In a 1‑year‑old with low‑grade fever (≈99‑101 °F), mucus‑containing loose stools and decreased appetite, and no chronic illness, immunodeficiency, recent antibiotic use, or sick contacts, what is the most likely diagnosis and first‑line management?

Acute Viral Gastroenteritis

This 1-year-old has acute viral gastroenteritis, and the first-line management is oral rehydration solution (ORS) to correct any dehydration, followed by immediate resumption of age-appropriate diet and continued breastfeeding if applicable. 1, 2

Most Likely Diagnosis

The clinical presentation—low-grade fever (99-101°F), mucus-containing loose stools, and decreased appetite—is classic for viral gastroenteritis, specifically norovirus or rotavirus. 1, 3

Key features supporting this diagnosis:

• Low-grade fever is typical of viral gastroenteritis, whereas high fever (>39°C) would be less common 3
• Mucus in stool can occur with viral gastroenteritis and does not automatically indicate bacterial infection 4
• Acute onset with duration likely <7 days fits the definition of acute gastroenteritis 4, 1
• No bloody diarrhea, which would raise concern for bacterial pathogens like Shigella or enterohemorrhagic E. coli 4, 3

The absence of sick contacts, recent antibiotics, or day care exposure makes bacterial or parasitic causes less likely, though viral gastroenteritis often occurs in clusters. 3

First-Line Management

Immediate Assessment

Evaluate hydration status clinically by assessing skin turgor, mucous membrane moisture, mental status, capillary refill time, and vital signs to categorize dehydration as mild (3-5%), moderate (6-9%), or severe (≥10%). 1, 2, 5

Rehydration Strategy

• For mild to moderate dehydration: Administer low-osmolarity ORS as first-line treatment, giving 100 mL/kg over 2-4 hours for moderate dehydration, then replace ongoing losses 1, 2
• For severe dehydration (≥10% fluid deficit or signs of shock): Initiate intravenous isotonic fluids (lactated Ringer's or normal saline) until pulse, perfusion, and mental status normalize, then transition to ORS 2
• If vomiting is prominent, give small volumes of ORS (5-10 mL) every 1-2 minutes, gradually increasing as tolerated 2

Nutritional Management

Resume age-appropriate diet immediately during or after rehydration rather than restricting foods or prolonged fasting. 1, 2 Offer starches (rice, potatoes, noodles, crackers, bananas), cereals, soup, yogurt, vegetables, and fresh fruits. 2

Continue breastfeeding throughout the illness without interruption. 1, 2

Avoid foods high in simple sugars including soft drinks, undiluted apple juice, Jell-O, and presweetened cereals, as these worsen symptoms through osmotic effects. 2 Also avoid high-fat foods as they may delay gastric emptying. 2

What NOT to Do

• Do not use antimotility agents (loperamide) in children under 18 years—they can cause serious side effects including ileus, drowsiness, and potentially fatal abdominal distention 2
• Do not use adsorbents (kaolin-pectin), antisecretory drugs, or toxin binders 2
• Do not prescribe antibiotics for presumed viral gastroenteritis—they provide no benefit and may cause harm 3, 2
• Do not delay rehydration while awaiting diagnostic testing 2
• Do not rely on the "BRAT diet" exclusively for prolonged periods, as it provides inadequate energy and protein 2

When to Consider Alternative Diagnoses or Further Testing

Red Flags Requiring Stool Testing

Obtain stool cultures if:

• Bloody diarrhea develops 3, 2
• White blood cells appear on stool examination 3
• Symptoms persist beyond 7-14 days 4
• The child appears toxic or has high fever (>39°C) 3

Consider Milk Protein Allergy

If symptoms persist despite appropriate management, trial an extensively hydrolyzed protein or amino acid-based formula for 2-4 weeks in formula-fed infants, as approximately 25% may have underlying milk protein sensitivity. 2

Indications for Hospitalization

Reserve hospitalization for:

• Severe dehydration or shock 2, 5
• Failure of oral rehydration therapy 2, 5
• Altered mental status 2
• Persistent vomiting preventing adequate oral intake 2
• Infants <6 months (lower threshold for admission) 1

Common Pitfalls to Avoid

• Do not assume viral gastroenteritis without considering bilious vomiting, which requires immediate imaging to rule out malrotation with volvulus 3
• Do not use inappropriate fluids like apple juice or sports drinks as primary rehydration solutions for moderate to severe dehydration 2
• Do not unnecessarily restrict diet during or after rehydration—early refeeding improves outcomes 2
• Do not use diluted formulas for extended periods, as this results in inadequate nutrition 2