Current situation of BVD in Spain: presence of genotype 2

Although the two BVDV species are genetically and antigenically different, they do share certain similarities, such as the fact that, based on their ability to induce changes in in vitro cell cultures, vacuolisation and cell lysis, isolates of this virus fall into two different biotypes (Birk et al., 2008; Neil et al., 2008). The cytopathic biotype (CP) causes changes in the cells that infect the in vitro culture, while the non-cytopathic biotype (NCP) does not cause any visible damage to the cell culture. Both biotypes can infect cattle and cause disease, but only the NCP biotype can cause persistent infection (Kelling, 2004). In addition, natural infections predominantly comprise the NCP biotype (Fulton, 2013). However, it should be pointed out that the virulence of the disease does not depend on the biotype, nor does it strictly depend on the genotype or the species. In fact, virulence depends on the specific strain of the virus and the immune status of the host (Ridpath, 2010). This is why disease outbreaks caused by both genotype 1 and genotype 2 have been reported with completely different manifestations, ranging from a completely silent disease onset with no apparent symptoms other than reproductive effects, to cases manifesting a broad spectrum of clinical symptoms (Carman et al., 1998; Bolin and Grooms, 2014). The way in which a strain manifests itself largely depends on its ability to affect lymphocyte count and the subsequent severity and duration of the state of immunosuppression induced. Thus, a highly virulent strain induces a significantly more severe and longer-lasting state of lymphopenia than less virulent strains (Liebler-Tenorio et al., 2003).