Infectious Complications of Bites¶
Chapter 146 | Part 5: Infectious Diseases
KEY CLINICAL POINTS¶
- Dog and cat bites are the most common animal bites, with ~4.7 million dog bites annually in the U.S. and 15–20% infection rates.
- Cat bites have higher infection rates (50%+) due to deep puncture wounds and anaerobic flora, often involving Pasteurella multocida.
- Human bites carry risk of infection based on wound depth (2–25% infection rates) and may transmit hepatitis B, HIV, and syphilis.
- Antibiotic prophylaxis is recommended for high-risk bites (e.g., facial, joint-involving, or crush injuries) with coverage against S. aureus, anaerobes, and MRSA.
- Rabies prophylaxis is critical for bites from animals with potential rabies exposure, while tetanus vaccination is mandatory for all bite wounds.
1. DEFINITION & OVERVIEW¶
Bite wounds introduce pathogens past the skin barrier, leading to localized or systemic infections. Infections vary by bite type, with dog/cat bites causing septic arthritis/osteomyelitis, human bites risking tetanus/HIV transmission, and venomous snakebites requiring antivenin.
Table 146-1: Management of Wound Infections Following Animal and Human Bites¶
| BITING SPECIES | COMMONLY ISOLATED PATHOGENS | PREFERRED A NTIBIOTIC(S)a | ALTERNATIVE IN PENICILLIN- ALLERGIC PATIENT | PROPHYLAXIS ADVISED FOR EARLY UNINFECTED WOUNDS | OTHER CONSI DERATIONS |
|---|---|---|---|---|---|
| Dog | Staphylococcus aureus, Pasteurella spp., anaerobes, Capnocytophaga canimorsus | Amoxicillin/clavul anate (875/125 mg PO q12h) or ceftriaxone 2 g IV once daily plus metronidazole 500 mg q8h | Clindamycin or metronidazole plus TMP-SMX (1 DS tablet PO bid) or ciprofloxacin (500 mg PO bid) | Sometimes | Consider rabies prophylaxis. |
| Cat | P. multocida, S. aureus, anaerobes | Amoxicillin/clavul anate, ampicillin/ sulbactam, or ceftriaxone plus metronidazole | Clindamycin or metronidazole plus TMP-SMX or fluoroquinolone | Usually |
| BITING SPECIES | COMMONLY ISOLATED PATHOGENS | PREFERRED A NTIBIOTIC(S)a | ALTERNATIVE IN PENICILLIN- ALLERGIC PATIENT | PROPHYLAXIS ADVISED FOR EARLY UNINFECTED WOUNDS | OTHER CONSI DERATIONS |
|---|---|---|---|---|---|
| Human, occlusional | Viridans streptococci, S. aureus, Haemophilus influenzae, anaerobes, Eikenella corrodens | Amoxicillin/clavul anate plus TMP-SMX (if MRSA coverage required) or ceftriaxone plus metronidazole | TMP-SMX plus metronidazole | Always | |
| Snake | Snake oral flora including Pseudomonas, Morganella spp., E. coli, anaerobes | Piperacillin/tazob actam 3.375 g IV q6–8h | Clindamycin or metronidazole plus fluoroquinolone | Evidence does not support benefit | Administer antivenin for venomous snakebite. Tetanus prophylaxis. |
| Rodent | Streptobacillus moniliformis, Spirillum minus, Leptospira spp. | Penicillin VK (500 mg PO qid) or ceftriaxone IV | Doxycycline (100 mg PO bid) | Sometimes | |
| Aquatic animal | Aeromonas hydrophila, marine Vibrio spp. | Third-generation cephalosporin (e.g., ceftriaxone 1 g IV q24h) plus doxycycline | Clindamycin or metronidazole plus levofloxacin plus doxycycline | Always | Obtain prompt surgical consultation for necrotizing infection risk. |
1.1 Pathogenesis¶
Bacteria from oral flora (Pasteurella, Capnocytophaga) or environmental sources (Aeromonas, Vibrio) enter via puncture wounds. Anaerobic bacteria dominate in deep wounds, while aerobic pathogens predominate in superficial injuries.
1.2 Complications¶
Systemic complications include septicemia, meningitis, and septic thrombophlebitis. Local complications include osteomyelitis, septic arthritis, and abscess formation. Venomous bites may cause necrotizing fasciitis or neurotoxicity.
2. EPIDEMIOLOGY¶
Dog bites: 4.7 million annually in U.S., 80% of animal-bite wounds. Cat bites: 15–20% infection rate. Human bites: 800,000 annual ED visits. Risk factors include immunocompromise, splenectomy, and prior antibiotic use.
2.1 Demographics¶
Children <4 years: 2/3 of injuries involve head/neck. Males more frequently bitten. Dog bites peak in boys 5–9 years (6 per 1000 population).
2.2 Risk Factors¶
Immunocompromise (e.g., HIV, SLE), splenectomy, hepatic dysfunction, and prior antibiotic use increase sepsis risk. Dehydration and hypercoagulable states worsen outcomes.
