Bovine viral diarrhea virus is a pest virus that can
cause severe clinical signs; Major economic losses continue to result
from reproductive losses and exacerbation of concurrent bacterial or
viral infections. Of less clinical importance is severe clinical disease
due to BVDV alone, either as result of acute infection by highly
virulent isolate or by development of mucosal disease in persistently
infected calves. Disease due to BVDV can arise at any stage of life and
is often dependent on the virulence of infecting isolate. Survival of
BVDV virus in population depended up on the characteristics of
persistently infecting its host. Persistent infection arises by the
unique ability of the BVDV to survive by immune tolerance in bovine
fetus through evasion of both innate and acquired immunity in utero.
Adaptive immunity is avoided by immune tolerance through infection of
fetus prior to development and maturation of adaptive immunity. The
pathogenesis of BVDV infection is complex, with infection pre- and
post-gestation leading to different outcomes. Infection of the dam
during gestation results in fetal infection, which may lead to embryonic
death, teratogenic effects, or the birth of persistently infected, (PI)
calves. PI animals shed BVDV in their excretions and secretions
throughout life and are the primary route of transmission of the virus.
These animals can usually be readily detected by virus or viral antigen
detection assays (RT-PCR, ELISA), except in the immediate post-natal
period where colostral antibodies may mask virus presence. Acute
infection with BVDV results in transient viraemia prior to
seroconversion and can lead to reproductive dysfunction and
immunosuppression leading to an increased incidence of secondary
disease. Antibody assays readily detect virus exposure at the individual
level and can also be used in pooled samples (serum and milk) to
determine herd exposure or immunity. Diagnostic tests can be used to
diagnose clinical cases, establish disease prevalence in groups and
detect apparently normal but persistently infected animals.
Keywords:Bovine viral diarrhea; Mucosal disease; Persistent infection; Virus
Abbreviations:BVD: Bovine
Viral Diarrheal; BVDV1: Bovine Viral Diarrhea Virus type1; BVDV2: Bovine
Viral Diarrhea Virus type2; BVDV: Bovine Viral Diarrheal Virus; Cp:
Cytopathogenic; CSFV: Classical Swine Fever Virus; ELISA: Enzyme Linked
Immune Sorbent Assay; FCS: Fetal Calf Serum; FCS-SM: Fetal Calf Serum
Supplemented Media; IHC: Immune Histocompatibility; MD: Mucosal Diseases
; MLV: Modified Live Vaccine NCP: Non cytopathogenic; OIE: Office of
International Epizootics; PI: Persistently Infected; RNA: Ribonucleic
Acid; RT-PCR: Reverse Transcriptase Polymerase Chain Reaction; VI: Virus
Isolation; VN: Virus Neutralizations
Bovine viral diarrhea virus (BVDV) is a Pest virus of
the family Flaviviridae capable of causing serious clinical diseases in
cattle [1]. The virus is divided into two genomes (BVDV-1 and BVDV-2)
because of antigenic and genetic differences [2]. And Infection with
BVDV is also known to have significant financial impact stemming
primarily from the reproductive and immunosuppressive effects of acute
infection [3]. As such, control and eradication programs are becoming
increasingly common across much of the cattle-producing world including
legislated, regional schemes and voluntary, herd-based schemes [4].
While healthy, immune-competent cattle (or late-term, immune-competent
fetuses) may suffer from acute BVDV infection resulting in
sero-conversion; the disease is primarily spread and maintained in
cattle populations by
persistently infected (PI) individuals that Persistency of infection
arises from fetal infection in early gestation Following acute infection
in the dam [5]. Most control programs aim to eliminate PI cattle and
hence the source of continuing infection [4].
This requires the efficient and effective use of
accurate diagnostic tests, and a wide range is readily available for the
detection of BVDV virus, antigen (Ag) or specific antibodies (Ab) that
the tests will return varying results depending on the current or
historical BVDV infection status and Animals that have never been
exposed to the virus will test negative for Ab, Ag, and virus with the
broad nature of disease, transmittance, and lack of treatment made BVD
global pandemic and one of the most significant cattle diseases in world
Coupled with vast clinical
symptom and associated death loss, gravity of BVD makes it a
disease no longer overlooked [6]. The overall effects of BVD are
difficult to examine, due to the breadth of the disease. Research
and data highlighting effects of the disease is very limited, simply
since too many variables are present for accurate measurement.
