Top 15 Differential Diagnoses

1. 🥇 Aeromonas hydrophila Infection

• GI: Watery secretory diarrhea (via enterotoxin/cytotoxin), abdominal pain, nausea, vomiting
• Soft tissue: Cellulitis (may be bullous), necrotizing fasciitis, myonecrosis, bacteremia/septicemia, wound infection
• Systemic: Edema, fever, septic shock
• Predisposing factors: water/soil exposure, trauma, liver disease, immunosuppression

"Cellulitis (may be bullous), fasciitis, myonecrosis, bacteremia." — Fitzpatrick's Dermatology, Vol. 1–2 (Waterborne Infections Table 182-8)

2. Vibrio vulnificus Septicemia & Wound Infection

• GI: Watery diarrhea, abdominal pain, vomiting (2–8 day incubation, raw shellfish exposure)
• Soft tissue: Rapidly expanding cellulitis, bullous skin lesions progressing to myonecrosis, necrotizing fasciitis
• Systemic: Life-threatening septicemia with edema and shock; fatality rate >50% once septicemic
• High-risk host: alcoholism, cirrhosis, diabetes, iron-overload states

"Cellulitis, sometimes with bulla formation; may progress to septicemia, especially in alcoholics, diabetics, and immunosuppressed patients; metastatic cellulitis, meningitis, and death may result from fulminant infections." — Fitzpatrick's Dermatology, Table 182-8 "Watery diarrhea, abdominal pain, vomiting, abdominal pain in diseased or sick individuals... undercooked or raw shellfish." — Tintinalli's Emergency Medicine

3. Clostridial Gas Gangrene / Myonecrosis (C. perfringens)

• GI (Type A food poisoning): Watery, non-bloody diarrhea with cramping (8–16 h after ingestion); typically without fever — but in necrotizing enteritis (Type C), bloody diarrhea, vomiting, peritonitis occur
• Soft tissue (Gas gangrene): Explosive myonecrosis with crepitus, edema, wound infection, bacteremia, septicemia, toxic shock
• Alpha-toxin (phospholipase C) and perfringolysin O drive tissue destruction

"Gas gangrene begins as a wound infection but progresses to shock and death in a matter of hours." — Sherris & Ryan's Medical Microbiology, 8e "Clostridium perfringens: food poisoning — rapid onset of abdominal cramps and watery diarrhea with no fever, nausea, or vomiting. Necrotizing enteritis: acute, necrotizing destruction of jejunum." — Medical Microbiology, 9e

4. Necrotizing Fasciitis (Type I — Polymicrobial)

• GI: Diarrhea, abdominal pain, fever — especially with abdominal-wall or perineal origin (Fournier's gangrene)
• Soft tissue: Rapidly spreading fascial necrosis, myonecrosis, edema, septicemia
• Organisms: Bacteroides, E. coli, Streptococcus, Staphylococcus, Clostridium spp.
• Fournier's gangrene (genitoperineal Type I NF) is a specific subtype in males

"Non-clostridial anaerobic cellulitis, synergistic necrotizing cellulitis, and Fournier's gangrene... many patients have underlying diabetes mellitus. Clostridial myonecrosis (gas gangrene)..." — Textbook of Family Medicine, 9e

5. Group A Streptococcus (GAS) Toxic Shock Syndrome + Necrotizing Fasciitis

• Soft tissue: Severe cellulitis → necrotizing fasciitis → myonecrosis (myositis); exquisite pain disproportionate to skin findings
• Systemic: Fever, shock, septicemia, renal failure, edema (capillary leak)
• GI: Nausea, vomiting, diarrhea as part of the toxic shock picture
• Streptococcal pyrogenic exotoxins (SPE-A, -B, -C) act as superantigens

"Shock, multiorgan failure, and localized skin lesion with severe pain in a middle-aged or older person → Toxic shock syndrome caused by Streptococcus pyogenes." — Textbook of Family Medicine, Classic Presentations Table

6. Vibrio cholerae / Cholera-like Illness

• GI: Profuse rice-water watery diarrhea, vomiting, abdominal cramping, severe dehydration, edema (hypoalbuminemia)
• Systemic: Fever, hypovolemic shock, electrolyte disturbances
• Can secondarily infect wounds in endemic areas

