Erythema infectiosum (fifth disease) is caused by the human parvovirus B19, a single-stranded DNA virus producing a benign viral exanthem in healthy children. The viral affinity for red blood cell progenitor cells makes it an important cause of aplastic crisis in patients with hemolytic anemias, including sickle cell disease, spherocytosis, and thalassemia. Parvovirus B19 also causes fetal anemia and hydrops fetalis after primary infection during pregnancy. The cell receptor for parvovirus B19 is the erythrocyte P antigen, a glycolipid present on erythroid cells. The virus replicates in actively dividing erythroid stem cells, leading to cell death that results in erythroid aplasia and anemia.
Erythema infectiosum is common. Parvovirus B19 seroprevalence is only 2% to 9% in children younger than 5 years of age but increases to 15% to 35% in children 5 to 18 years and 30% to 60% in adults. Community epidemics usually occur in the spring. The virus is transmitted by respiratory secretions and by blood product transfusions.
The incubation period is typically 4 to 14 days and rarely maylast 21 days. Parvovirus B19 infections usually begin with a mild,nonspecific illness characterized by fever, malaise, myalgias, andheadache. In some cases, the characteristic rash appears 7 to 10days later. Erythema infectiosum is manifested by rash, low-grade or no fever, and occasionally pharyngitis and mild conjunctivitis. The rash appears in three stages. The initial stage istypically a “slapped cheek” rash with circumoral pallor. An erythematous symmetric, maculopapular, truncal rash appears 1 to4 days later, then fades as central clearing takes place, giving adistinctive lacy, reticulated rash that lasts 2 to 40 days (mean, 11days). This rash may be pruritic, does not desquamate, and mayrecur with exercise, bathing, rubbing, or stress. Adolescents andadults may experience myalgia, significant arthralgias or arthritis, headache, pharyngitis, coryza, and gastrointestinal upset.
Children with shortened erythrocyte life span (e.g., sickle cell disease) may develop a transient aplastic crisis characterized by ineffective erythroid production typically lasting 7 to 10 days (see Chapter 150). Most children with parvovirus B19-induced transient aplastic crisis have multiple symptoms, including fever, lethargy, malaise, pallor, headache, gastrointestinal symptoms, and respiratory symptoms. The reticulocyte count is extremely low, and the hemoglobin level is lower than usual for the patient. Transient neutropenia and thrombocytopenia also commonly occur.
Persistent parvovirus B19 infection may develop in children with immunodeficiency, causing severe anemia resulting from pure red blood cell aplasia. These children do not display the typical manifestations of erythema infectiosum.
Hematologic abnormalities occur with parvovirus infection, including reticulocytopenia lasting 7 to 10 days, mild anemia, thrombocytopenia, lymphopenia, and neutropenia. Parvovirus B19 can be detected by PCR and by electron microscopy of erythroid precursors in the bone marrow. Serologic tests showing specific IgM antibody to parvovirus are diagnostic, demonstrating infection that probably occurred in the prior 2 to 4 months.