Transmission is commonly by droplet inhalation. Rarely, the transfusion of blood or blood products may lead to infection. At the peak of the viraemia, 1 week after exposure, there may be a mild flu-like illness with fever, chills and malaise, lasting 2-3 days. In many cases infection occurs without further symptoms. In erythema infectiosum the first sign is often marked erythema of the cheeks (slapped cheek appearance). A faint pink macular or maculopapular rash then develops on the trunk and limbs. As the rash fades a lacy or reticulate pattern may emerge. Recrudescence is common, especially after bathing or exposure to sunlight. In adult women symmetrical arthropathy is common (80% of cases), involving ankles, knees, wrists and fingers. This is normally resolved within 2-4 weeks. Low-grade fever may be present, and the rash may be pruritic. Leukopenia, reticulocytopenia and thrombocytopenia occur. Differential diagnosis. Other exanthems, especially rubella, also scarlet fever, echovirus and coxsackievirus infections, and allergy. The diagnosis can only be made with certainty by laboratory tests.
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