Viral Characteristics
Bovine Viral Diarrhea Virus

Family: Flaviviridae
Genus: Pestivirus
Species: Bovine viral diarrhea virus

Classification


Flavirivirus
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Bovine viral diarrhea virus (BVDV) belongs to the family Flaviridae. This family contains 3 genera: flavivirus, hepacivirus and pestivirus. These three genera are similar in virion morphology, genome organization and replication strategy however they do not share serological reactivity. BVDV itself is of the pestivirus genera. This virus exists as two biotypes, cytopathic and noncytopathic. Cytopathic biotypes cause microscopically visible cellular changes while noncytopathic biotype strains will not cause visible changes within host cells (Kelling 2004). The original BVDV strain was noncytopathic. Viral evolution lead to a mutation within this noncytopathic strain creating a new cytopathic strain. Greater than 95% of BVDV infections are however of the noncytopathic form. Both biotypes can cause disease in cattle however only noncytopathic strains can cause persistent infections in bovine calves (discussed later) (Kelling 2004). 
There also exists 2 genotypes, BVDV-1 and BVDV-2. BVDV-1 is considered the classic form of BVDV and exists as subtypes 1a and 1b. Presently, there are no subtypes of BVDV-2 however within these two genotypes both cytopathic and noncytopathic biotypes exist. Severe clinical disease is most commonly associated with noncytopathic BVDV-2 and thus it is this genotype and biotype that is used for the production of most BVDV vaccines (Kelling 2004).

Viral Structure and Morphology

Flaviridae viruses are enveloped with an icosahedral/polyhedral symmetry of their nucleocapsid. Infection of Flaviridae viruses occurs via receptor-mediated endocytosis. Interaction between host receptors and 2 viral envelop gene proteins faciliates attachment. The genome of Flaviridae viruses is a positive sensed single stranded RNA molecule. Upon entry into the host cytoplasm, the viral genome is translated by host ribosomes into a single, large polyprotein. This protein is then cleaved by both host and viral proteases to produce approximately 10 proteins. The complement RNA, or negative sensed RNA molecule, is synthesized by non-structural viral proteins and used as a template for progeny genomes. The details about the replication strategy of Flaviridae are not fully known and are under investigation.


                     BVDV Electron Micrograph
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ABBREVIATIONS