Diagnosing Bovine Viral Diarrhea in Persistently
Infected Animals
Bovine Viral Diarrhea (or BVD) is a disease complex with
many manifestations including subclinical infections, immunosuppression,
repeat breeding problems, abortion and mummification, congenital
defects, persistent infection (PI), and acute and chronic
mucosal disease. The PI animal results from a viral infection
of the fetus during the first four months of gestation. PI
calves may be born prematurely, grow poorly, exhibit lethargy,
have difficulty nursing, have reduced resistance to disease,
or die at a young age. Most PI calves will die within the
first 18 to 24 months of life; however, PI calves may appear
healthy and grow at a normal rate).
PI cattle in herds or newly introduced to herds are the
main source of BVD virus in cattle herds. Accurate identification
of these infected cattle is critical for eliminating infected
animals from herds. There are countries currently in the
European Union that are attempting to eliminate or have already
successfully eliminated BVD virus.
Clinically apparent PI animals and PI calves with mucosal
disease are not a diagnostic challenge, but accurate identification
of clinically silent PI animals requires an accurate herd
history and specialized diagnostic tests. The current “gold
standard” for demonstrating
BVD virus is virus isolation. A high level of viremia is
typical in most PI animals so virus is often easily isolated
from serum or buffy coats from whole blood. Virus can also
be isolated from nearly every tissue in the body, but greatest
results typically come from lymphoid organs (spleen, Peyer’s
patches, mesenteric lymph nodes, and thymus). This test can
be time consuming and very expensive if many animals are to
be tested. In addition, circulating maternal antibodies to
BVD virus can interfere with the virus isolation in very young
calves.
There are other molecular techniques available around the
country and across the globe that can aid in the diagnosis
of BVD. One of these techniques is immunohistochemical detection
of BVD viral antigen using frozen or fixed tissues. Since
most veterinarians submit fixed tissue, most immunohistochemistry
has been developed for formalin-fixed tissues. An antibody
is applied to the surface of fixed tissue that is tagged with
a signal. This antibody is specific for BVD antigen and,
if it finds its target, a chemical reaction takes place which
amplifies the signal. In PI animals, this technique can detect
the virus is nearly any tissue examined.
A second popular technique is Polymerase chain reaction
(PCR). This test is designed to be specific for a portion
of the viral genetic material. When the viral RNA is detected,
the signal is amplified again so that the signal can be visualized.
This method is very sensitive and can detect extremely small
amounts of BVD virus. PCR can be used on pooled samples,
such as bulk milk tank samples, thus screening large numbers
of animals in lots of several animals and, if a positive signal
is detected, the individual animals in that lot can be retested
to determine which animal or animals are positive.
In summary, there are many different tests available that
can detect PI animals in a herd. Blood, milk or tissues can
be submitted for virus isolation, or can be sent to various
diagnostic labs that routinely perform other molecular diagnostic
techniques such as immunohistochemistry or PCR. Virus isolation
remains the gold standard for definitive diagnosis of BVD
virus, but can become cost prohibitive if applied to large
herds and can yield false results when testing very young
calves that have circulation colostral antibodies.
If you have questions regarding diagnosis of BVD please
do not hesitate to call and we will discuss this with you
further.
-by Lorrie Culver, Class of 2000
-edited by Brad Njaa, DVM, MVSc
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