What are the main differences in patient presentation between 5th disease and measles, particularly in terms of clinical symptoms, demographics, and vaccination history?

Distinguishing 5th Disease from Measles

The key clinical differences are that measles presents with high fever (≥38.3°C), severe prodromal symptoms (cough, coryza, conjunctivitis), and a descending maculopapular rash that becomes hyperpigmented, while 5th disease (erythema infectiosum) typically presents with mild or absent fever, a "slapped cheek" facial rash followed by a lacy reticular rash on the extremities, and lacks the severe respiratory prodrome.

Clinical Presentation Differences

Fever Pattern

• Measles: High fever ≥38.3°C (≥101°F) is a defining feature, typically appearing during the prodrome and persisting through rash onset 1, 2, 3
• 5th Disease: Low-grade or absent fever; when present, it is mild and brief

Prodromal Symptoms

• Measles: Characterized by the classic triad of cough, coryza (runny nose), and conjunctivitis ("the 3 Cs"), which are severe and prominent 1, 2, 3, 4
• Measles: Koplik spots (small white spots on red buccal mucosa) appear during prodrome and are pathognomonic for measles 3, 5
• 5th Disease: Minimal or no prodromal symptoms; may have mild malaise

Rash Characteristics

Measles rash progression:

• Begins on the face (hairline/behind ears) and spreads downward in a cephalocaudal pattern to trunk and extremities over 3-4 days 2, 3, 4
• Maculopapular rash that becomes confluent, particularly on face and upper body 1, 4
• Rash duration ≥3 days is part of the clinical case definition 1
• Characteristically turns hyperpigmented as it resolves, with 89% of cases showing this feature 5

5th Disease rash progression:

• Begins with bright red "slapped cheek" appearance on face
• Followed 1-4 days later by lacy, reticular rash on trunk and extremities
• Facial rash typically spares the nasolabial fold
• Rash may wax and wane with temperature changes, exercise, or sun exposure

Contagiousness and Epidemiology

Transmission Period

• Measles: Highly contagious from 4 days before to 4 days after rash onset; airborne transmission with high attack rate 2
• 5th Disease: Contagious during prodromal phase before rash appears; once rash develops, patient is no longer contagious

Vaccination History

• Measles: Unvaccinated or incompletely vaccinated individuals are at risk; those born before 1957 generally considered immune 1
• 5th Disease: No vaccine available; immunity develops after natural infection

Complications

Measles Complications

• Diarrhea (most common), otitis media, bronchopneumonia are frequent 2, 3
• Encephalitis occurs in approximately 1 per 1,000 cases 2, 3
• Death in 1-2 per 1,000 reported U.S. cases, primarily from pneumonia and encephalitis 2, 3
• Subacute sclerosing panencephalitis (SSPE) is a rare but fatal late complication appearing years after infection 3

5th Disease Complications

• Arthralgia/arthritis, particularly in adults (more common in women)
• Aplastic crisis in patients with hemolytic anemias
• Hydrops fetalis if infection occurs during pregnancy

Diagnostic Approach

Clinical Diagnosis

• Measles: The combination of fever, maculopapular rash, and hyperpigmented rash has 90.7% sensitivity and 93.2% positive predictive value in outbreak settings 5
• Measles: Adding cough, coryza, and Koplik spots increases specificity 5

Laboratory Confirmation

• Measles: Serum IgM antibody is the primary diagnostic test; becomes detectable 1-2 days after rash onset, peaks at 7-10 days 1, 6
• Measles: All suspected cases should be laboratory confirmed, particularly isolated cases not part of an outbreak 1
• 5th Disease: Parvovirus B19 IgM antibody or PCR; however, diagnosis is often clinical

Common Pitfalls

Misdiagnosis Risk

• Rubella can be confused with measles but presents with milder symptoms, lower fever (>37.2°C), and prominent lymphadenopathy 1, 2
• In low-prevalence settings, false-positive measles IgM results can occur; confirmatory testing with direct-capture IgM EIA method is recommended when no epidemiologic linkage exists 6
• Atypical measles presentations in immunocompromised patients may lack the characteristic rash 3

Critical Action Points

• Measles requires immediate isolation: Airborne precautions with N-95 respirators for all staff, regardless of immunity status, due to ~1% vaccine failure rate 2
• Measles is reportable: Suspected and confirmed cases must be reported immediately to local health departments 1
• Healthcare workers exposed to measles without immunity should be excluded from work days 5-21 after exposure 2