Bovine Viral Diarrhoea (BVD)

 Bovine Viral Diarrhoea(BVD)

BVD is a common cause of respiratory and reproductive issues in the herd. It is a economically important disease in many countries, and problems realted to the disease are thought to be increasing in some areas.

Although a disease in itself, BVD causes a number of transient infections which are often the cause of animal health and economic problems.

Cause

Bovine viral diarrhea is a viral disease of cattle and other ruminants that is caused by the bovine viral diarrhea virus (BVDV).

BVD is transmitted in a number of ways. Either through a congenital infection of the fetus or after birth. Congenital infections may cause resorption, abortion, stillbirth, or live-birth. Congenitally infected fetuses that survive in utero infection (i.e., the live-births) may be born as BVDV-infected calves. The BVDV infection in these calves will persist during the entire life of the calf, and they will shed BVDV continuously in the farm environment.

Symptoms

In adults, clinical signs are highly variable. Signs of acute infection include fever, lethargy, loss of appetite, ocular dishcharge, nasal dischargem oral lesions, diarrhea and decreasing milk production. Chronic infection may lead to signs of mucosal disease.

In calves, the most commonly recognised birth defect is cerebellar hypoplasia. The signs of this are:

• Ataxia/ lack of voluntary coordination of musle movements;
• Tremors
• Wide stance
• Stumbling
• Failure to nurse

In severe cases the calf may die.

Transient infections include diarrhea, calf pneumonia, 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.

Treatment

Treatment of BVD is limited primarily to supportive therapy. Once identified, infected animals should be culled.

Prevention

Research is ongoing to look at the potential for breeding animals that are less susceptible to the disease. At the moment, no one breed has an advantage.

One strategy to minimise BVD transmission is to make infected cattle less infectious, and this can be achieved by increasing the antibody titer. Cattle that have antibodies at the time that they acquire acute BVDV infection do not shed as much virus, and they will shed virus for a shorter period of time.

On farm there it is important for producers to cull persistantly infected animals from the herd. Blood tests will identify Housing calves in individual hutches as opposed to group housing will decrease contact and risk of infection, as will reducing stocking density.

Strategic vaccination and high-quality colostrum could also decrease the proportion of susceptible cattle.

A BVD control programme on farm would aim to prevent fetal infections, to eliminate reproductive loss and decrease losses due to transient infections. Control is achieved with a combination of removal of PI cattle, vaccination and enhanced biosecurity.

Vaccines

Vaccines for BVD are available. The two categories are modified live virus (MLV) vaccines and killed virus (KV) vaccines. Generally speaking, MLV vaccines require only one dose during the initial immunization step, however they are more difficult to handle.

KV vaccines are usually more expensive and more than one dose is required during immunization. However, KV vaccines are less susceptible to deactivation by temperature extremes, and they are less susceptible to deactivation by chemicals.

Diagnostics

Because persistently infected (PI) calves look and act normal, it is very important that producers use diagnostic testing to identify PI animals and remove them from the herd.

PCR is the most sensitive tool for early identification of PI calves, so farmers can remove these highly shedding animals from the herd as soon as possible. Detection of BVDV in newborn calves using antibody-based tests can be difficult as calves can have very high maternal antibody titers to BVDV, mainly via ingested colostrum. These maternal antibodies bind the virus and prevent its detection in this type of test. This is generally called shielding.

PCR tests use either blood or tissue (ear notch) samples, can be completed in a very short period of time and provide very accurate results.

Antibody-based tests are used for bulk-milk screening and allow diagnosis of BVDV infections in large scale screening and eradication programs.

(November 2, 2021. From “ The Cattle Site”)

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