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Human parvovirus B19‐induced transient red cell aplasia in a patient with vitamin B12 deficiency

A 27-year-old man presented with a 2-week history of malaise and cough followed by a fever of 39 0°C. A full blood count showed leucopenia (leucocytes 2 2 9 10/l), macrocytic anaemia (haemoglobin concentration 87 g/l, MCV 123 2 fl), thrombocytopenia (platelet count 30 9 10/ l) and decreased reticulocytes (0 3%). Laboratory tests showed increased lactate dehydrogenase, undetectable serum vitamin B12 and positive anti-parietal cell antibodies. Bone marrow aspiration showed a normocellular marrow with hypersegmented neutrophils, giant myeloid progenitor cells and giant proerythroblasts in the absence of maturing erythroid cells (top). Trephine biopsy sections showed some proerythroblasts, some with round, centrally situated intranuclear inclusions (bottom left, arrow). Immunohistochemical staining for human parvovirus (HPV) B19 was positive in early erythroid cells (bottom right). Although no anti-HPV B19 immunoglobulin (Ig)M or IgG antibodies were detected, high copy numbers of HPV B19 DNA were detected by polymerase chain reaction. Based on these findings, the diagnosis made was of a transient red cell aplasia induced by an acute HPV B19 infection with underlying vitamin B12 deficiency. A single dose of 200 mg/kg intravenous immunoglobulin was administered and treatment with methylcobalamin was initiated. The reticulocyte count began to increase soon after initiation of methylcobalamin followed by an increase in leucocytes and platelets. Both anti-HPV B19 IgM and IgG antibody became positive and 2 months later his blood count was normal. The bone marrow findings of this patient, i.e., presence of giant proerythroblasts and absence of maturing erythroid cells, are typical of acute HPV B19 infection. Given that erythrocytes have a long lifespan, acute HPV B19 infection causes anaemia only when the red cell lifespan is shortened. In this patient with vitamin B12 deficiency, the anaemia was not disproportionate to the reduction in leucocyte and platelet counts and the finding of red cell aplasia was an unexpected finding that led to the diagnosis of acute HPV B19 infection.