Experimental infection with bovine viral diarrhea virus (BVDV) genotype 2 in calves with neutralizing antibodies to BVDV genotype 1

The aim of this study was to determine the consequences of the infection of BVDV genotype II (BVDV-2) in calves with preexisting immunity to BVDV-1 (induced by infection with wild type BVDV (Group A, 4 calves), or induced by vaccination with a killed BVDV-1 product (Group B, 2 calves). Group A and B were intranasally exposed to a field isolate of BVDV-2 (25ml x 10(-7.8) TCID50/ml). Calves of Group A had erosions on the oral mucosa from 3 to 4 day post-challenge (DPC); moreover, intense salivation, mucous nasal secretion and sporadic diarrhea were observed from 4-14 DPC. Only one calf of Group B had diarrhea at 5 DPC. The average value of peripheral blood leukocytes (PBL) previous to BVDV-2 challenge was 8,775 PBL/ml (+/-603) in calves of Group A and 9,975 PBL/ml (+/-1308) in calves of Group B. After BVDV-2 challenge there was leukopenia in Groups A and B. The minimal number of PBL of 5,875 PBL/ml (+/-311) was observed in Group A at 6 PDC (P <0,05). There were differences (P <0,05) in the average of PBL during the 17 DPC among challenged animals and uninfected controls. Moreover, a difference in the number of blood lymphocytes was observed at 6-7 DPC between virus exposed and control calves. BVDV was isolated from ocular swabs of 3 calves of Group A at 13 and 17 DPC and from PBL at 9 and 11 DPC. The neutralizing antibody titer (NT) to both genotypes of BVDV was higher (P <0,05) in calves of Group A. Previous to virus challenge calves in this group had NT of 1: 125 (geometric mean) and 1:128 to BVDV-1 and -II, respectively. The NT rose to 1:2048 at 17 DPC to both Genotypes of BVDV. Calves in Group B had a NT of 1: 16 to both BVDV-I and -II previous to BVDV-2 challenge. At 17 DPC the NT rose to 1:64 and to 1:181 to BVDV-1 and -II respectively. These results allow us to conclude that antibodies generated by natural infection or vaccination with BVDV-1 increased for BVDV-1 and -2 after the experimental challenge with BVDV-2.