What is the diagnosis and treatment for a 13-month-old with a 3-day history of fever, fussiness, and a rash resembling blisters on the buttocks, hands, and thighs, following recent travel and a history of diaper rash?

Diagnosis and Management of a 13-Month-Old with Fever and Vesicular Rash

This clinical presentation is most consistent with hand-foot-and-mouth disease (enteroviral infection), but you must immediately rule out Rocky Mountain Spotted Fever (RMSF) given the recent international travel, as RMSF can be fatal within 9 days and requires immediate doxycycline treatment. 1, 2

Immediate Life-Threatening Differentials to Exclude

Start empiric doxycycline immediately if ANY suspicion exists for RMSF, without waiting for laboratory confirmation. 1, 2

• RMSF can be fatal within 9 days of illness onset, with 50% of deaths occurring in this timeframe 3, 1
• Up to 40% of RMSF patients report no tick bite history, so absence of known tick exposure does NOT exclude this diagnosis 3, 1, 2
• Early serology (IgM and IgG) is typically negative in the first week of illness, so negative antibody testing does NOT rule out RMSF 3, 1
• RMSF should be considered endemic throughout the contiguous United States regardless of geography 3, 2
• The rash in RMSF typically begins on wrists, ankles, and forearms as small blanching pink macules that evolve to maculopapules and can involve palms and soles 2

Consider meningococcemia, which cannot be reliably distinguished from RMSF on clinical grounds alone—administer intramuscular ceftriaxone pending blood culture results if the child appears ill. 1, 2

Most Likely Diagnosis: Enteroviral Infection (Hand-Foot-and-Mouth Disease)

Given the clinical description of blisters on buttocks, hands, and thighs following 3 days of fever in a 13-month-old, the most likely diagnosis is an enteroviral infection. 1, 2

• Enteroviruses commonly cause fever with vesicular rash that can involve palms, soles, and diaper area 1, 2
• The timing (rash appearing after several days of fever) and distribution are classic for this diagnosis 1

Critical Physical Examination Findings You Must Document

Examine for these specific features to differentiate benign from life-threatening conditions:

• Palms and soles involvement: If present with petechiae or purpura, strongly suggests RMSF or meningococcemia 1, 2
• Type of lesions: Vesicles on an erythematous base suggest viral exanthem; petechiae or purpura suggest serious bacterial infection 1, 2
• Mucous membrane involvement: Oral erosions or ulcers support enteroviral infection; conjunctival injection with oral lesions may suggest Kawasaki disease 3, 1
• Check scalp, axillae, and groin for attached ticks 2
• Assess for signs of toxicity: Altered mental status, hypotension, respiratory distress, or decreased level of consciousness indicate medical emergency requiring immediate hospitalization 1

Mandatory Immediate Diagnostic Workup

Obtain these tests before initiating treatment:

• Complete blood count with differential, C-reactive protein, comprehensive metabolic panel 1
• Blood culture before antibiotics 1
• Urinalysis and urine culture (urinary tract infections cause >90% of serious bacterial illness in this age group) 1
• Acute serology for Rickettsia rickettsii if any tick exposure possible or geographic risk exists 1, 2

Key laboratory findings that suggest RMSF rather than viral illness:

• Thrombocytopenia (platelet count <150,000/µL) 3
• Elevated liver enzymes (AST >40 U/L, ALT >55 U/L) 3
• Leukocytosis or leukopenia 3

Treatment Algorithm

If ANY Suspicion for RMSF:

Start doxycycline immediately, regardless of age—doxycycline is indicated in children <8 years for suspected RMSF. 1, 2

• Do NOT wait for serologic confirmation, as early serology is typically negative 3, 1
• Broad-spectrum antimicrobials including penicillins, cephalosporins, and erythromycin are NOT effective against rickettsiae 3, 1
• Consider adding intramuscular ceftriaxone to cover meningococcemia pending blood cultures 1, 2

If Enteroviral Infection is Confirmed (After Excluding RMSF):

• Supportive care with adequate fluid intake during high fever 4
• No antibiotics are indicated, as they are not effective against enteroviruses 4
• Counsel parents that the illness is self-limited and benign 4
• Instruct parents to return if warning signs develop 4

Critical Pitfalls to Avoid

Never exclude RMSF based on:

• Absence of reported tick bite (40% have no history) 3, 1, 2
• Geographic location (endemic throughout contiguous US) 3, 2
• Negative early serology (antibodies not detectable in first week) 3, 1, 2

Never assume a benign diagnosis based solely on "diaper rash" appearance—many serious conditions can present with rash in the diaper area. 1

Never wait for positive serology to treat suspected RMSF—antibodies are not detectable in the first week of illness. 3, 1, 2

Disposition Criteria

Admit to hospital if:

• Child appears toxic or has signs of sepsis 1
• Petechial or purpuric rash develops 1
• Hypotension, altered mental status, or respiratory distress present 1
• Suspected meningococcemia or RMSF with systemic symptoms 1
• Progressive clinical deterioration 4

Outpatient management acceptable if:

• Child appears well and playful despite fever 4
• No petechiae, purpura, or signs of toxicity 1
• RMSF and meningococcemia definitively excluded 1
• Reliable follow-up ensured with clear return precautions 4