3. ETIOLOGY & PATHOPHYSIOLOGY¶
Pathogens vary by bite type: dog/cat bites (Pasteurella, Capnocytophaga), human bites (Eikenella, anaerobes), snakebites (Pseudomonas, Vibrio). Anaerobes dominate in deep wounds, while aerobic bacteria predominate in superficial injuries.
3.1 Microbial Sources¶
Dog/cat oral flora (Pasteurella, Capnocytophaga), human oral flora (Streptococcus, Eikenella), and environmental sources (Aeromonas, Vibrio) contribute to infections.
3.2 Mechanisms¶
Deep punctures allow anaerobic bacteria to proliferate. Bacterial spread via venous sinuses (e.g., cavernous sinus thrombosis) occurs via emissary veins. Venomous bites cause direct tissue damage and systemic toxicity.
4. CLINICAL FEATURES¶
Symptoms vary by bite type: dog/cat bites (septic arthritis, osteomyelitis), human bites (tetanus, HIV transmission), snakebites (neurotoxicity, necrosis). Systemic signs include fever, lymphadenopathy, and sepsis.
4.1 Local Signs¶
Pain, swelling, cellulitis, purulent discharge, and abscess formation. Cat bites may present with severe inflammation and rapid progression.
4.2 Systemic Signs¶
Fever, chills, sepsis, meningitis, and multiorgan failure. Venomous bites may cause neurotoxicity, coagulopathy, and systemic inflammation.
5. DIFFERENTIAL DIAGNOSIS¶
Septic thrombophlebitis, bacterial meningitis, septic arthritis, and tetanus. Differentiate from non-infectious causes like cellulitis or foreign-body reactions.
5.1 Key Differentiators¶
Systemic symptoms (fever, sepsis) vs. localized infection. Presence of venomous symptoms (neurotoxicity) vs. bacterial infection. Imaging findings (thrombosis, abscess) vs. non-infectious inflammation.
6. INVESTIGATIONS & DIAGNOSIS¶
Imaging (MRI, CT) for thrombosis; cultures (blood, wound) for pathogens; Gram stain for rapid identification. Diagnostic criteria include presence of purulent discharge, abscess, or systemic signs.
6.1 Diagnostic Tests¶
MRI/CT for venous sinus thrombosis; blood cultures for bacteremia; wound cultures for localized pathogens. Gram stain identifies aerobic/anaerobic organisms.
6.2 Imaging¶
MRI detects intracerebral hemorrhage and thrombosis. CT venography confirms sinus thrombosis. Ultrasound guides abscess drainage.
7. MANAGEMENT & TREATMENT¶
Wound debridement, irrigation, and antibiotics. Prophylaxis for high-risk wounds. Antivenin for venomous bites. Surgical intervention for abscesses or septic thrombosis.
7.1 Wound Management¶
Primary closure avoided for high-risk wounds. Debridement of devitalized tissue and foreign bodies. Abscess drainage required for purulent infections.
7.2 Antibiotic Therapy¶
Empirical coverage for S. aureus, anaerobes, and MRSA. Duration 5–14 days. Penicillin-allergic patients use clindamycin/metronidazole. Ceftriaxone/fluoroquinolone for severe infections.
7.3 Antivenin¶
Administer for venomous snakebites. Tetanus prophylaxis mandatory. Rabies prophylaxis required for suspected rabid animal bites.
8. PROGNOSIS & COMPLICATIONS¶
Mortality 1–2% for uncomplicated infections. Severe complications include septicemia, meningitis, and septic thrombophlebitis. Long-term sequelae include joint damage and chronic infection.
8.1 Mortality¶
Rabies and septicemia account for most deaths. Tetanus mortality ~30% without treatment.
8.2 Long-Term Effects¶
Chronic osteomyelitis, joint deformities, and neurologic deficits from septic arthritis. Psychological sequelae from severe bites.
9. SPECIAL CONSIDERATIONS¶
Immunocompromised patients require extended antibiotic courses. Pregnancy: avoid certain antibiotics (e.g., fluoroquinolones). Pediatric patients need careful wound closure to prevent scarring.
9.1 Pregnancy¶
Avoid fluoroquinolones and tetracyclines. Use penicillins or cephalosporins for antibiotic coverage.
9.2 Pediatrics¶
Close facial wounds for cosmetic outcomes. Monitor for tetanus in infants. Avoid prolonged antibiotic use to prevent resistance.
10. KEY POINTS & CLINICAL PEARLS¶
- Cat bites have 50%+ infection rates due to deep punctures and anaerobic flora.
- Prophylactic antibiotics recommended for facial, joint-involving, or crush wounds.
- Rabies prophylaxis required for bites from animals with potential rabies exposure.
- Tetanus vaccination mandatory for all bite wounds.
- Venomous snakebites require antivenin and surgical consultation for necrotizing infections.