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BVDV
– Bovine Virus Diarrhea Virus
Raymond
G. Lien ,DVM
Staff Veterinarian
Technical Service
Bovine Virus Diarrhea is a significant
disease in cattle throughout the world. Sweden and Denmark
have instituted eradication programs of testing, removal of PI
(persistently infected) animals and strict biosecurity
restrictions. The Academy of Veterinary Consultants have
adopted the resolution: “That the beef and dairy industries
adopt measures to control and target eventual eradication of
BVDV from North America”. The BVDV has been found to cause
clinical disease in sheep. Wild ruminants (mule deer, elk and
bison) may harbor and transmit the virus without exhibiting
significant disease.
The Bovine Viral Diarrhea Virus (BVDV) can
cause a variety of clinical diseases. Primary acute infection
may affect both the respiratory and the intestinal system,
resultant in the shedding of active virus. The BVDV causes an
immune suppression by destroying white blood cells. Dual
respiratory infections respond poorly to treatment and have a
higher mortality when caused by BVDV and any combination of
other pathogens: IBR (Infectious Bovine Rhinotracheitis
virus), BRSV (Bovine Respiratory Syneytial virus), Mannheimia
haemolytica, Pasteurella multocida. This synergistic
effect is likely due to the immunosuppression by the BVDV. A
chronically infected animal with a suppressed immune system
may be diagnosed as mucosal disease and eventually die. The
BVDV may only survive in the environment for a short period of
time, however, these chronically infected and persistently
infected animals are a constant source of virus for other
animals in close contact.
The effect on the developing fetus depends
upon the stage of gestation. The viremia from an acute
infection or exposure in the pregnant heifer/cow during the
first 60 days of gestation can cause death of the embryo with
few noticeable signs other than being open or late in calving.
Abortion can occur throughout the pregnancy and BVDV is
frequently isolated in cases of late term abortions or
stillbirths. Fetal exposure to BVDV between the fourth and
sixth month of gestation may result in congenital defects.
Of primary concern is the Persistent
Infection (PI) that occurs when a fetus is exposed to a non-cytopathic
strain of BVDV between 60 and 120 days of gestation. Non-cytopathic
strains of the virus do not cause fetal death and because the
fetal immune system does not recognize the virus as a foreign
organism, the fetus is carried to term and will continue to
shed the virus throughout its life. Often the PI individual is
noticed as the “poor doing” or “unthrifty” calf.
However, the PI individual may also be an unnoticed heifer or
bull that stays in the herd for a period of time, continuing
to be a reservoir of virus. It is estimated that potentially
1% of all cattle are persistently infected. Pregnant females,
who are not adequately immunized, and calves less than 6
months of age, with waning maternal antibodies, are quite
susceptible to this constant virus exposure.
Control efforts are hampered by the fact
that the BVDV readily mutates as it replicates in the animal.
There are over 140 identified strains though we currently deal
with just a few in natural disease and vaccine production. The
biotype designation, cytopathic (CP) or non-cytopathic (NCP)
is based on the visual effects of how the virus grows on cells
in the laboratory. The majority of the viruses isolated from
infected animals are non-cytopathic. The BVDV has been
categorized into different genotypes and subtypes: type 1a,
type 1b and type 2 BVDV. Each subtype has a predilection for
different organs in the body and differs in the severity of
disease associated with each.
Different manufacturers use various
combinations of the virus antigens in their vaccine
production. Currently there are many studies being conducted
to determine the immune response provided by the different
strains, biotypes and genotypes of BVDV in different vaccines.
A primary goal is to provide adequate fetal protection in the
pregnant female. Diagnostic laboratory testing can provide
your veterinarian with data to advise which vaccine antigens
should be most effective in protecting your herd. There are
advantages to the use of either the killed virus (KV) or the
modified live virus (MLV) vaccines. KV vaccines require two
primary doses spaced three to four weeks apart and are safe in
pregnant females or calves nursing pregnant females. MLV
vaccines tend to stimulate a longer lasting immunity and it is
being recommended that female replacement cattle receive at
least two MLV vaccinations prior to first breeding. As with
all vaccines, these products should be used according to the
product label instructions and indications in conjunction with
your local veterinarian’s counsel. Vaccines are an important
part of disease control when used in conjunction with
effective biosecurity.
Several diagnostic tests are available to
determine the presence of BVDV in a herd. Serological testing
on blood samples has limitations due to confusing titers from
vaccinations. Virus isolation can be quite specific but is
more costly and requires more laboratory time.
Immuno-histochemistry (IHC) on a skin biopsy or ear-notch is a
reliable and cost-effective diagnostic tool to identify and
eliminate the PI calf shortly after birth or provide assurance
that the new replacement heifers or new bull are not an
unsuspected PI virus shedder.
Quarantine of new animals before inclusion in the herd
allows time for diagnostic testing to determine the presence
of virus or PI. Quarantine allows the animals to be properly
immunized in accordance with the health plan for the herd.
Biosecurity must consider the exposure to neighboring
livestock and wildlife. Biosecurity is most effective when it
becomes a community effort with all of the producers utilizing
a unified plan.
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