Bovine Viral Diarrhea Virus
Viral Diarrhea (BVD) and Mucosal Disease (MD) are two diseases caused by the Bovine Viral Diarrhea Virus (BVDV), a
member of the Pestivirus genus of the
family Flaviviridae. To date, three
species of pestiviruses are recognized: BVDV, Hog Cholera Virus, and Border
Disease Virus (Coetzer and
Bovine Viral Diarrhea Virus is a single stranded RNA virus
with positive polarity. Virions are spherical particles that measure
approximately 50nm in diameter and are encapsulated by a tightly adherent
envelope containing glycolipids (Coetzer and
Bovine Viral Diarrhea Virus exists as one of two biotypes that are identified in the laboratory: Non-cytopathic isolates and Cytopathic isolates.
Non-cytopathic biotypes represent the majority of BVDV isolates collected from the field and do not affect the integrity of cultured cells (Murphy et al, 1999). It is important to note that non-cytopathic viruses are still pathogenic however they cannot cause overt pathological changes in affected cells. This is important diagnostically because infected cells may appear normal. Non-cytopathic biotypes are the most important economincally because they are capable of crossing the placenta and producing persistently infected (PI) calves (Deregt and Loewen, 1995). The immune system of PI calves is unable to recognize the virus as foreign and thus will not mount an immune response, allowing for continual shedding of the virus without clinical signs of infection. Non-cytopathic isolates in PI animals are capable of mutation to cytopathic forms that cause severe mucosal disease (Deregt and Loewen, 1999).
Cytopathic isolates are most often collected from animals
with mucosal disease and will alter the integrity (or cause death) of cultured
cells. Cultured cells may appear vacuolated (see photo). The viral protein NS3 is considered its
molecular marker (Coetzer and
In addition, two genotypes of BVDV (BVDV1 and BVDV2) are identified, within which non-cytopathic and cytopathic isolates exist. Recent studies show that genotype 2 isolates are more virulent than genotype 1 (Deregt and Loewen, 1995). Severe disease is most often caused by non-cytopathic BVDV-2 and is therefore used for the manufacture of most commercial vaccines.