Highly infective, presents with an acute catarrhal illness, fever, characteristic Koplik's spots on the buccal mucous membranes followed by a distinctive maculopapular rash. In large cities and towns, measles is most likely to occur in infants and preschool children, but in rural and less crowded urban areas the principal incidence is between the ages of 5 and 10 years. Measles is extremely rare under 3-4 months of age, because of protective maternal antibody, but authenticated cases have occurred.
Measles has an incubation period of 8-14 days. A mild illness may occur at the time of infection but most cases develop a prodromal illness some 3-5 days before the eruptive stage. The main features of this illness are pronounced catarrh, characterized by a constantly running nose, conjunctivitis and a harsh dry cough. Fever and irritability are usually present and there may be a fleeting scarlatiniform or morbilliform rash. Koplik's spots, the most pathognomonic sign of measles, appear during this stage and are seen as small, grayish white lesions on the buccal mucosa close to the posterior molar teeth; they are usually quite numerous but may be scanty or occasionally cover the entire lining of the cheek. They can be difficult to demonstrate and the angle of the inspecting light is critical; having faded, they are replaced by a dry, matte appearance on the mucosa which has a ground-glass-like surface.
The true rash of measles starts behind the ears and along the hair line. Fever, which will have lessened at the end of the prodromal period, may now rise again to 39-40degC, and the eruption spreads rapidly to involve the face. The lesions are maculopapular in character and of a dusky hue. Over the next 2 days the eruption spreads downwards and becomes generalized; marked confluence of the spots develops and this gives a blotchy appearance.
The extent and severity of the rash show wide variation. In some, especially the younger cases, the eruption may be unusually sparse and modification by maternal antibody has been suggested. There is frequently some degree of hemorrhage or diapedesis into the rash giving it a purpuric quality and subsequent skin staining. This should not be confused with the rare and usually fatal hemorrhagic measles in which extensive bleeding occurs into the skin and from the mucous membranes.
Fading of the rash can be surprisingly rapid but it usually disappears quite slowly beginning to fade on the third day in the order of appearance; the rash may be largely gone from the face and upper trunk by the fourth day though persisting on the lower extremities. After a further 3-4 days a brownish staining appears, probably due to capillary hemorrhage and on occasions this staining can be very intense. In severe cases a fine desquamation may occur at the site of the rash but this does not usually involve the hands and feet like scarlet fever.