What are the characteristics of a measles rash?

Measles Rash Characteristics

The measles rash is a maculopapular or morbilliform eruption that begins on the face and spreads downward to the trunk and extremities in a cephalocaudal pattern, appearing 3-4 days after fever onset and lasting ≥3 days. 1

Rash Appearance and Distribution

• The rash is maculopapular (flat red spots with raised bumps) or morbilliform (measles-like) in character 1, 2
• It begins on the face and behind the ears, then spreads downward to the trunk and outward to the extremities 2, 3, 4
• The rash becomes more confluent (merges together) as it spreads in a cephalocaudal (head-to-toe) direction 4
• The rash must last ≥3 days to meet the clinical case definition for measles 1

Timing and Associated Features

• The rash appears approximately 14 days after exposure (range 7-18 days) and 3-4 days after the onset of prodromal symptoms 2, 3
• Rash onset coincides with peak fever (≥38.3°C or ≥101°F) and the height of systemic symptoms 1, 3
• The appearance of the rash represents the adaptive immune response with T cell infiltration into tissue sites of viral replication 5

Prodromal Features Before Rash

• A 2-3 day prodrome of fever, cough, coryza (runny nose), and conjunctivitis ("the three C's") precedes the rash 1, 2, 3
• Koplik spots (small white spots on a red background on the buccal mucosa) appear during the prodrome and are pathognomonic for measles, providing diagnostic opportunity before rash emergence 1, 2, 3, 4

Clinical Significance

• The rash marks the transition from the infectious prodromal phase to viral clearance, though patients remain contagious from 4 days before to 4 days after rash onset 1
• Clearance of infectious virus coincides with rash appearance, but viral RNA persists in blood and tissues for weeks to months after rash resolution 5

Important Caveats

• Vaccinated individuals may present with milder or atypical rash patterns, or even no rash at all 6
• Immunocompromised patients may develop severe, prolonged infection sometimes without the typical rash 2, 7
• The generalized maculopapular rash lasting ≥3 days combined with fever ≥38.3°C and at least one of cough, coryza, or conjunctivitis constitutes the clinical case definition 1