DEFINITION OF TERMS

Noncytopathic BVDV

A characteristic of BVDV which applies to how the virus interacts with cells in an artificial cell culture system. In cell culture, noncytopathic BVDV does not cause visible cell damage and death. In nature, noncytopathic BVDV is the most common biotype isolated. It causes acute infections with various clinical outcomes, intrauterine infections of fetuses, and persistent infections.

Cytopathic BVDV

A characteristic of BVDV which applies to how the virus interacts with cells in an artificial cell culture system. In cell culture, cytopathic BVDV causes characteristic visible cell damage and death. Cytopathic BVDV arises from genetic changes in noncytopathic virus. Isolation of cytopathic BVDV in association with acute infections or intrauterine infections is rare. Cytopathic BVDV has not been shown to cause persistent infections. Cytopathic virus is most important in the pathogenesis of mucosal disease.

Type I and Type II BVDV

Classification of type I and type II BVDV are based on significant differences in the genetic sequence of the 5^' untranslated region of the genome. Correlated with this grouping are differences in the antigenic makeup of the virus and the pathology manifested by the virus.

^{Mucosal Disease}

Mucosal disease is a rare, yet dramatic manifestation of BVDV infection. Two forms of mucosal disease have been describe: acute and chronic. Both arise under similar circumstances and are invariably fatal. Mucosal disease arises when cattle who are persistently infected with noncytopathic BVDV become superinfected with cytopathic BVDV. The cytopathic virus is most commonly derived from specific mutations that occur in the persistent noncytopathic virus. A key for mucosal disease to occur is that both the noncytopathic and cytopathic virus must be antigenically very similar.

Persistent Infection

Cattle persistently infected with BVDV are the major virus reservoir for the spread of the virus. Cattle become persistently infected when they are infected as a fetus between the 1st and 4th month of gestation with noncytopathic BVDV. At this stage of gestation, the fetal immune system is developing. Included in this process is the recognition of self antigens. BVDV circulating during this period is distinguished as a self antigen, preventing an immune response from eliminating the virus. Persistently infected animals who survive are constantly producing and shedding virus into the environment through nasal and oral secretions, urine, and feces. Live calves born to persistently infected dams will also be persistently infected with BVDV. The detection and elimination of persistently infected animals is key to the control of BVDV on a farm.

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