Bovine virus diarrhoea is a highly contagious viral disease that is caused by the Bovine Viral Diarrhoea Virus. Because the virus is so contagious, it causes substantial damage on dairy farms. The BVD virus is split into two types, depending on its genetic makeup: BVDV-1 and BVDV-2. These types can be subdivided into different subtypes. The virus can be introduced to a farm in various ways. When animals come into contact with the virus for the first time, they start to produce antibodies. If a pregnant animal is infected during the first four months of pregnancy, the calf is born as a BVD carrier. BVD virus carriers are the main source of spread of the virus within the population. The BVD virus carriers secrete the virus for life and continuously. BVD virus carriers are also referred to as persistently infected animals (PI).
BVD infections are not that obvious and often overlooked. In 70-90 % of cases, only limited symptoms are visible. Some of the acute BVD infections will be accompanied by more obvious, clinical signs to a greater or lesser extent. Symptoms that can be seen are:
- Production drop
- Inflammation of the mucous membranes leading to diarrhoea, salivation, loss of appetite and/or dehydration
- Abortion and fertility disorders
- Birth of deviant and weaker calves
- Birth of BVD virus carriers
- The BVD virus causes a reduced immune function. Germs can easily strike, causing problems such as respiratory problems, diarrhoea and mastitis.
Diagnosis of a BVD infection can be done via blood tests (ELISA), as virus antibodies are a clear marker for infection. Furthermore, the ELISA test can be used to check whether there are antibodies present in the milk. If a BVD infection is suspected in a herd, the ELISA test is done on the blood and the tank. If an individual animal tests positive for the virus when it is tested for the first time, there are 2 possibilities; the animal is currently going thru an infection, or it is a BVD carrier. A second test after 3 weeks is needed for a conclusive diagnosis.
BVD can enter through the purchase of infected cattle, by infection during shows and via contaminated visitors to the farm. Cows carrying BVD virus are the main source of spread, as they continuously shed the virus. For this reason, it is important that all acquired cows are put in isolation for 2 weeks and tested. Infected animals should be traced and removed from the farm. If BVD gets into a non-vaccinated or improperly vaccinated herd, it will spread from animal to animal. Thus it is important to maintain a strong BVD vaccination program that will minimise this type of transmission and allow containment of the virus before it infects a large portion of the herd.