Furthermore, if considered the worldwide prevalence of BVD, it is
easily understood why BVD is an important cattle disease from an
economic standpoint [7].
The areas affected by BVD, specifically reproduction,
production and immunosuppression, are what make it such
an expensive disease; Reproductive disorders & syndromes
associated with BVDV include: abortion, early embryonic loss,
fetal defects, conception failure, normal to prolonged returns to
estrous, increase in return to service numbers, spread out calving
pattern, slow to grow and weak calves, birth defects such as blind
calves or calves with neurological problems, poor weaning rates
due to ill thrift and death and A good pregnancy rate does not
indicate a BVDV free herd as reproductive losses is associated with
the timing of infection [4].
Production effects span those regarding decline in feed to gain
efficiency, lower carcass quality, and decreased milk production
and because of the vague and diverse clinical signs observed
with the disease, it is very difficult to clinically diagnose BVD
infected cattle. The difficulty of clinically diagnosing BVD positive
cattle impedes the studying of these cattle in a clinical setting.
Regardless, understanding the nature of viral infections in animals
it should be noted that the side effects are not usually contained
to one specific area of the body. A virus such as BVD has been
noted to have potential complications with every body system in
cattle. Although rare, BVD viruses have manifested in many ways
as rare as causing skeletal deformities to severe neurological
damage (Houe, 1994). Finally, immunosuppression may also lead
to secondary disorders, along with a whole host of other [8].
With above background, the objectives of this paper are:
To provide a brief review on the economic importance of
BVD in production and reproduction.
To know the management systems of BVD infection
during any outbreak time and infectious period.
To understand the nature and characteristics of BVD.
To provide information on the epidemiology of BVD.
To emphasize diagnostic, prevention and control
methods of BVD.
Etiology
BVDV is a member of the pest virus genus caused by a virus
called Bovine Viral Diarrhea Virus (BVDV) that there are several
ways in which BVDV infection can manifest in a herd and cause
economic loss of that the most important craffied one’s are the
immunosuppressive effects, reproductive losses and losses
associated with persistently infected animals (Becher et al., 2011).
Viral taxonomy
There are four recognized species within the pest virus genus.
These species are BVDV-1, BVDV-2, border disease virus of sheep
and classical swine fever virus, previously known as hog cholera
virus [9]. (See box below)
Epidemiology
The occurrence of BVD infection
Acute infection is the result of horizontal transmission to
a susceptible animal following contact either with an acutely
infected animal that is shedding BVDV, or with a PI animal that
sheds BVDV continuously that it may also be acquired through
several routes of transmission such as inhalation, ingestion and
transplacental [10].
The spread of BVD
The greatest period of spread is during yarding, particularly
when cattle from different groups or herds are mixed or penned in
adjacent pens [11]. And the main source of infection is persistently
infected (PI) animals that shed many viral particles in all body fluids
and excretions (milk, urine, feces, saliva). Infection in susceptible
animals tends to occur from direct animal to animal contact that
may occur during yarding, mustering, agricultural shows and sales
and the most common ways that BVDV is introduced to a herd
through the introduction of replacement breeding animals, new
bulls and occasionally over the fence contact with infected animals
[3].
Persistence of BVD
Persistently infected (PI) calves are created when a fetus is
exposed to BVDV during the first half of gestation that on this time,
the fetal immune system is not developed enough to respond to
a BVDV infection so the fetus might be aborted, but if the fetus
survives it will likely develop into a PI calf. Some PI calves grow
poorly while others may look healthy and grow very well, making
it impossible to consistently detect PI animals visually. Most PI
animals die by 2 years of age, but some will survive for several
years and constantly shed BVDV throughout their life (Givens et
al, 2010).
