17.2 Infectious Causes of Wounds: Necrotizing, Mycobacterial, Fungal & Other Infections
Key Takeaways
- Necrotizing soft tissue infection is suggested by pain out of proportion, rapid spread, tense edema, bullae, skin anesthesia, crepitus, and systemic toxicity; it needs emergency surgical debridement with broad-spectrum antibiotics plus clindamycin or linezolid for toxin suppression, and a low LRINEC score does not rule it out.
- Saltwater and seafood exposure in a patient with liver disease suggests Vibrio vulnificus (hemorrhagic bullae, sepsis; treat with doxycycline plus a third-generation cephalosporin and debridement), while freshwater injuries suggest Aeromonas.
- Fish tank or swimming pool exposure followed by nodules spreading up the arm suggests Mycobacterium marinum; tissue culture needs lower incubation temperatures (about 30°C), and treatment usually combines clarithromycin with ethambutol for months.
- Nonhealing ulcers after travel or unusual exposures may be infectious: cutaneous leishmaniasis (painless ulcer with raised border after sandfly exposure), Buruli ulcer (Mycobacterium ulcerans), sporotrichosis (rose thorn; lymphocutaneous nodules), and mucormycosis (black necrotic eschar in diabetic ketoacidosis or immunosuppression).
- When an unusual infection is suspected, send tissue, not swabs, for histology with special stains and for bacterial, mycobacterial, and fungal cultures, and tell the laboratory what organisms to look for.
17.2 Infectious Causes of Wounds: Necrotizing, Mycobacterial, Fungal & Other Infections
Core Clinical Principle: Some wounds are not merely infected; they are caused by infection. Recognizing necrotizing infection within hours saves lives and limbs, and recognizing unusual organisms such as mycobacteria and fungi avoids months of ineffective wound care.
The CWSP outline lists infectious wounds under Etiological Considerations. Biofilm, local wound infection, diabetic foot infection, osteomyelitis, and antimicrobial stewardship are covered in other sections.
Necrotizing Soft Tissue Infections (NSTI)
| Type | Organisms | Typical Setting |
|---|---|---|
| Type I (polymicrobial) | Mixed aerobes and anaerobes | Diabetes, perineal infection (Fournier gangrene), postoperative wounds, injection drug use |
| Type II (monomicrobial) | Group A Streptococcus, sometimes with Staphylococcus aureus | Healthy people after minor trauma; streptococcal toxic shock |
| Other monomicrobial forms | Clostridium species (gas gangrene), Vibrio vulnificus, Aeromonas | Traumatic or surgical wounds; saltwater or freshwater exposure |
Clinical clues: Pain out of proportion to visible findings, tense or "woody" induration beyond the erythema, rapid spread over hours, hemorrhagic bullae, dusky or gray skin, skin anesthesia, crepitus, and systemic toxicity (fever, tachycardia, hypotension, confusion). Early cases can look like cellulitis.
Diagnosis: Clinical suspicion drives surgery. The LRINEC score (based on CRP, WBC, hemoglobin, sodium, creatinine, and glucose) uses 6 or more as concerning, but its sensitivity is too low to rule out NSTI. CT can show fascial gas or fluid but must not delay surgery. At operation, gray necrotic fascia, "dishwater" fluid, and fascia that separates easily with a finger confirm the diagnosis.
Treatment:
- Emergency surgical debridement of all necrotic tissue, with planned re-exploration within about 24 to 48 hours.
- Broad-spectrum antibiotics, such as vancomycin or linezolid plus piperacillin-tazobactam or a carbapenem, plus clindamycin (or linezolid) to suppress toxin production in streptococcal and clostridial infection.
- Intensive supportive care. Hyperbaric oxygen and intravenous immunoglobulin are adjuncts with uncertain benefit and must never delay surgery.
Gas gangrene (clostridial myonecrosis) follows deep contaminated trauma or surgery, with severe pain, bronze skin, bullae, crepitus, and rapid shock. Clostridium septicum infection without trauma should prompt a search for colon cancer.
