Immunoelectron microscopic identification of human parvovirus B19

A mildly macerated, hydropic fetus was delivered spontaneously at 25 weeks gestation by a multigravidous black mother. At autopsy, gross and microscopic evidence suggested fetal anemia, and excessive extramedullary erythroblastosis was present generally. Erythroid precursor cells in the pulmonary capillary circulation contained eosinophilic nuclear inclusions consistent with human parvovirus B19 infection. Transmission electron microscopy on osmicated Epon lung sections showed lucent, granular chromatin corresponding to the inclusions. In rare foci only these sections contained paracrystalline arrays of 20-nm virions. In the same cells, similar virions were seen within the cytoplasm, both randomly distributed and in paracrystalline aggregates. Postembedding immunoelectron microscopy, done on formalin-fixed lung embedded in a hydrophilic resin, showed positive labelling over the nuclear inclusions, and also localized the viral capsid antigen to cytoplasmic virion aggregates. The nuclear aggregates suggest that the virus would reach the circulation after cell lysis whereas those in cytoplasm suggest that parvovirus also may be excreted from infected cells before they lyse.