Changes

===Virus Genotypes===

===Virus Genotypes===

There are two antigenically distinct⁹ genotypes of BVDV, type 1 and type 2, which are most accurately characterised based on sequence variation. 11 and 3 subtypes exist within BVDV-1 and BVDV-2 respectively, which have been demonstrated by analysis of the 5’UTR¹⁰. Despite the large antigenic differences between the genotypes, cross-protection against type 2 viruses is afforded by type 1 vaccines^{11, 12, 13}.

There are two antigenically distinct⁹ genotypes of BVDV, type 1 and type 2, which are most accurately characterised based on sequence variation. 11 and 3 subtypes exist within BVDV-1 and BVDV-2 respectively, which have been demonstrated by analysis of the 5’UTR¹⁰. Despite the large antigenic differences between the genotypes, cross-protection against type 2 viruses is afforded by type 1 vaccines^{11, 12, 13}.

A split in virulence between genotypes is evident. BVDV1 species, including classical strains and common vaccine strains, tend to cause milder disease (Deregt, 2004) and are found worldwide. Alternatively, BVDV2 isolates typically cause more severe disease, characterised by fever, diarrhoea, thrombocytopaenia, haemorrhage, respiratory signs, and high abortion and mortality rates (Corapi et al., 1989; Carman et al, 1998). These viruses were first reported in Canada and the USA, although their distribution is widening. The first British case of type 2 BVDV was identified in 2002 (Drew et al., 2002).

In general, the genotypes differ in virulence. BVDV-1 species are found worldwide and tend to cause milder disease ¹⁴, wherease BVDV-2 isolates typically cause more severe disease which is often haemorrhagic and is associated with a high mortality rate^{15, 16}. Type 2 viruses were first reported in Canada and the USA and have a more limited distribution than type 1 isolates. The relationship between genotype and virulence is, however, not fixed: some type 2 strains cause mild or subclinical disease¹⁷, and the spectrum disease caused by type 1 viruses is broad.

 

However, genotype is not always an accurate indicator of virulence. Some type 2 strains cause only subclinical or mild disease (Ahn et. al, 2005), and the spectrum of type 1 disease is also broad.

===Virus Biotypes===

===Virus Biotypes===