Stages of Measles

Incubation Period

• Duration: ~10 days from exposure to onset of fever; ~14 days to appearance of rash
• The virus implants in the upper respiratory tract, replicates in the mucosal epithelium, then spreads via viremia to lymphoid tissues, bone marrow, skin, and CNS.
• No symptoms are present, but the patient becomes infectious from ~4 days before the rash appears.

Stage 1: Prodromal (Pre-Eruptive) Stage

• Timing: Days 10-14 after infection (lasts 2-4 days before rash)
• Features:
  • High fever
  • Coryza - sneezing, nasal discharge
  • Conjunctivitis (red eyes, lacrimation, photophobia)
  • Prominent cough
  • Vomiting or diarrhoea may occur
• Pathognomonic sign - Koplik's spots: Small, bluish-white spots on a red base (described as resembling table salt crystals) appearing on the buccal mucosa opposite the lower molars, 1-2 days before the rash. They may spread to other areas of the buccal mucosa and pharynx.

• Communicability: Maximum during this phase - virus is present in tears, nasal/throat secretions, urine, and blood.

Stage 2: Eruptive (Exanthematous) Stage

• Timing: ~Day 14 from exposure; rash lasts 6-7 days
• Rash characteristics:
  • Dusky-red, macular or maculopapular (morbilliform) lesions
  • Begins behind the ears and on the anterior scalp hairline
  • Spreads rapidly over the face and neck within hours, then down the trunk, reaching the lower extremities by day 2-3 (unlike rubella, which spreads more rapidly)
  • Lesions are most confluent in initially involved areas; may be more discrete on extremities
  • Often becomes confluent and blotchy
  • After 6-7 days the rash fades in the same craniocaudal order, leaving a brownish discolouration that may persist for 2 months or more
• Fever: Persists during the rash but falls as rash clears; leukopenia is common (pronounced leukopenia <2000 cells/mm³ is associated with poor prognosis)
• Pathophysiology of rash: The rash develops from interaction of immune T cells with virus-infected endothelial cells in small blood vessels. Patients with defective cell-mediated immunity do not develop a rash.
• As the maculopapular rash appears, circulating antibodies become detectable, viremia disappears, and fever falls.

Stage 3: Post-Measles Stage

• Timing: After rash resolution
• The child is weakened and has lost weight.
• Complications arising in this stage (occur in ~30% of cases):
  • Respiratory: Pneumonia (1-6%), laryngotracheobronchitis (croup), otitis media (7-9%)
  • GI: Diarrhoea (8%), protein-losing enteropathy (especially in developing countries)
  • Neurological: Post-infectious encephalitis (~1-4 per 1,000-2,000 cases)
  • Late neurological: Subacute sclerosing panencephalitis (SSPE) - develops 2-10 years after infection in ~1 per 10,000-100,000 cases
  • Other: Cancrum oris, reactivation of pulmonary TB, pyogenic infections, candidiasis, growth retardation
• Immune suppression: Profound depression of cell-mediated immunity (due to IL-12 downregulation by measles virus in monocytes/macrophages) persists for several weeks after acute illness, greatly increasing susceptibility to bacterial superinfections.

Summary Table

Classic diagnostic tip: The diagnosis of measles is practically incorrect in any febrile exanthem where red eyes and cough are absent. - Park's Textbook of Preventive and Social Medicine