The introduction of BVD virus to a herd can result in significant losses particularly if there are a large number of susceptible animals present. When cattle are first exposed to the virus they become infected and shed virus in their body fluids for about a week. During this time they will produce an immune response which helps clear the virus from their system. This is known as a transient infection. They may not show obvious signs of infection but occasionally there will be signs of diarrhoea or lethargy. However during this period of infection there is a suppression of their immune system, which can resulting in more severe outbreaks of calf scour and pneumonia occurring.
Poor herd fertility is commonly encountered. Increased returns to service or abortions can be some of the signs that BVD virus may have entered the breeding herd. Bulls exposed to the virus for the first time often have reduced fertility due to poorer semen quality, which can last for months.
If a cow or heifer that has never been previously exposed to the virus gets infected in the first 120 days of pregnancy then a persistently infected (PI) calf may be born. These are the danger animals as they are constantly shedding large amounts of virus.
PI animals may fail to thrive but they can often look healthy so it is easy to see why bought-in PI cattle are frequently responsible for BVD virus entering a herd.
PI animals are not cured by vaccination. They will continue to shed virus in large amounts so vaccination alone is insufficient to eliminate BVD virus from a herd. Eradication of BVD virus from a herd requires the removal of PIs and good biosecurity.
PIs tend to have short lives and usually die before they reach two years old from a fatal form known as mucosal disease where ulcers occur throughout the digestive tract from the mouth to the anus, resulting in severe diarrhoea.
Occasionally PIs survive for longer though, even up to 15 years old. As a PI cow will always produce a PI calf it is possible for generations of PIs to occur in a herd.
In herds that are free of BVD virus there will be a number of susceptible animals present and the proportion of these will increase over time. This can leave the herd vulnerable to infection and a potentially devastating outbreak of BVD virus, especially if the herd is not vaccinating and if biosecurity is inadequate. BVD breakdowns have occurred in herds that appeared to be well isolated and had tested any bought-in cattle. In these cases BVD virus may have been brought into the herd on clothing, equipment or vehicles.
Herds can gain BVD Accredited or BVD Vacccinated Monitored Free status through the health scheme.