Abstract. To examine the virus-host interaction in subclinical bovine viral diarrhea virus (BVDV) infections, the spread of a BVDV 2 strain of low virulence to different organs and the development of lesions were investigated. Eight colostrum-deprived, clinically healthy, 2-3-month-old calves were intranasally inoculated with 10 6 tissue culture infective dose of the naturally occurring BVDV 2 strain 28508-5 of low virulence, and 2 served as controls. Two calves each were euthanized at days 3, 6, 9, and 13 postinoculation (pi). Representative tissues were processed for histology and immunohistology. Signs of overt clinical disease were absent. However, a mild temperature elevation at days 7 or 8 pi and a moderate decrease of circulating lymphocytes occurred in all inoculated calves. The BVDV antigen was detected at day 3 pi in several lymphoid tissues. At day 6 pi, BVDV antigen was found widespread in lymphoid tissues and multifocally in intestinal epithelial cells but was associated with no or subtle lesions only. At day 9 pi, much less BVDV antigen was detectable, but there was severe depletion of lymphoid tissues. At day 13 pi, BVDV antigen had been cleared from most lymphoid tissues that were at variable phases of depletion and recovery. In conclusion, the BVDV strain of low virulence spread to lymphoid tissues and intestinal epithelial cells but was rapidly eliminated. Transient depletion of lymphoid tissues was followed by recovery.
Bovine viral diarrhea virus (BVDV) is a Pestivirusin the family Flaviviridae. 17 Infections with BVDV are widespread in cattle herds and cause severe economical losses in North America and Europe despite intense efforts to control or eradicate them. 19 On the basis of genomic differences in the 5ЈNTR region of BVDV, 2 species, BVDV 1 and BVDV 2, are currently differentiated. 4,17,33 The BVDV strains can grow in epithelial cell cultures with or without inducing a cytopathic (cp) effect, thus allowing further grouping in cp and noncytopathic (ncp) biotypes. 13,32 Infections with BVDV of both genotypes have been associated with a wide variety of clinical conditions including subclinical infection, reproductive failure, severe acute disease with or without hemorrhagic diathesis, and mucosal disease. 3 During the last decade, an increased number of field cases of severe acute BVD in all age groups of cattle has been reported. 8,10,18,30 The BVDV isolates from these outbreaks in North America predominantly belong to BVDV 2. 8 Besides the highly virulent strains of BVDV 2, there also exist strains causing subclinical infections only. 35 These strains might be even more important for the spread of BVDV because they may circulate undetected in a herd for an extended time period and may predispose to secondary infections or disease.Highly virulent isolates from field cases of severe acute BVD have been used to investigate the pathogenesis and pathology of acute BVD, 1,5,9,12,16,25,28,37 but there exists only 1 study using a BVDV 2 strain of low virulence. 27 It is still unclear how virus...