Bovine Virus Diarrhea and mucosal disease are clinically dissimilar disease syndromes, and were originally described as separate diseases, but they are now known to have a common viral etiology.

Bovine viral diarrhea describes the syndrome seen predominantly in 6- 18 month old cattle as a primary infection.

Mucosal Disease (MD) is usually sporadic, progressive and fatal. It would seem to occur in a small number of congenitally infected animals which are immunotolerant and harbor virus in all their tissues without showing any clinical symptoms

Bovine Viral Diarrhea Virus (BVDV) is a member of the Flaviviridae family in the genus of Pestivirus.

It is a positive sense, enveloped, single stranded RNA virus The structure of the virion is spherical polymorphic.

It has the ability to mutate into multiple variants depending on environmental pressures on the virus.

BVD viruses are grouped into two genotypes, Type 1 and Type 2, based on genomic characteristics and the severity of disease they produce in cattle.

Each genotype, 1 and 2, are divided into two biotypes- cytopathic (cp) and non-cytopathic (ncp) based on how they replicate in cell culture.

Cytopathogenicity does not correlate with the severity of disease in vivo (high and low virulence strains)

Some cp strains are recovered from animals with mucosal disease (MD), but most of the time ncp isolates are recovered from infected animals.

The classic virus is ncp; cp isolates are generated by mutations or genome rearrangements in the original/parental ncp strain. Most (>95%) of the field isolates are ncp.

Young calf persistently infected with BVD (right) compared to similarly-aged normal herd mate.

Host Range BVD is normally an infection of cattle,

but it has the ability to cause infections in pigs, sheep, goats,, deer, reindeer, bison,, and other wild ruminants.

Infections of BVDV are now seen throughout the world in all ages of cattle. Due to production and reproduction losses, this virus has a very significant economic impact.

TransmissionThe virus may be present in various secretions and semen. Spread is by direct and indirect contact. The mode of infection is by ingestion and inhalation. Transplacental infections are frequent and result in serious consequences for the embryo/fetus. Bulls may be persistently infected and the virus in semen is spread by coitus an artificial insemination.

Horizontal Transmission Fomites

BVDV has been shown to spread to susceptible animals through feed and equipments. One study showed that BVDV can be transmitted between animals during palpations if the same pair of gloves are worn for all exams.

EnvironmentBVDV is shed in most secretions and excretions (tears, urine, feces, mucus) and can survive outside the host for weeks.Calving pens and common shared areas can become highly contaminated.Crowding can also increase transmission if animals are infected with the respiratory type of BVDV.

Oronasal uptake is the most common route of transmission.

VectorsFlies have also been shown to transmit BVDV.

Horse flies, stable flies, head flies, face flies

If a cow is persistently infected it will infect the fetus. Other mechanisms of vertical transmission are: Contaminated Semen infects the cow, which leads to infection of the fetus.Embryo Transfer If the embryo is transferred to a cow that is infected, the fetus will also become infected. Modified Live Vaccine Vaccinating pregnant cows with live virus has been shown to infect the fetus.

Vertical Transmission

BVD virus can be spread in semen of persistently infected bulls or in bulls experiencing acute BVD with transient virus infection

Bulls are vigorously tested for BVD before entering AI

Pathogenesis: Bovine viral diarrhea describes the syndrome seen predominantly in 6-18 month old cattle as a primary infection. Passive immunity may protect up to 6 months of age explaining the prevalence of BVD in 6-18-month old cattle.

Infection is via the oropharyngeal route with primary replication in the epithelium of the oropharynx and a rapid uptake into the drainage lymph nodes. Viremia involves infection of lymphocytes giving rise to a leucopenia, and spreads to other lymphoid tissues, particularly Peyer's patches.

Concurrent replication in the epithelium of the alimentary tract results in discrete erosions which in the oral cavity cause excessive salivation, and in the small and large intestine induce diarrhea. Lesions in the nasal cavity and conjunctiva may also occur and be associated with discharges.

Occasionally concurrent respiratory disease occurs.. The majority of animals recover; morbidity is high, mortality low.

Primary infection of pregnant cows with BVDV may result in transplacental infection and may give rise to a range of problems according to the stage of pregnancy:

In early pregnancy it may be a cause of infertility and embryonic death. In mid-gestation it will produce congenital anomalies such as cerebella hyperplasia; it may also cause abortion; or may result in congenital infection of apparently normal calves. This latter group of animals do not have antibody to BVDV but frequently excrete virus and are therefore a source of infection; they also are likely to develop mucosal disease.

