[2][3] The causative agent, bovine viral diarrhea virus (BVDV), is a member of the genus Pestivirus of the family Flaviviridae.

Countries that had failed to implement any BVDV control and/or eradication programmes (including vaccination) had the highest PI prevalence.

This RdRp can undergo template strand switching allowing RNA-RNA copy choice recombination during elongative RNA synthesis.

[1] BVD is considered one of the most significant infectious diseases in the livestock industry worldwide due to its high prevalence, persistence and clinical consequences.

[18] Transmission of BVDV occurs both horizontally and vertically with both persistently and transiently infected animals excreting infectious virus.

Phagocytes take up BVDV or virus-infected cells and transport them to peripheral lymphoid tissues; the virus can also spread systemically through the bloodstream.

After 2–3 weeks, antibodies effectively neutralise viral particles, promote clearance of virus and prevent seeding of target organs.

This occurs because the fetal immune system has developed, by this stage of gestation, and has the ability to recognise and fight off the invading virus, producing anti-BVD antibodies.

These sites include ovarian follicles, testicular tissues, central nervous system and white blood cells.

Cattle with chronic infections elicit a significant immune response, exhibited by extremely high antibody titres.

However, clinical signs are frequently mild and infection insidious, recognized only by BVDV's immunosuppressive effects perpetuating other circulating infectious diseases (particularly scours and pneumonias).

In PIs the virus remains present in a large number of the animal's body cells throughout its life and is continuously shed.

It is vital that repeat testing is performed on positive samples to distinguish between acute, transiently infected cattle and PIs.

A standard test to assess whether virus has been circulating recently is to perform an Ig ELISA on blood from 5–10 young stock that have not been vaccinated, aged between 9 and 18 months.

A positive result in a pregnant female indicates that she has previously been either vaccinated or infected with BVDV and could possibly be carrying a PI fetus, so antigen testing of the newborn is vital to rule this out.

Negative results suggest that a PI is unlikely however this naïve herd is in danger of severe consequences should an infected animal be introduced.

Antibodies from wild infection or vaccination persist for several years therefore Ig ELISA testing is more valuable when used as a surveillance tool in seronegative herds.

Despite different conditions at the start of the projects in terms of legal support, and regardless of initial prevalence of herds with PI animals, it took all countries approximately 10 years to reach their final stages.

[25][26] Once proven that BVD eradication could be achieved in a cost efficient way, a number of regional programmes followed in Europe, some of which have developed into national schemes.