Bovine viral diarrhea
virus (BVDV) is widespread in cattle, with a high number of animals testing positive
to this disease. It has been estimated that 80 percent of cattle in this country
have been exposed to BVD, and that 70 to 90 percent of infections go undetected,
without visible symptoms. It is the most costly viral disease in cattle, inhibiting
conception, causing abortion and birth defects, and hindering the immune system
-- making the animals more susceptible to other diseases. In the U.S. cattle
industry BVD is costing producers an estimated $2 billion per year.
The first descriptions of BVD in North America (outbreaks of diarrhea in some
herds, and erosive lesions in the digestive tract -- sometimes with nasal discharge,
drooling, diarrhea and abortion) were reported more than 60 years ago. It got
its name from the profuse, watery diarrhea shown by weanling/yearling age cattle,
but we later learned that this was only one of many forms of disease caused
by this virus. The first attempts to isolate the cause of these various problems
were not successful; medical technology was not advanced enough to detect and
identify the virus. Eventually researchers found that all these symptoms were
caused by one virus.
TYPE I AND TYPE II VIRUS, STRAINS AND CYTOPATHIC/NON-CYTOPATHIC -- The BVD virus
is an elusive villian because it comes in two forms that have different effects
within the body cells (cytopathic and non-cytopathic). There are also two basic
genetic types of the virus (and several strains within each type), and each
of these types or strains may fall into either one of these categories regarding
how they affect the cells.
When scientists were trying to isolate the virus, they used certain body tissue
cells to culture it, and discovered that the cultures didn't respond the same
way. Some cells became infected but were not changed. In other cultures, the
virus caused obvious changes in the shape of the cells and killed them. This
version of BVD virus was termed cytopathic, meaning that it killed the cells.
The other version of BVD virus was called non-cytopathic, meaning that it did
not destroy or change the shape of the cells.
The two basic genetic types of the virus (Type I and Type II) are about 70 percent
similar in genetic make-up and can both be present in the same animal. They
can also change their genetic composition during the process of multiplication.
This explains why there can be such a variation in disease symptoms and in
how the animal's immune system deals with the virus. Each type also has both
cytopathic and non-cytopathic characteristics.
Thus there are several forms of BVD, ranging from subclinical infections (in
which the animal shows no obvious signs) to a severe and highly fatal form
called mucosal disease. BVD can affect the digestive, respiratory, immune,
nervous and reproductive systems. It can cause abortion in cows, stillborn
calves, calves born with birth defects or with immune deficiencies or persistent
The BVD virus can mutate or change somewhat as it multiplies, and there are several
strains of the virus, which means that the infected animal may or may not be
able to mount a protective immune response or be protected from the virus by
vaccination. This is why Type I and Type II viruses can produce different disease
signs even though they are both BVD viruses. Since they can also both be present
in the same animal this complicates the disease picture.
Some scientists subdivide the two types into more strains, with further subtle
changes in genetic composition of each type. Because the BVD virus can be so
varied, this is the reason some infections do not cause obvious disease, while
others affect the animal's ability to develop an immune response to fight off
the disease, and why BVD can appear in different systems (digestive, respiratory,
reproductive, etc.) in different situations.
MUCOSAL DISEASE -- The most serious form of BVD was first called mucosal disease
-- a sporadically occurring, highly fatal disease of the small intestine, in
which the virus has a cytopathic effect (changing and destroying the cells).
Researchers eventually discovered that mucusal disease only occurs in cattle
that are infected before birth with a non-cytopathic version of the virus (they
seem normal, but are persistently infected and have no immunity to the virus)
and then encounter a cytopathic type of BVD virus. Since these animals cannot
develop immunity (even if vaccinated), they are vulnerable to the severe effects
of the cell- killing version of the virus, if that virus is a strain that is
closely related to the persistently infecting virus. Not every combination
of non-cytopathic and cytopathic BVD virus in a PI (persistently infected)
animal results in mucosal disease however.
