aplastic crisis

Parvovirus B19 is a non-enveloped, single-stranded DNA virus that binds the P antigen (globoside) on erythroid progenitors and lyses them, halting erythropoiesis for around a week. In healthy people the red cell lifespan of 120 days absorbs this pause unnoticed, but in chronic haemolysis the lifespan may be only 10 to 20 days, so production arrest causes haemoglobin to plummet within days. The defining laboratory finding is reticulocytopenia in the face of severe anaemia, which distinguishes an aplastic crisis from a splenic sequestration crisis or an acute haemolytic crisis, both of which show a high reticulocyte count. Bone marrow shows giant pronormoblasts with intranuclear inclusions. In immunocompetent patients the crisis is self-limited and treated with transfusion as needed; in immunocompromised patients B19 can cause chronic pure red cell aplasia treated with IVIG, and in pregnancy it causes fetal anaemia and hydrops fetalis. The user submission's description of reduced red cell production is correct as far as it goes but omits the low reticulocyte count and the haemolytic-disease context that define the entity.