What is the likely cause and recommended management for a 6‑year‑old child who was sent home from school three days ago for vomiting and now presents with fatigue, anorexia, and painful oral lesions?

Primary Herpetic Gingivostomatitis (Herpes Simplex Virus)

This 6-year-old most likely has primary herpetic gingivostomatitis caused by herpes simplex virus (HSV), and should be treated with oral acyclovir 20 mg/kg four times daily (maximum 800 mg per dose) for 5-7 days, along with aggressive oral rehydration therapy.

Clinical Presentation and Diagnosis

The constellation of symptoms—vomiting followed by fatigue, anorexia, and painful oral lesions ("sores around his mouth")—is classic for primary HSV infection in school-aged children:

• Primary HSV gingivostomatitis typically presents with prodromal symptoms of fever, malaise, and sometimes vomiting, followed 1-2 days later by painful oral and perioral vesicular lesions that progress to ulcers 1
• The school exposure is significant, as HSV spreads readily through direct contact with infected saliva or lesions, making schools common transmission sites 1
• The 3-day timeline (vomiting initially, then systemic symptoms and oral lesions) matches the typical progression of primary HSV infection 1
• Fatigue and anorexia are expected due to the painful oral lesions making eating and drinking difficult, combined with the systemic viral illness 1

Immediate Assessment Priorities

Before confirming HSV as the diagnosis, rapidly exclude more serious conditions:

• Assess hydration status using clinical signs: check for dry mucous membranes, decreased skin turgor, capillary refill >2 seconds, and decreased urine output (fewer than 4 wet diapers/voids in 24 hours) 2, 3
• Rule out bacterial causes requiring antibiotics: streptococcal pharyngitis (though this typically lacks vesicles), meningococcemia (check for petechial rash elsewhere), or bacterial sepsis (assess vital signs, perfusion) 2
• Examine the oral lesions carefully: HSV causes painful vesicles and ulcers on the gingiva, tongue, buccal mucosa, and lips; hand-foot-mouth disease (enterovirus) causes lesions on hands/feet as well 1

Treatment Approach

Antiviral Therapy

Oral acyclovir is indicated for primary HSV gingivostomatitis when started early in the disease course:

• Dosing: Acyclovir 20 mg/kg orally four times daily (maximum 800 mg per dose) for 5-7 days 1
• Timing matters: Treatment is most effective when initiated within 72 hours of symptom onset, ideally within 48 hours 1
• Clinical benefit: Acyclovir shortens the duration of lesion healing, reduces pain duration, decreases viral shedding, and reduces the duration of fever and difficulty eating 1
• Renal adjustment: Ensure adequate hydration before starting acyclovir, as dosage adjustment may be needed if renal function is impaired 1

Hydration Management

Given the initial vomiting and likely poor oral intake due to painful mouth sores:

• Administer oral rehydration solution (ORS) in small, frequent volumes: 5 mL every minute initially using a spoon or syringe 3, 4
• For mild dehydration (3-5% deficit): give 50 mL/kg ORS over 2-4 hours 3
• For moderate dehydration (6-9% deficit): give 100 mL/kg ORS over 2-4 hours 3
• Replace ongoing losses: 10 mL/kg ORS for each vomiting episode 3
• Simultaneous correction of dehydration often lessens vomiting frequency, making oral acyclovir administration easier 3

Symptomatic Relief

• Pain control is essential to facilitate oral intake: acetaminophen or ibuprofen at weight-appropriate doses
• Avoid acidic or salty foods that exacerbate oral pain
• Offer cold, soft foods: popsicles, ice cream, yogurt, which may be better tolerated 3
• Antiemetic consideration: Ondansetron 0.2 mg/kg oral (maximum 4 mg) may be considered ONLY if persistent vomiting prevents oral acyclovir intake 3, 5

Critical Pitfalls to Avoid

• Do not dismiss oral lesions as "just a virus" without considering HSV—early acyclovir treatment significantly improves outcomes 1
• Do not confuse with hand-foot-mouth disease (enterovirus): HSV lesions are typically more painful, more extensive on the gingiva, and lack the characteristic hand/foot distribution 1
• Do not use antidiarrheal or antimotility agents if vomiting persists—these are ineffective and potentially harmful 2
• Do not withhold fluids or feeds—continue age-appropriate nutrition immediately once vomiting subsides 2, 3

Red Flags Requiring Immediate Escalation

Instruct parents to return immediately or call if:

• Vomiting becomes bilious (green color) or bloody—suggests intestinal obstruction 3, 6
• Severe dehydration develops: lethargy, altered mental status, no urine output for >8 hours, cool extremities 2, 3
• Inability to keep down any fluids despite small, frequent attempts 3
• Neurological symptoms emerge: severe headache, neck stiffness, altered consciousness—HSV can rarely cause encephalitis 2
• Lesions spread extensively or child appears toxic/septic 2

Follow-Up

• Recheck in 48-72 hours to ensure lesions are improving and hydration is maintained
• Expect gradual improvement over 7-10 days with treatment; lesions should begin healing by day 3-4 of acyclovir 1
• Monitor weight to ensure adequate nutritional intake despite oral discomfort 6