A study was made of some aspects of the pathogenic and antigenic
relationships between 2 reference strains of cytopathic (CP) pestivirus, namely
bovine virus diarrhoea (NADL strain) (BVDV-NADL) and border disease virus
(Moredun strain) (BDV-M).
Initial _in^ vitro studies showed that only cultured cells of bovine and
ovine origin allowed virus growth with the production of cytopathic effect.
BVDV-NADL gave comparable levels of virus in various foetal tissues from both
species, whereas ovine foetal tissues were superior for the growth and assay of
BDV-M. Ovine foetuses had the added advantage that only 2 of 105 (1.9%)
abattoir foetuses were contaminated with noncytopathic (NCP) pestiviruses as
against 15 of 113 (13.3%) bovine foetuses.
A severe acute disease resembling bovine mucosal disease was produced in 7
of 8 recovered hairy shaker (RHS) sheep which were persistently infected with
BDV and subsequently superinfected with high tissue culture passage BDV-M
derived from the infectious brain pool used to infect their dams at 54 days'
gestation. No disease occurred in RHS from the same origin superinfected with
BVDV-NADL or left unchallenged.
Superinfection with low passage BDV-M of RHS from field outbreaks of BD
produced serious acute or chronic disease in 4 of 10 cases but in none of 5
normal controls receiving the same inoculum. All surviving RHS lambs
seroconverted to BDV-M but NCP pestiviraemia was sustained and the animals
remained infectious to other lambs.
Low tissue culture passage BDV-M was used to infect 15 pregnant ewes at 54
days's gestation. Five produced 9 viable lambs, all of which had clinical BD
and an NCP pestiviraemia at birth. At 9 weeks of age a set of twins was killed
after exhibiting a mucosal disease-like syndrome, and CP BD virus was isolated
from many organs. The twins were the only live progeny of one of the 2 tups
used as sires. No such disease occurred in the other RHS in the same pen.
The pathogenicity for the placenta and foetuses of ewes infected with
plaque purifed stocks of the 2 reference strains or of 3 field isolates of CP
BVDV was very mild. Failure to recover virus easily was attributed to poor
replication of these stocks in vivo.
Antisera to the plaque purified viruses were produced in gnotobiotic lambs.
By cross-immunofluorescence the 5 viruses were indistinguishable whereas crossneutralisation segregated BDV-M as a distinct serotype from the 4 BVD viruses.
Western blotting demonstrated 2 virus-induced proteins (78K and 35K) in cells
infected with BVDV but only the 78K protein in cells infected with BDV-M.
Antiserum to this virus did not detect the 35K protein.