Measles Rash Does NOT Have Vesicles
The measles rash is characteristically maculopapular (flat red spots that become raised bumps), NOT vesicular, and this distinction is critical for differentiating measles from varicella (chickenpox) and other vesicular exanthems. 1
Key Characteristics of Measles Rash
The measles rash follows a predictable pattern that helps distinguish it from vesicular diseases:
- Rash morphology: The rash is erythematous and maculopapular, beginning as flat red spots that evolve into raised papules but never progress to vesicles or bullae 1, 2
- Distribution pattern: The rash classically begins on the face and spreads cephalocaudally (head to toe) to the trunk and extremities, becoming more confluent as it spreads 2, 3
- Timing: The rash appears after a prodromal phase of fever, cough, coryza (runny nose), and conjunctivitis, typically accompanied by pathognomonic Koplik spots (small white spots on the buccal mucosa) 1, 2, 4
Critical Distinction from Vesicular Rashes
This distinction has important clinical implications for diagnosis and infection control:
- Varicella (chickenpox) has vesicles: In contrast to measles, varicella presents with a characteristic vesicular rash with lesions in different stages of development, and the vesicular fluid is highly contagious 1
- Rickettsial infections may have vesicles: Some rickettsial infections can present with maculopapular OR vesicular rashes, but measles specifically does not 1
- Kawasaki disease lacks vesicles: The guidelines explicitly state that "bullous and vesicular eruptions have not been described" in conditions like Kawasaki disease that present with maculopapular rashes similar to measles 1
Pathophysiology Supporting Non-Vesicular Nature
The mechanism of measles rash formation explains why vesicles do not occur:
- Cellular infiltration pattern: Measles virus infects lymphoid and myeloid cells in the dermis first, then spreads to keratinocytes in the epidermis, causing hyperemia and edema that creates the maculopapular appearance rather than fluid-filled vesicles 5
- Immune-mediated clearance: The rash represents infiltrating immune cells clearing MV-infected cells, producing the characteristic morbilliform (measles-like) appearance without vesicle formation 5
Clinical Pitfall to Avoid
Do not confuse measles with varicella based on rash morphology alone—the presence of vesicles essentially rules out measles and should prompt consideration of varicella, herpes zoster, rickettsial infections, or other vesicular exanthems 1. The maculopapular nature of measles rash, combined with the classic prodrome of cough, coryza, conjunctivitis, and Koplik spots, provides the diagnostic framework 1, 2.