7. Salmonella Infection (Typhoid & Non-Typhoidal)

• GI: Watery → bloody diarrhea, abdominal pain, nausea, vomiting
• Systemic: High fever (stepwise), bacteremia/septicemia, hepatosplenomegaly, rose spots
• Typhoid fever: Enteric fever — 30 million cases/year worldwide; can cause intestinal perforation (peritonitis, myonecrosis of bowel wall), septicemia, edema

"Salmonella Typhi: bacteremia most commonly seen... Typhoid fever affects up to 30 million individuals worldwide each year." — Medical Microbiology, 9e; Robbins & Kumar

8. Staphylococcus aureus Infection (MRSA / MSSA)

• GI (food poisoning): Preformed enterotoxin → rapid-onset vomiting, watery diarrhea, abdominal cramps (no fever in pure food poisoning; fever with invasive disease)
• Soft tissue: Cellulitis, wound infection, abscess, septicemia
• Staphylococcal TSS: Fever, diarrhea, vomiting, edema, multi-organ failure
• CA-MRSA causes aggressive skin and soft-tissue infections with rapid necrosis

9. Clostridium difficile Colitis / Pseudomembranous Colitis

• GI: Profuse watery diarrhea (up to 15×/day), abdominal pain, fever, nausea, vomiting
• Systemic: Septicemia in fulminant colitis; toxic megacolon → perforation → myonecrosis of bowel wall; edema
• Predisposed by recent antibiotics; C. difficile enterotoxin A and cytotoxin B

"C. difficile: antibiotic-associated diarrhea and pseudomembranous colitis." — Jawetz, Melnick & Adelberg's Medical Microbiology, 28e

10. Pseudomonas aeruginosa Septicemia

• Soft tissue: Ecthyma gangrenosum (pathognomonic bullous necrotic lesions), cellulitis, wound infection progressing to myonecrosis
• Systemic: Fulminant septicemia, fever, edema, septic shock
• GI: Diarrhea, nausea, vomiting especially in immunocompromised/ICU patients

11. Edwardsiella tarda Infection

• GI: Watery to bloody diarrhea, abdominal pain, nausea, vomiting
• Soft tissue: Cellulitis, abscess, myonecrosis, osteomyelitis, bacteremia

"Edwardsiella tarda: Cellulitis, abscess, osteomyelitis, bacteremia." — Fitzpatrick's Dermatology, Table 182-8

12. Streptococcal Cellulitis / Streptococcus iniae (from aquaculture)

• Soft tissue: Cellulitis and bacteremia following skin injuries handling fresh fish
• Systemic: Fever, septicemia, edema
• Can produce diarrhea, vomiting, myalgia as part of bacteremic illness

"Streptococcus iniae: Cellulitis and bacteremia after skin injuries during the handling of fresh fish raised by aquaculture." — Fitzpatrick's Dermatology, Table 182-8

13. Enteric Fever with Secondary Bacteremic Complications (Shigella / Invasive Dysentery)

• GI: Watery then bloody diarrhea, severe abdominal pain, fever, vomiting, tenesmus
• Systemic: Bacteremia/septicemia in severe cases, reactive arthritis, HUS, toxic megacolon
• Secondary wound infections in immunocompromised or malnourished patients

14. Gas-Forming (Crepitant) Soft-Tissue Infection — Non-Clostridial

• Soft tissue: Cellulitis with gas, fasciitis, myonecrosis — clinically mimics Clostridial gas gangrene
• GI source: Perforated viscus, bowel flora contaminant
• Systemic: Fever, septicemia, edema, shock
• May follow bowel surgery, trauma, or diabetic foot infection

"Non-clostridial anaerobic cellulitis, synergistic necrotizing myonecrosis, and necrotizing fasciitis... differentiated from clostridial gas gangrene by Gram-stain and cultures." — Jawetz, Melnick & Adelberg's, 28e

15. Septicemia with Gut-Origin (Gram-Negative Enterobacteriaceae — E. coli, Klebsiella)

• GI: Watery diarrhea (ETEC, EPEC), abdominal pain, vomiting, fever
• Soft tissue: Secondary cellulitis, wound infection, myonecrosis via hematogenous seeding
• Systemic: Gram-negative septicemia, endotoxin-mediated edema, DIC, multiorgan failure
• Enterotoxigenic E. coli (ETEC) is the most common cause of traveler's diarrhea

Summary Table

Clinical Clues to Narrow Further