What is the likely diagnosis and treatment for an 18-year-old patient with persistent diarrhea, vomiting, and numerous pus cells in the stool exam?

Acute Invasive Bacterial Gastroenteritis

This 18-year-old with persistent diarrhea, vomiting, and numerous pus cells in stool most likely has acute invasive bacterial gastroenteritis requiring immediate stool culture for Salmonella, Shigella, Campylobacter, Yersinia, and STEC, with empiric antibiotics reserved only for specific high-risk scenarios. 1, 2

Likely Diagnosis

The presence of "too many to count" pus cells in stool examination is the critical diagnostic clue indicating invasive bacterial infection rather than viral or parasitic etiology. 1 The combination of persistent diarrhea with vomiting and inflammatory stool findings points specifically to:

• Primary suspects: Shigella, Salmonella, Campylobacter, or Yersinia species—these pathogens characteristically cause abdominal pain with high-grade fever and inflammatory diarrhea 1, 2
• STEC consideration: While STEC causes severe abdominal pain and bloody stools, patients are typically NOT febrile at presentation, which helps distinguish it from other bacterial causes 1, 2
• Clostridium difficile: Must be ruled out if any antibiotic exposure within the preceding 8-12 weeks 1, 2

Immediate Diagnostic Workup

Obtain single diarrheal stool specimen for: 1

• Bacterial culture or molecular multiplex PCR testing for: Salmonella, Shigella, Campylobacter, Yersinia, and STEC 1, 2
• STEC testing must use methods detecting Shiga toxin (or genes) and distinguish O157:H7 from other serotypes 3
• C. difficile toxin testing if any antibiotic use in past 8-12 weeks 1, 2
• Microscopic examination for red blood cells, pus cells, ova, and parasites 4

Do NOT routinely test for parasites unless diarrhea persists beyond 14 days, as parasitic infections (Giardia, Cryptosporidium, Entamoeba histolytica) typically present with chronic rather than acute inflammatory diarrhea. 1, 5

Blood cultures are indicated if: 1

• Signs of septicemia present (fever, tachycardia, hypotension, altered mental status) 6
• Patient appears systemically ill 6

Treatment Algorithm

Hydration (First-Line for ALL Patients)

• Reduced osmolarity oral rehydration solution (ORS) is the cornerstone of therapy for mild to moderate dehydration 1
• Intravenous isotonic fluids (lactated Ringer's or normal saline) required for severe dehydration, shock, altered mental status, or ORS failure 1
• Continue rehydration until pulse, perfusion, and mental status normalize 1

Antibiotic Decision-Making

DO NOT give empiric antibiotics while awaiting stool test results in immunocompetent patients with bloody/inflammatory diarrhea. 1, 3, 2 This is a strong recommendation that prevents unnecessary antibiotic exposure and potential complications.

EXCEPTIONS warranting empiric antibiotics: 1, 2

1. Documented fever (≥38.5°C) + bloody diarrhea + presumed bacillary dysentery (Shigella) 6, 2
2. Signs of sepsis (tachycardia, hypotension, altered mental status) 6, 2
3. Immunocompromised status (HIV, transplant, chemotherapy, chronic steroids) 1, 2
4. Recent international travel with temperature ≥38.5°C 6

Antibiotic choices when indicated: 2, 7

• Adults: Ciprofloxacin 500mg PO twice daily for 3-5 days OR azithromycin 500mg PO daily for 3 days 2, 7
• Adolescents: Azithromycin preferred (500mg PO daily for 3 days) OR third-generation cephalosporin 2

Critical Pitfalls to Avoid

NEVER give antibiotics for STEC O157 or Shiga toxin 2-producing STEC due to significantly increased risk of hemolytic uremic syndrome (HUS). 3, 2 This is an absolute contraindication.

Avoid loperamide in this patient with inflammatory diarrhea and pus cells, as antimotility agents can precipitate toxic megacolon in bacterial dysentery. 1

Watch for HUS red flags: bloody diarrhea with anemia, thrombocytopenia, or renal dysfunction requires urgent evaluation. 3, 2

Modify Treatment Based on Culture Results

Once bacterial pathogen is identified, antimicrobial treatment should be modified or discontinued accordingly. 1 Susceptibility testing guides definitive therapy, particularly important given rising fluoroquinolone resistance in Campylobacter and Salmonella. 1

If cultures remain negative after 48-72 hours and symptoms persist beyond 14 days, then pursue parasitic evaluation with stool ova and parasites examination specifically requesting Cryptosporidium and Cyclospora testing. 1, 5

Supportive Care

• Resume age-appropriate usual diet immediately after rehydration is completed 1
• Ondansetron may facilitate oral rehydration tolerance in adolescents with significant vomiting 1
• Probiotics may reduce symptom severity and duration but are not primary therapy 1

1, 6, 3, 2