Objective—To examine the role of bovine viral diarrhea
virus (BVDV) biotype on the establishment of
fetal infection in cattle.
Animals—30 mixed-breed pregnant cows.
Procedure—Pregnant cows were inoculated
oronasally with either i-VVNADL, originating from an
infectious BVDV cDNA clone of the National Animal
Disease Laboratory (NADL) isolate, or the parental
virus stock, termed NADL-A.
Results—All cows developed neutralizing antibodies
to BVDV, and virus was commonly isolated from
peripheral blood mononuclear cells or nasal swab
specimens of NADL-A inoculated cows; however,
virus was rarely isolated from specimens of i-VVNADL
inoculated cows. i-VVNADL did not cause fetal infection,
whereas all fetuses harvested from NADL-A
inoculated cows at 6 weeks after inoculation had evidence
of infection. Immunoblot analysis of fetal virus
isolates revealed the absence of NS3, confirming a
noncytopathic (NCP) biotype BVDV in the NADL-A
stock. The sequence of the NCP contaminant (termed
NADL-1102) and the i-VVNADL genome were virtually
identical, with the exception of a 270 nucleotide-long
insert in the i-VVNADL genome. Phylogenetic analyses
revealed that NADL-1102 forms a monophyletic
group with 6 other NADL genomes.
Conclusions and Clinical Relevance—These data
suggest that the contaminating NCP virus in the
NADL-A stock was the ancestral NADL virus, which
originally infected a bovine fetus and recombined to
produce a cytopathic (CP) variant. Following oronasal
infection of pregnant cows, viremia and transplacental
transmission of CP BVDV to the fetus is rare, compared
with the high occurrence of maternal viremia
and fetal infection observed with NCP BVDV.
(Am J Vet Res 2002;63:1455–1463)