International audience ; The enteropathogenicity of avian reoviruses (ARVs), isolated from chickens affected with malabsorption syndrome (MAS) from The Netherlands and Germany was studied. In the first trial seven different ARVs isolated from MAS cases were inoculated in 1-day-old specific pathogenic free (SPF) white leghorns. The pathogenicity was compared with 2 ARVs isolated from cases of tenosynovitis, namely reference strain S1133 and a Dutch strain. Although a difference in the severity of the clinical disease was observed, all reoviruses could induce vacuolar degeneration and sloughing of the epithelium of the small intestine at day 2 post inoculation (PI) till day 7 PI. Two Dutch and one German ARV derived from MAS causing the most severe intestinal lesions at day 2 PI, were further studied in the second trial using SPF broilers. These reoviruses did not cause weight gain depression in the broilers although lesions in the small intestine were present from day 1 up to day 4 PI and were more severe than in the white leghorn chickens. In one of the inoculated groups apical denuded villi were already present at day 1 PI. At day 7 PI the small intestine of the infected broilers appeared to be normal. Reovirus antigen was detected in the cytoplasm of the enterocytes at the tip and middle section of the affected villi both in layers and in broilers. To study the role of intestinal CD4$^+$ and CD8$^+$ T-cells and macrophages/monocytes in the pathogenesis of ARV, the numbers of these cells of the jejunal villi of one infected and the control broiler groups were compared. CD4$^+$ T-cells were detected in low numbers and only in the infected broiler group at day 14 PI. The numbers of CD8$^+$ T-cells and macrophages/monocytes were significantly higher in the infected broiler group than in the control broiler group at day 7 and 14 PI and at day 7 PI respectively. Our study indicates that the reovirus alone cannot induce intestinal lesions as found in MAS chickens. Moreover, CD8$^+$ T-cells may play a major role in ...