Characterization and phylogenetic analysis of Bovine viral diarrhea virus in brain tissues from nonambulatory (downer) cattle in Korea

Between August 2008 and May 2009, 386 brain and serum samples from adult cattle (2-7 years old) showing a variety of clinical signs of downer cow syndrome were received by the National Veterinary Research and Quarantine Service. All brain samples were tested for the presence of Bovine viral diarrhea virus (BVDV) by reverse transcription polymerase chain reaction (RT-PCR), immunohistochemistry (IHC), and antigen capture ELISA (Ag-ELISA). The BVDV nucleic acid was detected in 54 of 386 (15.5%) brain samples tested by RT-PCR. Positive results were detected in 14 (3.67%) and 13 (3.4%) of samples tested by IHC and Ag-ELISA, respectively. Both BVDV nucleic acid and antigen were detected in 11 cattle (2.9%) by all 3 diagnostic tests; however, antibodies against BVDV were not detected in these 11 cattle. A molecular classification of the identified viral strains (n 5 40) was also carried out. Neighbor-joining phylogenetic analysis revealed that most of the identified viruses belonged to BVDV genotype 1a (n 5 10), 1b (n 5 16), and 2a (n 5 8). The remaining strains were subtypes 1c (n 5 1), 1n (n 5 4), and 1m (n 5 1). Interestingly, most of the BVDV-1b strains (n 5 9) identified in brain samples were confirmed by all 3 diagnostic tests. Further studies should be performed to determine why the BVDV-1b strain was found in brain samples that were positive using all 3 diagnostic tests.