFERRED ANTIBIOTIC(S)a	ALTERNATIVE IN PENICILLIN-ALLERGIC PATIENT	PROPHYLAXIS ADVISED FOR EARLY UNINFECTED WOUNDS	OTHER CONSIDERATIONS
Dog	Staphylococcus aureus, Pasteurella spp. (mainly P. multocida and P. canis), anaerobes, Capnocytophaga canimorsus	Amoxicillin/clavulanate (875/125 mg PO q12h) or ampicillin/sulbactam (3.0 g IV q6h) or Ceftriaxone 2 g IV once daily plus metronidazole 500 mg q8h	Clindamycinb or metronidazole plus either TMP-SMX (1 DS tablet PO bid) or ciprofloxacin (500 mg PO bid)	Sometimesc	Consider rabies prophylaxis.
	P. multocida, S. aureus, anaerobes	Amoxicillin/clavulanate, ampicillin/sulbactam, or ceftriaxone plus metronidazole as above	Clindamycin or metronidazole plus TMP-SMX as above or fluoroquinolone	Usuallyd
Human, occlusional	Viridans streptococci, S. aureus, Haemophilus influenzae, anaerobes Eikenella corrodense	Amoxicillin/clavulanate plus TMX-SMX if consider including MRSA coverage or ampicillin/sulbactam or ceftriaxone plus metronidazole (consider adding vancomycin if MRSA coverage required)	TMP-SMX plus metronidazole	Always
	As for human bite	As for human bite	As for human bite	Always
Snake	Snake oral flora including Pseudomonas, Morganella spp., E. coli, group D streptococci, Salmonella spp., anaerobic organisms including Bacteroides fragilis, Clostridium spp.	Piperacillin/tazobactam 3.375 g IV q6–8h	Clindamycin or metronidazole plus a fluoroquinolone	Evidence does not support the benefit. Can consider in regions with high rates of infection such as Brazil.	Administer antivenin for venomous snakebite. Tetanus prophylaxis.
	Rat bite fever; Streptobacillus moniliformis, Spirillum minus, Streptobacillus notomytis, Leptospira spp., P. multocida	Penicillin VK (500 mg PO qid) or ceftriaxone IV	Doxycycline (100 mg PO bid)	Sometimes
Aquatic animal (alligator, piranha, shark, moray eel, barracuda)	Aeromonas hydrophila, marine Vibrio spp. (Vibrio vulnificus)	Third-generation cephalosporin (e.g., ceftriaxone, 1 g IV q24h) plus doxycycline (100 mg PO bid)	Clindamycin or metronidazole plus levofloxacin (750 mg PO qd) plus doxycycline	Always	Obtain prompt surgical consultation, as risk for necrotizing infection is high with Aeromonas and Vibrio spp.