BVD virus is most important when it infects susceptible breeding cattle during early pregnancy causing foetal death/abortion, and weak/premature calves.

Birth defects of the nervous system.  Note the low head carriage and wide stance.  This calf was also very unsteady on its feet

Birth defects of the nervous system.  Note the wide stance

Mucosal Disease (MD) is usually sporadic, progressive and fatal. It would seem to occur in the small number of congenitally infected animals which are immunotolerant and harbor virus in all their tissues without showing any clinical symptoms.

If they encounter a second strain of virus then superinfection occurs and extensive erosions are established throughout the alimentary tract. The outcome may be an acute fatal disease or a chronically progressive syndrome with the animals remaining antibody negative.

Congenitally infected excretor animals are an important source of dissemination of virus and difficult to detect since they are seronegative. Outbreaks are usually confined to a particular farm or to premises where cattle are frequently bought in. The virus survives quite well on fomites in warm weather or in indoor housing.

Double perimeter fence prevents direct contact with neighbours' cattle

Persistently Infected (PI) animals One of the main reservoirs for the virus is

PI cattle who are reported to shed several billion virions a day. Contact with a PI animal is more of a risk for transmitting the virus and causing infection than an acutely infected animal because the PI animals shed such a high amount of virions.

Poorly grown persistently-infected BVDv calf.  This calf has chronic pneumonia and ringworm infection

Epidemiology: The virus is transmitted easily from animal to animal and from herd to herd by indirect means through feed contaminated with urine or oral/nasal secretions, feces, or aborted fetuses and placentas.

The virus is transmitted directly to susceptible hosts, rather poorly from acutely affected cattle and very efficiently from persistently infected animals.

Some persistently infected females survive to breeding age and may give birth to persistently infected offspring, thereby perpetuating the transmission pattern. Where infection has been present in a herd for some time and the majority of cattle are immune, the introduction of susceptible animals, typically heifers, results in sporadic losses, often continuing over a period of years if husbandry practices remain unchanged.

Where infection is absent in a herd, the introduction of a persistently infected animal is often followed by dramatic losses. Since the infection also occurs in sheep and goats, as well as swine, deer, bison, and other wild ruminants, these species may also be sources of virus for the initiation of infection in cattle herds.

Clinical specimens: Nasal discharge, feces, blood, blood smears, spleen, kidney, lymph nodes, turbinates, intestine, lung, acute and convalescent sera, and fetal liver and kidney.

Diagnosis

BVDV is routinely diagnosed in laboratories via serology, antigen detection assays, virus isolation, and by viral RNA amplification such as polymerase chain reaction (PCR). Acute infections can be accurately diagnosed by viral isolation derived from Buffy coat or nasal swab samples, immunohistochemistry, and antigen-capture ELISA assays. It is essential for these samples to be collected while the animal is still shedding virus. If the animal is convalescent a positive diagnosis of the virus will not be achieved because virus is not present.

Serological evaluation of acute BVDV infection is most commonly performed via serum neutralization. It is important to note that for serology to be most effective there must be paired samples- acute and convalescent- for serum titers to have any significant meaning. Although serum titers can vary from lab to lab a four-fold change in titer, as well as clinical symptoms that may denote BVDV infection, are generally considered significant.

For an accurate diagnosis of viral infection via serology a clinical history of vaccination is essential. Due to the fact that most vaccines available to cattle producers are modified-live vaccines, when an animal is exposed to a field strain of virus that is similar to the vaccine strain, the antibody titers will be significantly higher, which may lead to inconclusive or false laboratory results.

TreatmentCurrently, there is no treatment available to

cure Bovine Viral Diarrhea VirusTreatment Options

Antibiotics for Secondary InfectionsPneumonia etc.

If the animal develops Mucosal Disease, there are treatments to alleviate the symptoms, but the animal will eventually die.IV fluids for diarrhea and water lossPI’s should be euthanized to prevent further

contamination and potential infection to the rest of the herd.

VaccinationVaccination does not provide complete protection

against BVDV infectionIt can help to reduce the number of infectionsVaccination is not long lasting

Regular booster shots should be kept up to date according to label or vetVaccination

Vaccinated cattle can thus become become infected and show some symptoms but death will not occur

Pregnant cattle that are vaccinated have less chance if infected, of transmitting the virus to the fetus.There is still a chance of abortions in those cattle that are

vaccinatedEven cattle that are vaccinated may still be able to

transmit the disease to other cattle even if there are no symptoms present.