Mucosal disease has several forms. There is an acute form with a very high mortality
rate and a chronic form that also has high mortality rate but a much longer
course. Cattle most likely to develop mucosal disease are usually six months
to two years old, and though only a small percentage of the herd may be affected,
nearly all of the affected animals die. Often these young animals were persistently
infected with the same strain of non-cytopathic virus and were then exposed
to an animal with a cytopathic virus.
Mucosal disease is often characterized by profuse, watery diarrhea (which may
contain blood and gut lining), which generally develops two to three days after
the animal exhibits fever, weakness, depression and lack of appetite. There
are usually lesions in the mouth that may involve the lips and tongue, or even
the nasal cavity. The animal becomes emaciated and dehydrated. Acute cases
usually die within a few days or weeks. A few become chronic and may survive
for up to 18 months, becoming progressively emaciated. Diarrhea in these cases
may be continuous or intermittent and some animals may develop chronic bloat.
Lesions in the mouth and skin are slow to heal and the animal may become lame.
Treatment is not recommended because even if there is a slight chance for recovery,
the animal will be infected for life and should be culled.
INFECTIONS SPREAD VIA BREEDING -- Semen from bulls that are infected may contain
the virus. Reduced conception rates have been reported in otherwise healthy
cows that were bred to (or inseminated with semen from) persistently infected
bulls. Bulls affected with acute BVD may shed the virus in their semen for
awhile after being infected. BVD infection at the time of breeding can result
in reduced pregnancy rates due to significant embryo or fetal loss.
SIDEBAR: BIRTH DEFECTS CAUSED BY BVD -- Infection of the fetus any time between
100 to 150 days of gestation can result in a variety of congenital defects,
since this stage is when the nervous system is in its final stages of development
and the fetal immune system is developing. BVD infection at any stage of gestation
may retard fetal growth, resulting in lower birth weight and poor bone growth.
Lung development may be incomplete. Skeletal defects may include a jaw too
short, or fused joints. Another defect sometimes seen is less than normal amount
of hair, curly hair, or hairlessness.
Defects involving the nervous system include inadequate brain development, incomplete
development of the cerebellum (the portion of the brain involved with coordination
of movements —affected calves have trouble standing up), water on the brain
and other brain problems. Congenital defects involving the eyes may include
cataracts, opaque cornea, inflammation of the optic nerve, atrophy or abnormality
of the retina, and varying degrees of blindness.
SIDEBAR: BVD CAN BE THE UNDERLYING CAUSE OF OTHER DISEASE
OUTBREAKS Since BVD
infection suppresses the immune system, infected cattle (especially young calves)
may have a higher incidence of other diseases, including pnuemonia, scours,
pinkeye, footrot, diptheria, etc. Pregnant cattle may abort--with outbreaks
of lepto, IBR and other diseases -- simply because they were not able to develop
immunity to those diseases. BVD infection can be difficult to diagnose because
it shows up in so many different ways. If a rancher has problems with several
types of calf scours and a high incidence of respiratory diseases, this may
be a clue that BVD is part of the problem. And besides affecting the immune
system, BVD by itself can cause illness in calves.
A herd health program may be ineffective because BVD infected cattle don't mount
a very good response to vaccinations. So even if a rancher diligently vaccinates
against lepto, IBR, pinkeye and other common diseases, some of the vaccinated
cattle may develop those diseases. The rancher may think the vaccine didn't
work, when in reality the animal was unable to develop a good response.
In one study, BVD virus was the virus most often found in the lungs of feedlot
cattle with pneumonia, and was usually found in conjunction with Pasteurella.
Infection with the BVD virus has been associated with In outbreaks of respiratory
disease complexes in feedlots, and on the ranch. In young calves with multiple
viral infections, BVD virus is the most frequently found pathogen. Infection
with BVD virus has been shown to impair the ability of calves to fight lung
infections caused by bovine herpesvirus 1 (infectious bovine rhinotracheitis
or IBR). Many vaccines combine IBR and BVD, to give protection against both
of these viruses.