PI cattle are much more efficient at transmitting the disease
due to the higher amount of systemically circulating virus present
within the animal. With higher amounts of circulating virus these
animals will secrete higher levels of the virus. Additionally, PI
cattle will secrete the virus in various forms from inoculation of
the virus until their death. The potency of transmission is of major
concern, and it has been shown that horizontal transmission from
a PI animal to a seronegative animal can be detected within 20
hours after direct contact [9] (Figure 1).
Transmission and source of infections
Transmission
Bovine pest virus is spread by close contact (nose-to-nose)
between cattle. The greatest period of spread is during yarding,
particularly when cattle from different groups or herds are
mixed or penned in adjacent pens [11]. BVDV does not usually
survive in the environment very long (less than 3 weeks), so
direct transmission between animals is the most common route
of transmission that Acutely infected animals are a temporary
source of BVDV transmission, but PI animals shed millions of viral
particles every day; therefore, serve as a constant source of BVDV
exposure in a herd because they continuously shed virus in saliva,
mucous, tears, milk, feces, urine, and any other bodily secretion and
its transmission may occur vertically (i.e., before birth), leading to
congenital infection of the fetus, or the transmission may occur
horizontally (i.e., after birth) that Congenital infections may cause
resorption, abortion, still birth, or live-birth and Congenitally
infected fetuses that survive in utero infection (i.e., the live-births)
may be born as BVDV-infected calves [12] (Figure 2).
Source of infection
Persistently infected carrier calves play a key role in spreading
the virus as they can shed viruses throughout their life, acting as a
source of infection for other animals. This most commonly occurs
when these cattle are between 6–24 months of age. However,
some PI’s have been known to survive and enter the breeding
herd. They may die from a range of diseases caused by the
immunosuppressive effects of BVDV infection and occasionally
they may develop Mucosal Disease [13].
Risk factors and host range
Risk factors for BVDV infections.
The risk factors for BVDV infections are often a reflection of
the risk of direct or indirect contact with PI cattle. The suspected
risk factors include livestock trade, pasturing animals, use of
common pasture for infected and susceptible cattle, cattle density,
number of infected neighboring herds, broken fences, animal
contact between herds (e.g., livestock exhibitions), weak hygiene,
and presence of other susceptible ruminants, such as sheep and
wildlife [14].
Host range
Other than cattle carrying the BVD virus, domesticated farm
animals have been shown to have the potential of becoming
infected with BVD as well. So, the virus has been found in sheep,
pigs, and wildlife, although the chance of transmission to or from
cattle has not been fully established but transmission between
sheep and cattle has been experimentally proven [15]. The BVD
virus has been found in pigs as classical swine fever virus (CSFV),
but once again the transmission to cattle is considered doubtful.
Initially this was thought to be a possible mode of transmission to
and from cattle, and some even thought that the disease may have
originated from wildlife populations such as serological evidence
exists that deer can become infected with BVD [16].
Pathogenesis
BVDV infection has a wide range of clinical presentations
and unique diagnostic challenges; so, to select an appropriate
diagnostic test, it is important first to understand the pathogenesis
of the disease. BVDV is divided into noncytopathogenic (ncp), and
cytopathogenic (cp) biotypes based on effects on cultured cells
rather than in the infected host. Cp biotypes induce apoptosis in
cultured cells while ncp biotypes do not [17]. Non-cytopathogenic
BVDV, however, appears to be the cause of acute infections and
can be transmitted in a wide range of body fluids, including nasal
discharge, urine, milk, semen, saliva, tears, and fetal fluids [18].
Also, Cp BVDV has been shown to be capable of inducing acute
infection under experimental conditions [19].
