In this study, a new BVDV JL-1 strain was isolated from calves afflicted with BVD, a recently discovered BVDV present in northeastern China. The identification of the isolate has been confirmed by IFA. Many other studies have demonstrated that the BVDV-1b subgenotype is the most common of the BVDV-1 field isolates. The trend of relative increase in BVDV-1b isolates was found not only during comparison of different geographic regions but also during comparison of the isolates found over the past 20years in one diagnostic laboratory [19, 20]. In this study, the phylogenetic relationships among BVDV JL-1 and other BVDV wild-type strains from GenBank were clarified. Genetic and phylogenetic analysis showed that the virus BVDV JL-1 belonged to BVDV1b, and a high degree of identity was detected among all studied wild-type strains in China.
The present pathogenesis study was performed upon conventionally reared calves. Results indicated that BVDV JL-1 can induce mild clinical disease. Clinical presentation after infection with BVDV JL-1 was similar to that observed after infection with the BVDV NY-1 strain. Clinical manifestations included mild short-term pyrexia (basal temperatures between 39.2 and 40.0°C for 1–3days), pronounced and prolonged febrile response, pronounced reduction in circulating white blood cells (>40%), and viral excretion in the buffy coat. All of the virus-infected calves developed virus-neutralizing antibodies by day 14. Strain NY-1 belongs to the BVDV1 pestivirus species (subgenotype BVDV1b). It is the first noncytopathic BVDV strain to be isolated. In recent years, the NY-1 strain has become an accepted laboratory reference strain. It has also been offered as a challenge strain for vaccine efficacy studies .
The clinical symptoms observed in calves from which BVDV JL-1 was isolated mainly showed anorexia, diarrhea, dehydration, and marasmus. The present pathogenesis study, which was performed upon BVDV JL-1, only mild clinical signs associated with BVDV infection were observed. These included depression, fever, leucopenia, and viremia. None of the treatment calves showed diarrhea. This might be related to the immune state and age of experimental animals. In this report, the BVDV JL-1 virus isolates were confirmed to be free of contamination by viruses, mycoplasmas, bacteria, and fungi. The experimental animals were 6 to 9 months old and immunocompetent. No secondary infections occurred during the animal experiment. Histopathological observation showed lymphoid depletion and shedding of the villi of the small intestine. If the treatment calves experienced a secondary infection at that time, the results led to an exacerbation of the inflammatory response and to the development of more intense clinical symptoms and lesions than those observed in healthy animals . Various authors have demonstrated that BVDV can induce lymphopenia and has a range of effects on the immune response. These can allow secondary infection to take place [22, 23].
Ever since the first isolation of BVDV in 1980, no effective measures have been developed to control the spread of BVDV in China. This is the first report of a pathogenic BVDV strain in China, here called BVDV JL-1. It is an NY-1 strain, so infection of immunocompetent adult calves can cause clinical manifestations. These findings may contribute to the development of a vaccine for the prevention and control of BVDV in China.