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10 THE RURAL VOICE
DIAGNOSTICS
BOVINE VIRUS DIARRHEA II
by DR. G. K. A. JOSEPHSON
VETERINARY LABORATORY SERVICES
In last month's article, we discuss-
ed the clinical aspects of BVD. In this
article we will outline control and
preventative measures, as they exist at
the present time.
The tremendous amount of re-
search that has been conducted into
this costly disease over the past few
years has resulted in the development
of control programs that show a great
deal of promise. In the past, BVD was
either allowed to run its course within
a herd or a herd vaccination program
was instituted. Vaccination with mo-
dified -live BVD virus vaccines pro-
bably created as many problems as it
solved. Vaccine induced immunosu-
ppression often caused severe disease
problems. In other cases, outbreaks of
actual BVD followed the use of these
vaccines.
We now know the main drawback
from these practices was that persis-
tently infected animals were not iden-
tified or removed from the herd, as we
realized a single persistently infected
animal can perpetuate the disease to
future generations. The identification
and removal of these animals is nece-
ssary to prevent the spread of BVD.
A control program for infected
herds has recently been announced by
the Veterinary Laboratory Services
Branch. This program is aimed at
identifying persistently infected ani-
mals through virus isolation, removing
them from the herd, and then protect-
ing this closed herd through a vaccina-
tion program. Although the economic
benefits of this program have not yet
been investigated, it shows great
promise.
This program involves:
a) Identifying carriers by virus
isolation
Serum antibodies (indicating that
the animal has been exposed to the
disease and is now immune to it) exist
in 50 to 90 per cent of all cows. How-
ever the cattle that are negative may
either be unexposed and therefore
susceptible, or they may be persistent
car-riers and unable to mount an anti-
body response. It has been known for
a long time that the virus causing
BVD can be isolated from many body
fluids and tissues, including saliva,
tears, urine, and, most importantly,
blood.
In this program, blood samples are
collected from all animals in the herd
and submitted to the laboratory in
Guelph, where virus isolation is
performed on all of these samples.
This is a costly and time-consuming
procedure, taking up to four or more
weeks to perform. Noncytopathic
strains must also be identified.
All positive animals must be retest-
ed three or more weeks after the first
sample was taken in order to rule out
animals with acute BVD infections.
All animals that are positive (persis-
tently infected) on the retest are then
removed from the herd. All calves
that are born into the herd over the
next nine months must also be tested
to detect those that are persistently
infected.
b) Protecting the herd
Once the carriers have been re-
moved, it is important to prevent rein-
fection of the herd. Vaccination with
a killed BVD virus vaccine is a major
component of this protection plan.
The first dose should be given follow-
ing collection of the first blood sample
for virus isolation. All replacement
animals should be given three injec-
tions, consisting of an initial injection,
a booster, and a prebreeding injection.
Mature cows should be vaccinated be-
tween calving and rebreeding. Since
there are many different strains of the
virus, it is recommended that a differ-
ent manufacturer's vaccine be used
each year to take advantage of the
different strains that are in the differ-
ent vaccines.
To prevent entry of the BVD virus