Clinical Signs and symptoms
Clinical signs
Clinical signs of infected cattle cover a wide spectrum from
showing no signs to death. The varying clinical signs depend on
what form of the virus the animal is carrying along with the extent
of the infection. Cytopathic strains of BVD will result in more severe
reproductive effects including embryonic loss, stillbirths, and
congenital defects. Additionally, it forms of the virus contracted
in older cattle will cause these animals to present themselves
critically in a clinical setting causing fever, depression, lack of
appetite, labored breathing, oral ulcerations, and diarrhea to be
observed also in the most severe cases Bleeding of internal organs
and death has been observed. Supportive care such as fluids and
anti-inflammatory drugs may be used, but the likelihood of losing
the animal is still relatively high. Noncytopathic strains of the virus
will cause infected cattle to show mild clinical signs if any at all. So,
this type of virus will create PI calves if they are infected in utero
and will cause transiently affected animals if contracted by older
animals. In general, a noncytopathic form of the virus will have
less detrimental effects than a cytopathic strain of the virus [20].
Secondary Diseases
Other than the consequences of reproductive disorders, Bovine
Respiratory Disease (BRD) is a major disease that is often coupled
with BVD. It is much like BVD, an umbrella term for a wide range
of bacterial and viral caused respiratory diseases. So, those that
contained high levels of BVD seropositive cattle were at a higher
risk of Bovine Respiratory Syncytial Virus (BRSV) [14]. Viruses are
the principle pathogenic instigators of BRD, making prevention of
BVD key to keeping BRD herd levels at a minimum and should be
noted that immunosuppression is not the only explanation for
BRD being a secondary disease to BVD (Hansen et al, 1996).
Clinical syndromes associated with BVD.
There is a wide spectrum of outcomes associated with BVDV
that its Infections may be subclinical (no outward signs of disease),
clinical or chronic. This depends on the immune status of the
animal, the stage of gestation of female animals and the amount
or strain of virus that the animal is exposed to. Common outcomes
of exposure to BVDV in cattle include one or more of the following
Reproductive Syndromes such as embryonic loss, stillbirths, and
congenital defects the most common once; conception failure,
normal to prolonged returns to estrous, increase in return to
service numbers, spread out calving pattern, slow to grow and
weak calves; birth defects such as blind calves or calves with
neurological problems, poor weaning rates due to ill thrift and
death [14].
Pathology
Pathologically, PI cattle are viremic, yet clinically they often
show little or no signs of being infected. The classical PI animal is
the one that is never shows signs clinically and may seem fine but
is the silent culprit for regenerating the disease; so, these reasons
that PI animals are of greatest concern to producers. The common
pathology of the disease is cerebral hypoplasia, ulcerative
esopagitis, hemorrhagic necrosis of peyers patchs on ileum, colitis
and crypt dilatation unilateral corneal opacity are good examples
of BVD [21] (Figure 3(a,b,c)).
Diagnostics
In general, diagnostic assays for detection of BVDV infections
can be divided into Serological methods, i.e. methods aimed at
detection of virus-specific antibodies as an indication of exposure
to the pathogen, and methods aimed at detection of infectious virus
or viral components (antigen or nucleic acids), as an indication of
current infection [22].
Serological methods
In the context of BVDV control, the main objectives of
serological methods are to Differentiate between exposed and
non-exposed herds, to monitor progress and Drawbacks within an
ongoing control programme, and to investigate immune status in
individual animals in infected herds, to identify possible PI animals.
A PI animal is, as previously mentioned, generally seronegative,
unless it has persisting maternal antibodies, or has been exposed
to and infected with a heterologous strain. Serological methods
can also be used to diagnose acute infection by detection of
seroconversion in paired samples [8]. Virus neutralization (VN):
is the reference test for antibody detection. VN is sensitive and
specific, but cell culture dependent and labour demanding and
will typically take 5-6 days to perform. Thus, it is mostly used as a
reference test for back-up and calibration purposes [8].
