Clinical response and immunomodulation following experimental challenge ofcalves with type 2 noncytopathogenic bovine viral diarrhea virus

Eight calves between 16 and 18 weeks of age that were seronegative to bovin e viral diarrhea virus (BVDV), bovine leucosis virus and bovine immunodefic iency-like virus were infected (day 0) intranasally with the type 2 noncyto pathogenic Canadian 24515 field isolate of BVDV in order to evaluate the ef fect of BVDV infection on certain clinical, hematological and immunological parameters. All virus-exposed animals developed fever and showed a signifi cant (P < 0.05, 0.01 or 0.001) drop in the number of circulating leucocytes (neutrophils, lymphocytes and monocytes) by day 3 or 5 post-exposure (PE), which continued to the end of the experiment at day 12 PE. BVDV was consis tently isolated from the peripheral blood buffy coat cells from day 5 PE, a nd also from selected tissues (spleen, thymus, mesenteric and submaxillary lymph nodes, small intestine, lungs and thyroid gland) that were collected at the time of euthanasia of the animals at day 12 PE. Diminished significa nt (P < 0.05) percentages of peripheral blood mononuclear cells (PBMCs) exp ressing at their surface either B7 and MHC II molecules were observed in vi rus-exposed carves at days 7, 10 and/or 12 PE. when compared to virus-nonex posed control calves (n = 5). However, no changes in the percentages of PBM Cs expressing either B4 or MHC I molecules were observed throughout the exp eriment. Finally, a significant (P < 0.05 or 0.01) enhanced phagocytic capa bility of the PBMCs, as analyzed by flow cytometry, was observed in virus-e xposed animals at days 3, 5, 7, 10 and 12 PE, when compared to control calv es. These results demonstrated the virulence of the 24515 isolate of BVDV i n 4 to 4.5 month-old calves, and suggest that type 2 BVDV infection in calv es is associated with dysregulation of certain immunological functions.