Extravascular haemolytic anaemia in chicks infected with highly pathogenic Marek's disease viruses

Chicks were infected at 1 day of age with highly pathogenic AC-1 or RB-1B isolates of Marek's disease virus and 14 to 36 days post-exposure they were bled for haematocrits and euthanized. At necropsy, specimens were collected for light and electron microscopy. Both viruses caused significant reductions in packed cell volume (P<0.01). Hyperplasia of reticuloendothelial (RE) cells in the liver was a common finding in chicks that were ill from infection with AC-1 or RB-1B viruses. Erythrophagocytosis by sinusoidal macrophages was common and some of these cells contained an overload of haemosiderin. RE cells in the spleen were also active in destruction of erythrocytes. In chicks that were severely affected by AC-1 virus(s) there were often groups of RE cells in liver that had undergone lytic necrosis. Some of these foci appeared to have been replaced by proliferative lesions that ranged from being lymphogranulomatous to neoplastic. Tumour development was more apparent with RB-1B than with AC-1 and the multiple tiny lymphomas observed in RB-1B infected livers appeared to have developed from sinusoidal mononuclear cells. Ultrastructurally, Kupffer cells often contained stacks of electron light cisternal tubules that were densely packed with parallel fibrils. There were also groups of short electron opaque cisternal tubules that sometimes contained ghosts of fibrils packed in parallel. These organelles gave positive reactions for catalase and peroxidase and might have been peroxisomes that developed from the cisternal system due to the overload with iron. It was concluded that extravascular haemolytic anaemia resulted from erythrophagocytosis by hyperplastic RE cells.