Antibody ELISA: For testing of large series of samples,
enzyme-linked immunosorbent assays (ELISAs) have many
advantages over VN. They are rapid, relatively inexpensive both to
establish and run, and are suitable for automation. Two principally
different ELISA formats are common in use for antibody detection:
indirect or blocking (competitive) assays. In the indirect format,
specific antibodies are trapped by immobilized viral antigen,
and subsequently detected using enzyme conjugated [22]. Virus
isolation (VI): in bovine cell cultures, followed by identification
of the viral isolate by immunoperoxidase or immunofluorescence
staining is considered the standard reference test for detection of
infectious virus. Although VI demands time, resources, and skill, it
is a reliable and widely used method. The presence of antibodies,
or cell toxic substances, or both in the sample, however, can yield
a false negative result. Furthermore, because BVDV is a common
contaminant of biological by-products of bovine origin such as
fetal calf serum (FCS) and, consequently, bovine cell cultures
grown in FCS-supplemented media, the cell culture system must
be monitored to ensure freedom from contamination with virus
or antibodies [23].
Detection of antigen
Assays for detection of viral antigen rely on existing BVDVencoded
antigens in the sample material, i.e., there is no
amplification of the target, as is the case for VI and RNA detection
protocols. This minimizes the risk of cross contamination of
Samples, and flavors detection of PI over acutely infected animals
[22]. Several formats of ELISAs are commercially available for
detection of viral antigens. The basic principle consists of the
use of virus-specific monoclonal antibodies to capture viral
antigens, followed by detection of antigen-antibody complex with
enzyme-conjugated antibodies. Antigen ELISAs are widely used
for identification of PI animals and can be used for detection of
viruses in serum, Buffy coat cells or skin biopsies (e.g., ear notch
samples). Like VI, antigen ELISAs may yield false negative results
if antibodies are present in the sample. This should be considered
when testing young animals that might have persisting maternal
antibodies [8].
Immunohistochemical detection of antigen in skin biopsies,
particularly ear notch samples, is another method that is used for
identification of PI animals [24]. By use of separate sets of primers
and probes, BVDV-1 and -2 can also be discriminated between in
the same assay [25]. Genotyping: The genetic typing of BVDV has
most frequently been based on sequence comparison of partial
sequences of the 5’ NCR, Npro or E2 regions [26]. Analysis of the
5’ NCR, a highly conserved region of the genome, has shown to be
a reliable and reproducible method for genetic characterization of
BVDV isolates even though the genetic resolution obtained is not
as high as that obtained with more variable regions as Npro or
E2 [27]. Furthermore, it is the target region for most PCR-based
diagnostics, and as such a suitable target for direct sequencing
from the PCR product [28].
Differential diagnosis of BVD
BVD is differentially diagnosed with several diseases like:
Malignant catarrhal fever bilateral corneal opacity(edema) or
uveitis, dermatitis are typical; Rinderpest- tiger or zebra striping
lesion is prominent in large intestine on chronic case; Bluetongue-
Hemorrhage on the base of pulmonary artery and aorta with
large quantity of plasma like fluid in thorax and pericardial sac;
Salmonellosis-no oral lesion, diarrhea but have is zoonotic bluereddish
discoloration of extremities, cyanosis; Johne’s disease
corrugated appearance of intestine & also has no oral lesion and
bottle jaw are typical and prominent mucosal folds [21].
Treatment
Reducing Clinical Effects
It is common for producers to treat BVD animals with
antibiotics, thinking it will help the cattle “get over the hump”
of whatever they are experiencing. In animals that are showing
clinical effects of BVD, the only true treatment available is
supportive. Optimizing the animal’s ability to neutralize the
virus may consist of reducing fever, countering any nutritional or
metabolic disorders, and providing basic husbandry practices that
one would apply to a diseased animal. Use of antibiotics may be an
option to prevent secondary diseases such as BRD and in a form
may help reduce clinical effects of BVD. An animal with an acute
form of BRD will cause the immune system to weaken, allowing
a BVD virus to overcome the animal’s immune system, causing
an acute form of BVD. The use of antibiotics has been proven to
be effective at fighting respiratory infections in cattle. Broad
spectrum agents such as Nuflor, Excenel, LA-200, and Micotil, can
reduce the severity and length of a bacterial infection. In the likely
case that an animal is fighting BRD along with BVD, by reducing
the extent of the respiratory infection it will reduce the likelihood
that the animal shows clinical signs of BVD [29,30].
