Chapter 56

### INTRODUCTION

Human parvovirus B19, a single-stranded DNA virus, is a member of the erythrovirus genus within the Parvoviridae family and the only erythrovirus that is a pathogen in humans.1 Human parvovirus was first identified by electron microscopy in 1975 and was associated with clinical disease approximately a decade later.2 It is the etiologic organism that manifests clinically as erythema infectiosum (fifth disease), a common childhood viral exanthem. In immunocompetent adults, infection with B19 is generally asymptomatic or mild. It is most commonly transmitted through respiratory droplets, and primary B19 infection in pregnant women can also result in vertical transmission to the fetus. Although most commonly asymptomatic for both mother and fetus, in some cases primary or acute infection in pregnancy may lead to adverse perinatal outcomes, including spontaneous abortion, hydrops fetalis, and fetal demise.

### EPIDEMIOLOGY

Parvovirus B19 infection occurs worldwide, and cases may be sporadic or may occur in clustered outbreaks and even as epidemics.3, 4 The infection is easily transmitted from person to person via the respiratory route. In the United States, B19 infection occurs more frequently between late winter and early summer. Cycles of local epidemics have also been reported, with case numbers peaking approximately every 4 years.5, 6 Hematogenous transmission can also occur from administration of blood or blood products containing B19. Individuals requiring regular infusions of blood products that are made from large plasma pools are at greater risk for acquiring the virus compared to those individuals receiving single units.7

Seroprevalence rates of parvovirus B19 vary based on age and geographic location. The percentage of people with measurable levels of B19-specific immunoglobulin (Ig) G increases with increasing age, with most individuals becoming infected during their school years. Seroprevalence is approximately 15% of preschool children, 50% in adults, and 85% in the elderly3, 8 and is high (82%–89%) in women who live or work with young children.9 The secondary attack rate for household contacts may be as high as 50%.3 Immunity is lifelong in immunocompetent individuals.10

Nonimmune pregnant women have the same susceptibility to B19 infection as other immunocompetent adults. In the United States, 50%–75% of women in the reproductive age group are immune and demonstrate antibodies to human parvovirus B19.11, 12, and 13 Therefore, approximately 30%–50% of pregnant women are susceptible to B19 infection, which then places their fetus at risk of infection. The incidence of acute B19 infection in pregnancy may range from 1% to 3.8% and may be as high as 10% during epidemics.14, 15, 16, and 17

### PATHOPHYSIOLOGY

Parvoviridae are small, nonenveloped DNA viruses that infect a variety of animals, usually in a species-specific fashion; only parvovirus B19 is known to cause disease in humans. The virus replicates in erythroid progenitor cells (late erythroid cell precursors and ...

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