Virus bvdv

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. Acutely infected animals are a temporary source of BVDV transmission, but PI animals shed millions of viral particles every day. PI animals, 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 figure 4.

Figure 4. A PI calf constantly exposes herdmates to BVDV via saliva, urine, feces, tears, mucous, and any other bodily secretion. Control of BVDV currently involves a combination of biosecurity, testing and culling of PI animals diagnostic surveillance , and vaccination. Specific BVDV control protocols will differ from one cattle farm to the next depending on herd goals, herd health history, BVDV exposure risk factors, etc.

Work with your veterinarian and Extension agent to create an effective BVDV control program tailored to your farm. For more information on Bovine Viral Diarrhea Virus, visit www. Bovine Viral Diarrhea Virus. Acute Infections Acute transient infections only last from a few days to a couple weeks but can result in fever, diarrhea, respiratory disease, reproductive problems, and much more depending on the age and immune status of the animal infected, as well as the strain of BVDV involved.

For reproductive-age cattle, vaccination to prevent viremia and birth of persistently infected offspring is considered more important, while also more difficult to achieve than prevention of clinical disease. Recent advances have been made in the understanding of BVDV vaccine efficacy. Due to the nature of viral infections, there is no treatment to fully cure an animal of a viral infection.

All control programs which are in use in many countries of the world, mainly depend upon the detection of PI animals, eliminating them and preventing their return into the herds.

Detection of PI animals at early stage, particularly soon after birth is of significant benefit to implement BVDV control programs. Each method to detect BVDV has advantages, disadvantages, and applicability for different diagnostic situations. The reliability of diagnostic tests is optimized by choosing the appropriate sampling strategy on the basis of animal age. BVDV is a common infection of cattle worldwide so that most herds are at risk for infection. The broad nature of the disease, transmittance, and lack of treatment have made it a globally enzootic, and one of the most significant cattle diseases Tautz et al.

Although BVDV is named for its primary host, its prevalence in non-bovine species has become increasingly recognized. To date, the virus has been isolated in over 40 species and serological evidence indicates that most wild ruminants are susceptible to BVDV infection. In addition to wildlife, multiple domestic non-bovid species have also been reported to carry and spread the disease. There is evidence of transient infection TI within most of these species, resulting in the familiar BVDV syndromes of reproductive insufficiency, respiratory disease, and immunosuppression Nielsen et al.

With regard to potential of BVDV infection in farmed and free-ranging wildlife, the risk of transmission of the disease from wildlife to cattle remains unknown Uzal et al. Bovine viral diarrhea virus has been found in sheep, goats, pigs, buffaloes and wildlife, although the chance of transmission to or from cattle has not been fully established.

Transmission between sheep and cattle has been experimentally proven Deregt et al. Isolation of the virus in wild ruminant animals such as deer and elk in North America has been reported Vilcek et al. Epidemiological investigations have shown that demographic factors such as herd size and density are significant predictors for the prevalence of infection in populations where BVDV is endemic Ezanno et al. Therefore, there is a range of virulence among BVDV isolates, varying from subclinical infections or mild clinical disease to severe fatal syndromes.

Persistently infected cattle are an important reservoir of virus and shed large amounts of virus throughout their lives spreading virus among cattle herds. This paper aims to review various aspects and complications of detection and control of persistent BVDV infections in cattle herds.

The genus Pestivirus is composed of four recognized species, BVDV-1 and BVDV-2 previously referred to as genotypes 1 and 2 , classical swine fever virus and border disease virus Tautz et al.

There are hundreds of different strains of the virus, characterized by viral nucleotide sequence comparison or by monoclonal antibody MAb serotyping, which can also be categorized under two biotypes based on their growth characteristics in cell cultures Ridpath et al. The rare cytopathic CP biotype will damage tissue cultures and the much more common non-cytopathic NCP will not.