No True Cure
Due to the nature of viral infections, there is no treatment
to fully cure an animal of a viral infection. As we have discussed
there is a limited amount of treatment that may be performed
with BVD animals; the key lies in prevention of disease that are: All
purchased animals should be tested for PI status and quarantined
to give time for acute infections to clear; Any animal leaving the
farm (shows, leased bulls, etc.) should be quarantined for 30 days
before re-entering the herd. Purchased pregnant animals should
not only be tested themselves for PI status, but also their calves
should be tested at birth to make sure they are not a PI [31].
Prevention and control
Prevention
The global nature of the disease, along with the ease of
transfer between carriers, makes prevention of BVD a formidable
challenge. The challenge of eradicating, or even just controlling
BVD, can be seen by the state-wide and national attempts made by
groups of producers to control the disease. When the time comes
for a producer to implement a prevention program, one must keep
in mind that the program must be specifically attuned to the type
of operation (cow-calf, stocker, feedlot, dry lot dairy, pastured
dairy cattle, dairy calf ranch, etc.). There is no “one size fits all”
prevention program. It is crucial that the Producer design and
continuously adapt a program that specifically fits their operation
[22]. Animal Husbandry: The start of any effective prevention
program begins with basic animal husbandry practices. Scoring of
the body condition may be done with relative ease, especially with
the help of a veterinarian and records should be maintained to
compare past results. The importance of not underestimating the
body condition of the animals is a crucial step in preventing BVD
within a herd. Animals under nutritional deficiency may become
immunocompromised, hindering the immune response time [32].
Passive Immunity: Adequate quantity and quality of colostrum
within the desired 3-hour post parturition window. The wellknown
benefits of adequate transfer of immunoglobulins from
dam to offspring can be summed up simply by the formation of
a competent and responsive immune system. The rewards of
immunoglobulin absorption of offspring therefore are linked
directly to the protection of clinical diseases including prevention
of BVD [28,29]. Active Immunity and Vaccination: Although
removal of a PI animal is the primary form of halting the shedding
of BVD, it has been shown that transmission of the disease will
be hindered by an adequate vaccination program [7]. Wide ranges
of vaccine-induced neutralizing titers were formed in calves
with a two-series BVD specific vaccine of either an inactivated or
modified live vaccine [7]. Other important aspects of the study
were that the study population was of colostrum-deprived calves;
although the importance of colostrum and passive immunity
should not be underestimated, this shows that regardless of
degree of passive transfer, vaccines are still effective at stimulating
titers in immunoglobulin naive calves [28,29]. It is apparent that
vaccination and passive immunity both offer degrees of protection
against BVD but, when coupled together; the combination
conferred a potent efficacy against shielding viruses [28, 29].
Testing: Testing cattle for BVD is the most important aspect
of prevention for the disease in white blood samples will be
necessary to detect BVD in some PI animals. Different test
methods used by laboratories, veterinarians and producers will
be highlighted in the following discussion. Immunohistochemistry
(IHC) is the most practical approach to testing a herd for BVD.
Ear notch samples are the most common tissue sample used
for this test. This requires little labor in obtaining samples from
large numbers of cattle with the use of an instrument such as ear
notch pliers and the samples are also suitable for transport. This
test is very accurate at identifying PI animals but cannot provide
accurate results for transient animals [33]. Also, Polymerase
Chain Reaction (PCR) tests used to detect BVD have been shown
to be timelier compared to virus isolation tests. Additionally, PCR
has the potential to detect BVD in its antigen-antibody complex.
Therefore, unlike IHC, this test can be performed on calves under 6
weeks of age, because it makes no difference if colostral antibodies
are bound with the antigen [24].
