| What
is BVD? |
| Bovine
viral diarrhoea (BVD) is a highly damaging, contagious,
worldwide prevalent viral disease of cattle. The
disease has been listed by OIE as a priority disease
in cattle. It has many clinical presentations although,
interestingly enough, diarrhea is not the most common
observation. BVD is related to both border disease
(BD) of sheep and goats and classical swine fever
CSF) of pigs. |
|
What is the causative agent?
|
BVD
is caused by either bovine viral diarrhoea virus
type 1 (BVDV-1) or BVDV-2, which belong to Pestivirus
genus within the family Flaviviridae. BVDV can infect
buffaloes, sheep, goats and many other domestic
and wild ruminants and sometimes pigs. There are
certain BVDV-1 and BVDV-2 viruses that cause severe
disease and some cause very mild disease. There
are two biotypes of BVDV, cytopathic (CP) and noncytopathic
(NCP) depending on the cytopathogenicity in cultured
cells. NCP strains are more common and in mucosal
disease both CP and NCP strains occur. In India
both BVDV-1 and BVDV-2 occur. BVDV-1b strains are
predominantly prevalent in cattle. In sheep and
goats, border disease is caused by BVDV-1 and BVDV-2.
BVDV-1c has been identified in Himalayan yaks.
|
|
What
does BVD cause?
|
It
causes gastrointestinal, respiratory and reproductive
diseases including scouring, ill thrift, coughing,
rough coats, mouth ulcers. It can also cause poor
calf health, lameness, infertility, abortions,
congenital defects and it may suppress the immune
system to make other diseases more likely. Remember
however that in the majority of infected animals,
there are little or no clinical signs at all.
Also it is very difficult to diagnose BVD only
on the basis of clinical signs. In India, pneumonia,
glomerulonephritis and testicular degeneration
has been reported in experimentally acutely infected
calves. Mucosal disease should be suspected whenever
rinderpest like symptoms appear in cattle.
|
| Possible
outcomes to BVD virus infection in cattle. |
| Fetal
Infection |
| 1 |
Early Embryonic Death |
| 2 |
Abortion |
| 3 |
Stillbirths |
| 4 |
Congenital Birth Defects (eye defects, brain
defects) |
| 5 |
PI Calves (immunotolerant and persistently infected
shedders) |
| 6 |
Normal Calves born with antibodies to the BVD
virus |
| Acute
Infection |
| 1 |
Subclinical–no signs of disease |
| 2 |
Severe BVD signs with diarrhea and lesions in
the gut (mouth to anus) |
| 3 |
Hemorrhagic Syndrome (failure of blood to clot
normally) |
| 4 |
BVD infection and respiratory disease (pneumonia) |
| 5 |
Venereal disease |
| Mucosal Disease |
| 1. Infection of PI calves
with a CPE virus which causes severe diarrhea,
weight loss, damage to the gastrointestinal system,
and death. |
|
What
is a PI?
|
Persistently
infected (PI) animals are born infected with BVD
virus and will die infected with BVD virus. Clinically
they can appear perfectly normal. During their
lifetime, they shed large quantities of virus,
spreading infection to every naïve animal
they come in contact with. Persistent infection
in cattle can only occur if the dam is exposed
to ncp BVD virus for the first time during pregnancy
(around day 90-125). Although many die within
one year of birth, many PI calves (up to 20%)
survive to breeding age. PI sheep and goats also
transmit infection to cattle.
|
|
How
do PI animals spread the virus?
|
Virtually
all secretions and excretions - nasal discharges,
saliva, semen, urine, tears, milk, blood, foetal
fluids and faeces spread the virus. Other animals
come in contact with the virus usually orally.
PI animals are extremely efficient at spreading
BVD.It takes as little as one hour for a susceptible
animal to become infected if in direct contact
with a PI animal.
|
|
Are
PIs the only source of BVD?
|
No
- animals can also acquire infection by contact
with other acutely infected cattle. An acute infection
is a self-limiting virus infection similar to
human flu, typically lasting 10-14 days. Acute
(transient) infections are less efficient at transmitting
the disease than PI animals because they shed
much less virus and do not shed it very long.
However, these acute infections can cause severe
disease and sometimes high fatalities.
|
|
What
is mucosal disease?
|
MD,
an invariably fatal disease, affects only PI animals
and usually occurs between 6 and 24 months of
age. It is characterised by severe damage to the
mucosal surfaces of the gastrointestinal tract.
Afflicted animals have fever, persistent diarrhoea,
chronic wasting, mouth ulcers, etc. Mucosal disease
cases have not yet recorded in India.
|
|
How
big a role does the bull play in BVD transmission?
|
Semen
is an excellent transmission vehicle for BVD virus.
When a PI bull serves a naïve cow, early
embryo loss is the most likely outcome until the
dam develops immunity to the virus. Once immune,
she should conceive as normal. It is much more
common for them to cause delays in conception,
thereby lengthening the calving interval. Semen
from PI bulls is always of poor quality than that
from non-PI bulls, and so is less effective at
getting cows pregnant. Both persistently and acutely
infected bulls can transmit the virus through
semen unless tested by laboratory methods.
|
|
Can
man spread BVD virus?
|
Blood
transfusions, nose tongs, rectal gloves, contaminated
needles, vaccines, biologicals and even biting
flies can all transmit BVD experimentally. The
important source for all of these is the PI animal.
Remove the PI animal and you will remove the vast
bulk of your BVD problem. In India many of the
cell lines and bovine serum used for production
of live attenuated vaccines are contaminated with
BVDV and can be a potential source of spread of
infection. Live attenuated PPR, sheep pox and
goat pox vaccines should be verified BVDV free
before releasing for mass vaccination.
Once an animal has been infected, has it lifelong
immunity?
|
Most
animals, which have experienced an acute BVD infection,
are immune and protected for life against the
similar type of BVDV (1 or 2). However considerable
antigenic diversity within BVDV-1 subtypes have
been reported which can provide only partial protection.
The amount of immunity stimulated by carrying
a PI calf, will persist for the remainder of the
dams life.
|
|
How
to control BVD in cattle?
|
BVD
vaccination should be timed so as to give maximum
protection in the first four months of pregnancy.
The animal should be fully vaccinated before the
breeding season commences. But vaccine protection
is unlikely to reach 100% due to individual animal
factors (age, nutrition, genetics, stage of pregnancy,
etc). This is why the removal of PIs (the virus
factories) and biosecurity is so important in
BVD control. Vaccination is a huge help but can
not control it alone. Inactivated vaccines are
commonly in use in many countries around the world
but not in India.
|
|
Is
there any human risk from BVD?
|
| BVD
is not a zoonosis. So, humans cannot contract
BVD infection. |
Can
BVD be treated?
|
Once
an animal is acutely infected, the disease runs
its course. Affected animals can be treated for
their particular clinical signs – fluids
if scouring, antibiotics to keep off secondary
bacterial infections etc. There is no effective
treatment for persistently infected or mucosal
disease animals. Immediate removal from the herd
is by far the best advice.
|
|
Where
to send the samples for diagnosis of BVD?
|
The samples
from suspected animals (whole blood, serum and tissues
kept in ice) should be sent to High Security Animal
Disease Laboratory, IVRI, Bhopal for confirmation
of BVD and other ruminant pestiviruses. More information
on collection and dispatch of samples for BVD diagnosis
is available
|
