Page 733 - Veterinary Immunology, 10th Edition

In general, the response to these viruses is determined by the
VetBooks.ir state of immunological development of the fetus. For example, if

live bluetongue virus vaccine, which is nonpathogenic for normal
adult sheep, is given to pregnant ewes at 50 days post-conception, it

causes severe lesions in the nervous system of fetal lambs,
including hydranencephaly and retinal dysplasia, whereas if it is
given at 100 days post-conception or to newborn lambs, only a mild
inflammatory response is seen. Bluetongue vaccine virus given to

fetal lambs between 50 and 70 days post-conception may be isolated
from lamb tissues for several weeks, but if given after 100 days,
reisolation is not usually possible. Akabane virus acts in a similar
fashion in lambs. If given before 30 to 36 days post-conception, it

causes congenital deformities. If given to older fetuses, it provokes
antibody formation and is much less likely to cause malformations.
Piglets infected by parvovirus before 55 days post-conception will
usually be aborted or stillborn. After 72 days, however, piglets

develop high levels of antibodies to the parvovirus and survive.
Prenatal infection of calves with BHV-1 results in a fatal disease, in
contrast to postnatal infections, which are relatively mild. The
transition between these two types of infection occurs during the

last month of pregnancy.
The effects of the timing of viral infection are well seen with
bovine viral diarrhea virus (BVDV). If a cow is infected early in
pregnancy (up to 50 days), she may abort. On the other hand,

infections occurring between 50 and 120 days, before the fetus
develops immune competence, lead to asymptomatic persistent
infection because the calves develop tolerance to the virus (Fig.
23.2). These calves are viremic yet, because of their tolerance, fail to

make antibodies or T cells against the virus. Some of these calves
may show minor neurologic problems and failure to thrive, but
many are clinically normal. If the cow is infected with BVDV
between 100 and 180 days post-conception, calves may be born

with severe malformations involving the central nervous system
and eye, as well as jaw defects, atrophy, and growth retardation.
Calves infected after 150 to 180 days' gestation are usually clinically
normal.

733