Biotypes apparently behave differently in vivo. Non-cytopathic strains have a tropism for leukocytes, lymphoid organs and the respiratory tract, while CP strains are more restricted to the digestive tract Bezek et al. The syndromes caused by the two biotypes differ mainly in the occurrence and severity of disease that they cause upon infection. Cytopathic biotypes have only been isolated in connection with outbreaks of mucosal disease MD and the NCP biotype is commonly found in nature and causes PI in animals Bezek et al.

In general, transient BVDV infections can be divided into five categories: acute, severe acute, hemorrhagic infection, bovine respiratory disease, and immuno-suppression-only. The importance of acute TI in the transmission and maintenance of BVDV within a population of animals domestic and wild should not be underestimated.

Furthermore, both genotypes are divided into subtypes. Genotypes 1 include the classic isolates, which are commonly used in laboratory reference and vaccine strains. Genotype 2 includes BVDV strains associated with high mortality acute and peracute infections, thrombocytopenia and hemorrhaging Vilcek et al. These viruses are genetically and antigenically related to BVDV-1 and 2 and cause disease similar to that traditionally associated with BVDV infections.

The main transmission route in infected herds is direct contact with a PI animal. Common mechanisms of horizontal transmission include: fomites feed, water, and equipment such as nose tongs, milk bottle nipples, and needles , palpations if the same pair of gloves are worn for all exams , secretions and excretions urine, faeces, mucus, milk , crowding can also increase transmission if animals are infected with the respiratory type of BVDV , and vectors horse flies, stable flies, head flies, face flies have also been shown to transmit BVDV Niskanen et al.

If a cow is PI, its fetus will become infected. The virus has the ability to cause transplacental infection resulting in different outcomes depending on the stage of gestation at which the acute infection takes place, leading to fetal death, malformations, acute syndromes of the neonate, immune tolerance and lifelong viral persistence Peterhans et al. Recently, BVDV antigen was detected in two neonate calves with clinical signs of congenital tremor Taghipour Bazargani et al.

Other mechanisms of vertical transmission include: contaminated semen, embryo transfer, and contaminated modified live vaccines. Infected bulls can shed BVDV in semen for prolonged periods, and cattle have been infected following insemination with frozen semen from these animals Schlafer et al.

Bovine viral diarrhea virus gains access to the oropharyngeal mucosa by ingestion or inhalation. Following contact with the mucosal lining of the mouth or nose, initial replication occurs in epithelial cells with a predilection for the palatine tonsils and newly assembled viruses egress via exocytosis. The virus is able to spread systemically through the blood stream.

Spread can occur through both free virus in the serum and virus infected leucocytes. In males, BVDV replicates in the seminal vesicles and the prostate gland. Infected animals shed virus in nasal and oral secretions, less so in feces and urine. In a few situations, animals, mainly more than 6 months old, a clinical syndrome as classical BVD develops. After an incubation period of days, the affected animals develop fever, leukopenia and viremia that may persist up to 15 days. The virus is present in leukocytes buffy coat , especially lymphocytes and monocytes, and in plasma.

The clinical symptoms include lethargy, anorexia, mild oculonasal discharge, diarrhea, mild oral erosions and ulcers Uzal et al. Since the early s, a syndrome of severe acute BVD has been recognized with high morbidity and mortality in susceptible animals.

Primary infections with a few highly virulent BVDV-2 strains caused this syndrome with peracute to acute course and signs of fever, sudden death, diarrhea, or pneumonia. Production of inflammatory cytokines, in response to widespread infection of mononuclear phagocytes has been postulated as a cause of this severe disease Chase et al.

In some cases, a thrombocytopenic syndrome with clinical symptoms including epistaxis, hyphema, mucosal hemorrhages, bleeding at injection sites and bloody diarrhea, is superimposed on the alimentary syndrome.