Culling: Within any herd attempting to prevent BVD a strict
“test and cull” policy must be practiced. Regardless of other factors
including the worth of animals, market prices, labor involved, these
animals must be removed from the herd. Every day a transient or
PI animal is with its herdmates, the potential for the disease to
spread will not decrease [34]. Closed Herd: Once the herd has
been tested and is clean of BVD, the next challenge in preventing
the disease is maintaining a closed herd. Simply put, any animal
being introduced to the herd should be properly quarantined
until tested as negative for BVD. At this time, it is then safe for the
animal to be turned out with other animals. Replacement heifers
and bulls raised on a dairy or ranch are obviously exempt from this
protocol, because of their origination from the herd. Maintaining
a closed herd is a difficult task for producers to accomplish due to
the transition of the beef and dairy industry to fewer and larger
farms. This means that a larger number of animals are being
moved in and out of farms in a shorter period [35].
Control
For long, attempts to control BVDV infections were limited to
prophylactic treatment practices by using modified live vaccine
and killed virus vaccine, implemented primarily to reduce, or
prevent clinical disease on a herd basis. Today, however, it is
well established that the benefit of preventing clinical disease in
transiently infected animals is negligible when considering the
overall prevention and control of the disease [27].
Bovine Viral Diarrhea Infection in Ethiopia
In Africa, few studies conducted on seroprevalence of the
disease have indicated that the prevalence’s were varied from
70–83% [36]. Although Seroepidemiology of BVD were reported
in European and some African countries (OIE, 2008), there is no
research or published information available as to these authors
knowledge on the investigation of BVDV infection in Ethiopia.
But there was one study had done to detect the presence of BVDV
infection and to determine the Seroepidemiology and associated
factors in three agroecological zones in Ethiopia such as Jimma,
Southwest Shoa, and West Shoa where BVD is more prevalent
where due to the following factors tropical climate and humidity
of the area with bimodal heavy annual rainfall ranging and
traditional mixed crop-livestock agriculture of areas makes the
infection more prevalence on these areas as a study [37].
Economic Significance Of BVD
BVDV adversely affects both health and productivity. The
losses due to infection are diarrhea, decreased milk production,
reproductive disorders, increased occurrence of other diseases,
and death. The losses from fetal infection include abortions;
congenital defects; weak and abnormally small calves; unthrifty,
persistently infected (PI) animals; and death among PI animals.
[6]. As we have introduced, the effects of BVD are wide-ranging
that vague and diverse clinical signs observed with the disease,
it is very difficult to clinically diagnose BVD infected cattle. The
difficulty of clinically diagnosing BVD positive cattle impedes
the studying of these cattle in a clinical setting. Regardless,
understanding the nature of viral infections in animals it should
be noted that the side effects are not usually contained to one
specific area of the body. So, a virus such as BVD has been noted
to have potential complications with every body system in cattle.
Although rare, BVD viruses have manifested in ways as rare as
causing skeletal deformities to severe neurological damage [34-
46].
An understanding of the pathology and pathogenesis of BVDV
infection will guide and optimize diagnostic approaches. As the
age of the animals and stage of infection varies, different tests
and testing protocols need to be utilized. Highly sophisticated,
but relatively cheap diagnostic tests, such as a combination of RTPCR
and Ab monitoring on pooled material allow classification of
the BVDV infection status of individuals and herds. A molecular
epidemiological approach may be used to trace routes of BVDV
transmission, and to identify and prevent risky behaviour within a
BVDV control programme.
A single introduction of BVD into a susceptible population
may result in widespread infection, and an introduced strain may
circulate for many years and remain genetically unaltered. The
significance of a single animal being ill may be overlooked, as often
occurs with animals that are persistently infected. It is important
to realize that this single case could be an indicator of the potential
for greater losses, due to the reproductive, productive, and other
effects of the pestivirus. Awareness of the presence of pestivirus,
followed by appropriate management strategies, will minimize
losses. Finally, the immune suppression, Persistent infection,
production loss, reproduction defect and neurological defects all
these make BVD as one the most important economical disease in
veterinary world.
Based on the above conclusion the following recommendations
are forwarded.
- It should be maintained on closed herd (not always
practical).
- Don does not purchase replacements from auction.
- Quarantine new arrivals and show stock for 30 days and
test new arrivals for BVD- PI.
- Test purchased pregnant animals and their calf at birth.
Keep records (reproductions and health).
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