Replication of PPV in vitro is cytocidal and characterized by "rounding up," pyknosis, and lysis of cells (Fig. 1A). Many of the cell fragments often remain attached, eventually giving the affected culture a ragged appearance. Intranuclear inclusions develop but they are often sparsely distributed. Infected cultures may hemadsorb slightly (Fig. 1B). Cytopathic changes are extensive when cell culture-adapted virus is propagated under appropriate conditions. However, on initial isolation several serial passages of the virus or, better, the infected culture may be necessary before the effects are recognized. The use of immunofluorescence (IF) microscopy greatly increases the likelihood of detecting minimally infected cultures.