Histopathology of chicks inoculated with chicken anaemia agent (MSB1-TK5803 strain).

One-day-old chicks, inoculated intramuscularly with the MSB1-TK5803 strain of chicken anaemia agent (CAA), showed a decrease of haematocrit value and inhibition of body weight gain, particularly between days 12 and 20 post inoculation (pi). Macroscopically, yellowish bone marrow, atrophy of the thymus and bursa of Fabricius, enlargement and discoloration of liver were observed in most chicks at the peak of infection. Histologically, the lesions appeared first in bone marrow and thymus on day 6 pi, and then in the bursa, spleen and liver. Swelling and intranuclear inclusion bodies in the haematopoietic precursor cells, thymocytes and reticular cells of the thymus were considered to be characteristic lesions. Hypoplasia or aplasia of bone marrow, depletion of lymphocytes in the lymphoid organs and swelling of hepatocytes occurred during the anaemic phase. Repopulation of the haematopoietic cells and lymphocytes occurred between days 16 and 24 pi, and most tissues of the surviving chicks recovered by day 32 pi.