The mechanism of thrombocytopenia is not completely defined, although infected megakaryocytes in the bone marrow undergo necrosis Peterhans et al. During pregnancy, BVDV has the ability to cross the placenta and cause intrauterine infections. In fact, most new PI detected in an infected herd will be the result of TI in dams with a normal immune response Moennig et al.

Several factors have influenced the persistence of BVDV in cattle. A non-lytic infection produced by NCP BVDV strains and the ability to evade the host immune response is the primary mechanism of persistence. When NCP biotype of BVDV infects the dam during the first trimester, the immature immune system of fetus is not able to develop a sufficient immune response yet and the virus produces the PI.

Because the immune system of the fetus now recognizes the virus as part of its make-up, the virus will remain in the calf as long as it lives. Persistently infected animals are viremic virus-positive and antibody-negative or seronegative , continually shed large amounts of BVDV in all body secretions including: nasal discharge, saliva, semen, faeces, etc.

These animals serve as a major reservoir of virus for within the herd as well as the mechanism for maintaining BVDV in the cattle population Fig. Because the PI calves serve as one of the main reservoirs that maintain BVDV within the cattle population and BVDV spreads through most organs in the animal, but no lesions are present, it is exceedingly important to identify and remove these animals from the herd.

PI cattle excrete the virus throughout the life being a source of infection for other animals in a herd. The age-specific prevalence of PI is greatest at birth and decreases with age. Some PI calves can survive until maturity and if they are retained for breeding, their offspring is always PI but often fails to survive.

BVDV infection is diagnosed on the basis of the clinical signs plus confirmation through necropsy findings and laboratory tests of blood samples. If blood is drawn during the acute phase of the disease the laboratory can often isolate the virus from the white blood cells buffy coat. If two serum samples are obtained, one in the acute phase and one a few weeks later, a rise in serum antibodies SN test between the two samples also confirms BVDV infection.

When abortion is the only sign, diagnosis is often more difficult. In these cases it is important for your veterinarian to submit the aborted fetus and placenta, in addition to serum samples from the dam, to the laboratory for testing.

The persistently infected PI carrier animal is easy to detect. This animal sheds so much virus that a viral antigen in its serum readily confirms its condition. Therefore, other test methods have been developed, including skin notch testing and whole blood viral DNA detection that can be applied to baby calves and also older animals. If BVDV gets into a non-vaccinated or improperly vaccinated herd, it will spread from animal to animal.

Thus it is important to maintain a strong BVDV vaccination program that will minimize this type of transmission and allow containment of the virus before it infects a large portion of the herd.

Keep in mind that cattle exposed to the virus at less than days of pregnancy may give birth to persistently infected calves. These calves, if not removed from the herd, will serve as a continuous source of the virus that will perpetuate the disease in the herd. Consequently, once BVD occurs in the herd, all calves born over the next 12 months should be tested for persistent infection.

If any such carriers are detected, they must be removed from the herd immediately. It is best to test calves for persistent infection at birth or as soon as possible thereafter.

An effective BVDV prevention program is based on maximizing immunity and minimizing exposure of the herd to the virus. BVD vaccines are an important part of the prevention program. Vaccines are available in two forms - modified live and killed. Both forms have their advantages and disadvantages. An advantage of modified live BVD vaccines is that they stimulate the entire immune system both cell- mediated and humoral immunity. Therefore, it is generally recommended that every animal receive a modified live BVD vaccine at least once in its lifetime, preferably when it is months of age.

A disadvantage of modified live BVD vaccines is there may be label restrictions against using them on some or all pregnant animals, thus their use in mixed populations of bred and open animals must be carefully controlled. An advantage of killed BVD vaccines is that they can be used on all open and pregnant animals, thus the entire herd can be vaccinated at any time. Disadvantages of the killed BVD vaccine include: a shorter duration of immunity, a reduced ability to stimulate cell-mediated immunity, and sometimes a slight drop in milk production for a day or two after vaccination.

As indicated on the label, killed vaccines must be administered twice two or three weeks apart if the animal is being vaccinated for the first time.