What are the likely causes and recommended management for a new rash that began on the trunk of a 1‑year‑old infant?

Rash Starting on Trunk in 1-Year-Old

The most likely diagnosis is roseola infantum (HHV-6), which characteristically presents with a maculopapular rash appearing on the trunk after 3-4 days of high fever in children aged 6 months to 2 years. 1

Immediate Assessment Priorities

Before assuming a benign viral exanthem, you must urgently exclude life-threatening conditions that require immediate treatment:

Critical Red Flags Requiring Emergency Action

If the child has fever PLUS rash PLUS any of the following, start doxycycline 2.2 mg/kg twice daily immediately without waiting for laboratory confirmation:

• Headache, myalgias, or known tick exposure (even without recalled tick bite—40% of Rocky Mountain Spotted Fever cases report no bite history) 2, 3
• Rash that began 2-4 days after fever onset, particularly if involving ankles, wrists, or forearms 2, 3
• Petechial progression or rash spreading to palms and soles 2, 3
• Thrombocytopenia or hyponatremia on laboratory testing 2, 3

The mortality rate for untreated RMSF is 5-10%, with 50% of deaths occurring within 9 days of illness onset, and young children can deteriorate within days. 2, 3

Kawasaki Disease Evaluation

If fever has persisted ≥5 days, immediately assess for Kawasaki disease, which causes coronary artery aneurysms if untreated: 4, 3

Check for 4 of 5 features:

• Bilateral conjunctival injection (non-purulent)
• Oral mucosal changes (strawberry tongue, cracked lips, pharyngeal erythema)
• Cervical lymphadenopathy ≥1.5 cm
• Extremity changes (erythema/edema of palms/soles, later desquamation)
• Polymorphous rash (often truncal with groin accentuation) 4, 2

Young infants aged 6-12 months may present with fever and minimal clinical features ("incomplete Kawasaki disease"), making this age group particularly high-risk for missed diagnosis and coronary complications. 4

If Kawasaki disease is suspected, obtain ESR, CRP, albumin, CBC, and echocardiography, then start IVIG 2 g/kg plus high-dose aspirin 80-100 mg/kg/day within 10 days of fever onset. 4, 3

Most Likely Benign Diagnoses

Roseola Infantum (Exanthem Subitum)

This is the most probable diagnosis in a well-appearing 1-year-old with trunk rash: 1, 5

• High fever (often 39-40°C) for 3-4 days, then rash appears as fever resolves (pathognomonic feature) 1, 5
• Discrete rose-pink macular or maculopapular lesions, 2-3 mm diameter, starting on trunk then spreading to neck and proximal extremities 1
• Rash blanches with pressure, lasts 2-4 days, and resolves without treatment 1
• Child appears well, active, alert, and playful despite rash 1
• Febrile seizures occur in 10-15% during the febrile period 1

Other Viral Exanthems

Enteroviral infections are the most common cause of maculopapular rashes in children, typically involving trunk and extremities while sparing palms, soles, face, and scalp. 2

Additional considerations:

• Parvovirus B19 (fifth disease): "Slapped cheek" facial erythema with possible truncal involvement 2, 5
• Echovirus: Pink maculopapular rash on trunk and face, lasting 3-4 days, may have mild gastroenteritis 6

Management Algorithm

Step 1: Assess Fever Pattern and Timing

• Rash after fever resolves → Roseola infantum (no treatment needed) 1, 5
• Fever ≥5 days with rash → Evaluate for Kawasaki disease 4, 3
• Rash 2-4 days after fever onset → Consider RMSF, start doxycycline if any red flags 2, 3

Step 2: Examine Rash Distribution

• Trunk-predominant, blanching, maculopapular → Likely viral exanthem 1, 5
• Ankles/wrists spreading centrally to trunk → RMSF until proven otherwise 2, 3
• Groin accentuation with systemic features → Kawasaki disease 4, 2

Step 3: Assess General Appearance

• Well-appearing, playful, eating/drinking normally → Supportive care with antipyretics 1
• Ill-appearing, irritable, poor feeding → Urgent laboratory evaluation (CBC, CMP, ESR, CRP) 3

Step 4: Laboratory Testing (When Indicated)

Order immediately if considering RMSF or Kawasaki disease:

• Complete blood count (looking for thrombocytopenia, leukopenia) 2, 3
• Comprehensive metabolic panel (hyponatremia, elevated transaminases) 2, 3
• ESR and CRP (elevated in Kawasaki disease and RMSF) 3
• Urinalysis (sterile pyuria in Kawasaki disease) 4

Never delay empiric treatment for RMSF or Kawasaki disease while waiting for laboratory confirmation—both require immediate therapy based on clinical suspicion. 3

Common Pitfalls to Avoid

• Do not dismiss rash as "viral" or "antibiotic reaction" without excluding RMSF and Kawasaki disease first—these diagnoses are frequently missed in young children, leading to preventable mortality and morbidity 4, 2, 3
• Do not exclude RMSF based on absence of tick bite history—40% report no bite 2, 3
• Do not wait for rash to involve palms/soles before treating RMSF—only 50% develop rash in first 3 days, and 20% never develop one 2
• Do not use penicillins, cephalosporins, or sulfa drugs for suspected RMSF—they are completely ineffective against rickettsiae 3
• In infants <6 months with fever ≥7 days and no clear source, obtain echocardiography to evaluate for incomplete Kawasaki disease 4

Supportive Care for Benign Viral Exanthems

Once life-threatening conditions are excluded:

• Antipyretics (acetaminophen or ibuprofen) for fever and discomfort 1
• Adequate hydration 1
• Parental reassurance that most viral rashes resolve spontaneously in 2-4 days 1, 5
• No specific antiviral treatment needed 1
• Return precautions: worsening rash, petechiae, persistent high fever, irritability, poor feeding, or any concerning symptoms 1