Ultrastructural changes in follicles of small-intestinal aggregated lymphoid nodules in early and advanced phases of experimentally induced mucosal disease in calves

Objective-To investigate ultrastructural changes in follicles of small-inte stinal aggregated lymphoid nodules (Peyer's patches) of calves with early a nd advanced phases of experimentally induced mucosal disease (MD). Animals-Twenty 2.5- to 7-month-old Holstein-Friesian calves (11 females, 9 males). Procedure-MD was induced in 13 of 18 calves that were persistently viremic with bovine viral diarrhea virus (BVDV). Eight of the 13 calves were euthan atized before the onset of clinical signs of MD, and 5 were euthanatized af ter becoming moribund with MD, Five persistently viremic calves and 2 carve s without BVDV served as controls. Specimens of small-intestinal aggregated lymphoid nodules were prepared for transmission electron microscopy. Results-The ultrastructure of follicles of small-intestinal aggregated lymp hoid nodules from healthy carves was consistent with that in sheep. In the early phase of MD, changes were characterized by numerous apoptotic lymphoc ytes and macrophages with apoptotic bodies. In more advanced lesions, affec ted lymphoid follicles consisted of macrophages and variable numbers of fol licular dendritic cells (FDC), whereas others did not contain FDC. In morib und carves, small follicles consisting predominantly of FDC and follicles w ith central cavities surrounded by macrophages, and few neutrophils were ob served. Conclusions and Clinical Relevance-The ultrastructural changes in lymphoid follicles of small-intestinal aggregated lymphoid nodules indicate apoptosi s of lymphocytes as an initial event. The development of small follicles co nsisting predominantly of FDC or the complete loss of follicular architectu re in advanced phases of MD is determined by the intensity of apoptosis of lymphocytes, the capacity of the macrophages for uptake, and the reorganiza tion of a stromal network.