BVDV is an enveloped,
single-stranded, positive-sense RNA virus in the genus Pestivirus and
family flaviviridae. Other pestiviruses include Classical Swine Fever
virus and Border Disease virus. With genetic sequencing, isolates can be
grouped into genotype 1 or genotype II; each genotype has at least two
subgenotypes. The genotypes can be further classified as a cytopathic biotype
or noncytopathic biotype, based on their effects in infected cell culture.
In cattle, clinical
manifestations of BVDV infections include subclinical disease, diarrhea,
respiratory infections, ill thrift, infertility, abortion, congenital defects
and gastrointestinal ulcerations, mainly in the abomasum and ileal Peyer's
patches (mucosal disease). Birth defects include cerebellar hypoplasia,
hydrocephalus, porencephaly, ocular anomalies, and cutaneous and skeletal
defects. Persistent BVDV infection can occur in a fetus infected with
noncytopathic BVDV infection in utero at less than four months
gestation. Persistently infected (PI) calves shed large amounts of viral
antigen and are at increased risk to the herd. If PI calves are infected with
a cytopathic BVDV biotype, or their non-cytopathic biotype mutates to a
cytopathic biotype, they are susceptible to fatal mucosal disease.
BVDV infected camelids
exhibit clinical signs similar to those in cattle; clinical manifestations
include abortions, early pregnancy loss, stillbirths, PI crias, diarrhea,
respiratory disease, and ill thrift. In one reported case, a PI alpaca cria
had a thin fleece with long hair-like fibers (from primary follicles) that
extended past the woolly fibers of secondary follicles. These clinical findings were similar to fleece
abnormalities in PI lambs infected with Border disease. Reportedly, in utero BVDV infection in NWC results in
PI crias; and, like PI calves, PI crias are viremic and shed large amounts of
viral antigen. However, the stage of gestation during which a camelid fetus is
persistently infected is unknown.
A study involving more
than 12,000 alpacas in North America was conducted to determine the most
prevalent subgenotype of BVDV in PI alpacas. RT-PCR identified 46 isolates
which were non-cytopathic biotype, type 1b subgenotype. Only one of the 46
isolates was phylogenetically indistinct from cattle isolates; 45 of the 46
isolates had over 99% identity to cattle isolates.
A separate study of 63
alpaca herds from the US identified 25.4% herds with a seropositive cria and
6.4% with a PI cria. In some cases, seropositivity was associated with
administration of bovine colostrums. Farms with PI crias reported economic
losses associated with abortions, treatment of weak crias, diagnostic testing,
and lost sales.
Some diagnostic tests for
BVDV in cattle are valid in NWC. Viral antigens can be detected by virus
isolation, PCR, or immunohistochemistry (IHC). BVDV can be isolated from lymph
nodes, whole blood, fetal tissue and placenta. Serology is also available to
detect antibodies. Detection of virus or viral antigen without antibody can
indicate a PI animal or an animal in the acute viremic stage of infection.
Detection of antibodies alone indicates previous exposure to the virus.
Because PI crias are believed to be the
main source of infection, efforts should be made to identify these animals. If
current diagnostic methods are used to identify PI crias, virus should be
detected in serum or buffy coat cells twice (21 days apart). Virus can be
isolated from buffy coat cells, genome can be detected via PCR of buffy coat
cells, and antigen can be detected in tissue by IHC. IHC of cutaneous biopsies
can react positively to germinal cells of the epidermis because hair follicles
can retain BVDV antigen in a low percentage of non-PI animals. If IHC is
positive, this test should be followed with a separate diagnostic test 30 days
later. Detection of an antibody titer on an animal that is also positive for
antigen does not rule out persistent infection. PI animals can have antibodies
to heterologous vaccines or natural infections; however, titers are usually
low. In cattle, it is recommended to test all new calves for 8-9 months after
a PI animal is detected and removed. Because the gestation period of NWC is
longer than in cattle, it is recommended to test all crias born 11 months after
removal of a PI animal. Seroprevalence
studies show that there are few seropositive NWC; these findings indicate a
large naïve population of NWC.
Biosecurity is critical for keeping herds free of BVDV. Naïve animals, especially pregnant females, should not
come into contact with infected animals. In addition to other NWC, cattle,
goats and sheep are also primary carriers of the virus. Currently, vaccinating
NWC with bovine vaccines for BVDV is not recommended. A bovine vaccine could
interfere with diagnostic testing and the efficacy or safety of these products
is not understood in camelids.
Bovine viral diarrhea
virus (BVDV) should be considered in the differential diagnosis in NWC that
have ill thrift, diarrhea, respiratory infections, abortion, and stillbirths.
Introduction of BVDV into a NWC herd can lead to devastating economic losses.
New additions to the herd should be screened for BVDV and biosecurity measures
should be in place to prevent introduction of this disease.
-by Erica Twitchell, PUSVM
Class of 2010
-edited by Dr. Tiffany
Reed, ADDL Graduate Student
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