P arvoviruses (family Parvoviridae) are small (18-to 26-nm), nonenveloped, icosahedral viruses encapsidating a linear single-stranded DNA genome of approximately 5,000 nucleotides (1). Parvovirus B19, currently designated B19V, is a member of the family Parvoviridae, genus Erythrovirus (2). The B19V viral capsid has an icosahedral structure and is comprised of two proteins, VP1 (84 kDa) and VP2 (58 kDa). VP2 is the major protein (95% of the capsid composition) and contains receptor-and coreceptor-binding domains which together with self-assembly domains lead to the formation of highly stable particles (3). The minor capsid protein, VP1, differs from VP2 only in an N-terminal unique region (VP1u) composed of an additional 227 amino acids and has phospholipase A 2 (PLA 2 ) activity (4). VP1u elicits a dominant immune response (5).B19V infection in many instances is subclinical or presents as the mild and self-limiting fifth disease, or erythema infectiosum, typically manifesting as a "slapped cheeks" rash (6). Infection by B19V during pregnancy can result in several serious complications in the foetus, such as anemia, nonimmune hydrops, and ultimately intrauterine death (7). B19V is a potent inhibitor of hematopoiesis because it lytically infects erythroid progenitor cells (8), and in immunodeficient individuals, chronic B19V infection can result in chronic anemia (9). In individuals with underlying chronic hemolytic disorders, transient aplastic crisis following B19V infection can be a life-threatening complication (10). Finally, there is some evidence to suggest that B19V may infect other cell types, suggesting a possible involvement in the pathogenesis of a broad range of medical conditions, including idiopathic arthritis, vasculitis, meningoencephalitis, hepatitis, and myocarditis (11).Initially, the laboratory diagnosis of B19V infection was hampered by the lack of a cell culture system to grow the virus, necessitating the use of diagnostic tests lacking in sensitivity (12). Recently, recombinant DNA technology using prokaryotic (e.g., Escherichia coli) or eukaryotic (e.g., insect cells) expression systems has been applied to produce selected B19V antigens, in particular, VP1 and VP2. Prokaryotically expressed VP1/VP2 antigens undergo denaturation, and the antigen epitopes expressed are linear, whereas eukaryotic expression using baculovirus vectors generates empty capsids that are antigenically analogous to the native virus and include conformational epitopes. Assays using linear and those using conformational B19V VP1/VP2 can produce different results, leading to the general conclusion that conformational epitopes should be used for diagnostic purposes (13). Such assays need to be designed so that the presentation of conformational epitopes is optimized (14). Antibodies to linear VP2 epitopes are found mostly during acute infections and early convalescence, whereas those to conformational epitopes persist