Other Bacterial Causes of Necrotic or Ulcerated Skin
| Condition | Clues | Treatment Points |
|---|---|---|
| Ecthyma gangrenosum | Neutropenic or immunocompromised patient; hemorrhagic bulla becoming a black necrotic ulcer; usually Pseudomonas aeruginosa bacteremia | Blood cultures, antipseudomonal antibiotics, debridement |
| Vibrio vulnificus | Saltwater or brackish water wound or raw oysters; liver disease or iron overload; hemorrhagic bullae and sepsis | Doxycycline plus a third-generation cephalosporin (such as ceftazidime), early debridement |
| Aeromonas | Freshwater injuries, medicinal leech therapy | Fluoroquinolone or trimethoprim-sulfamethoxazole based on susceptibility |
| Erysipelothrix | Fish, shellfish, or meat handlers; violaceous plaque on the hand | Penicillin |
| Ecthyma (streptococcal) | Punched-out crusted ulcers on the legs in poor hygiene or warm climates | Antistaphylococcal and antistreptococcal therapy |
| Cutaneous anthrax | Painless black eschar with marked surrounding edema; animal hides or wool exposure | Report to public health; antibiotics such as ciprofloxacin or doxycycline |
| Syphilis | Painless chancre, or late gummatous ulcers | Serology; penicillin |
Mycobacterial Infections
- Mycobacterium marinum ("fish tank granuloma"): Nodules or ulcers on the hand weeks after aquarium, fish, or pool exposure, often spreading up the lymphatics (sporotrichoid pattern). Culture tissue at about 30°C; treat with clarithromycin plus ethambutol for months, with debridement for deep infection.
- Rapidly growing mycobacteria (M. abscessus, M. chelonae, M. fortuitum): Infections after cosmetic surgery, injections, tattoos, and nail salon footbaths; need multidrug therapy guided by susceptibility testing.
- Buruli ulcer (M. ulcerans): Painless nodule that becomes a large ulcer with deeply undermined edges, in parts of West Africa and Australia; WHO recommends 8 weeks of rifampicin plus clarithromycin, with surgery for selected cases.
- Cutaneous tuberculosis and leprosy (Hansen disease): Leprosy causes neuropathic plantar ulcers similar to diabetic ulcers and requires multidrug therapy.
Fungal Infections
| Condition | Clues | Treatment |
|---|---|---|
| Sporotrichosis | Rose thorn, sphagnum moss, or hay injury; ulcerated nodules spreading along lymphatics | Itraconazole |
| Mucormycosis | Diabetic ketoacidosis, neutropenia, transplant; rapidly spreading black necrotic eschar (including after trauma) | Emergency surgical debridement plus liposomal amphotericin B; correct acidosis |
| Mycetoma (Madura foot) | Chronic swelling with sinus tracts draining grains, in tropical regions | Distinguish fungal (eumycetoma) from bacterial (actinomycetoma) causes; long-term therapy and surgery |
| Chromoblastomycosis | Warty plaques on the legs of rural workers in tropical areas | Itraconazole, surgery or cryotherapy |
| Blastomycosis, coccidioidomycosis | Verrucous or ulcerated lesions with lung disease in endemic regions | Azoles or amphotericin B |
| Candidiasis | Red, macerated skin with satellite pustules in folds and periwound skin | Topical or oral azoles, moisture control |
Parasitic and Viral Ulcers
- Cutaneous leishmaniasis: Painless ulcer with a raised, indurated border weeks after travel to endemic areas (sandfly bites). Diagnose with biopsy, smear, or PCR; treatment depends on species and location.
- Herpes simplex virus: Chronic, painful perianal, genital, or oral ulcers in immunocompromised patients; confirm with PCR and treat with antivirals.
- Varicella zoster virus: Dermatomal vesicles that can ulcerate and leave scars, especially in older or immunocompromised patients.
- Mpox: Firm, deep-seated, painful pustules and ulcers, often anogenital; confirm with PCR and follow public health guidance.
Getting the Right Specimen
- Take tissue (punch or incisional biopsy) from the active edge, not a surface swab.
- Split the sample: formalin for histology with special stains (acid-fast, fungal stains such as PAS and GMS), and sterile saline for bacterial, mycobacterial, and fungal cultures.
- Tell the laboratory about suspected organisms and exposures, because some need special media, temperatures, or long incubation.
- Add PCR for viruses, leishmaniasis, and some mycobacteria.
Clinical Traps
Trap 1: Waiting for Imaging in Suspected NSTI
A CT scan should never delay surgical exploration when the clinical picture suggests necrotizing infection.
Trap 2: Months of Antibacterial Therapy for a Sporotrichoid Arm Lesion
Nodules spreading up the arm after fish tank cleaning or gardening point to M. marinum or sporotrichosis. Biopsy for special cultures.
A 52-year-old man with diabetes scraped his thigh 2 days ago. He now has severe pain far beyond the modest erythema, tense woody swelling extending past the red area, a few hemorrhagic bullae, and a heart rate of 124 beats per minute. His LRINEC score is 4. What is the most appropriate next step?
A 44-year-old aquarium hobbyist has had three painless, slowly enlarging nodules on the dorsal hand and forearm for 2 months, arranged in a line along the lymphatic path. Two courses of cephalexin did not help. Which diagnostic approach is most appropriate?
A 61-year-old man with alcohol-related cirrhosis cut his leg while wading in the Gulf of Mexico. Within 24 hours, he develops hemorrhagic bullae spreading up the leg, severe pain, and hypotension. Which pathogen and treatment approach are most likely?