The Bovine Viral Diarrhoea (BVD) Eradication Scheme phase 4: guidance for vets

Part 4 What Constitutes A Separate Group

The effectiveness of the check-test depends on the correct identification of each separate group of youngstock. A separate group consists of those animals that can freely achieve nose-to-nose contact with all others within the group.

For example, in most beef herds the minimum number of animals to sample will be five from the heifer group, five from the bull or steer group and where there are spring and autumn calving blocks care should be taken to include groups of calves from each block. To achieve this more than one sampling visit may be required in some herds.

The point of testing separate groups is to show if there is a PI animal within the herd, and where it is. PI animals spread the virus very efficiently when in close contact with other cattle. Nasal discharges and saliva are the most potent sources of infectious virus so that any husbandry system which permits nose-to-nose contact will hasten the spread of virus from PI cattle to susceptible animals. Intensive housing with trough feeding will ensure rapid spread whereas spread will be slower among cattle at grass.

Because of the efficient spread it is not necessary to bleed the whole group. A sample of animals is sufficient providing the following conditions are met:

• All the animals in the group have been together for at least two months.
• They will have had nose-to-nose contact during that period.
• They are in the appropriate age range (see testing options described previously).

The second point above is important when deciding about housed animals. Animals either side of a central passageway for example are separately managed groups.

It is not always necessary to test every pen in a shed. Consider the layout of a shed shown in the diagram below:

Diagram 1

In the example above, cattle in each pen have nose-to-nose contact with cattle in adjoining pens. This means that a PI animal in any pen is likely to cause transient infection in any directly neighbouring pen. So, provided the groups in the diagram above have been together for more than two months, taking a sample of five calves aged 9-18 months from Pen 2, Pen 4 and Pen 6 will be sufficient. This is because enough calves in Pen 1, Pen 3 and Pen 5 would be infected by a PI calf in a neighbouring pen.

Careful consideration should be given to the degree of separation between groups. For instance, in the example on the previous page, if there were walls between the pens, this could prevent sufficient nose-to-nose contact from occurring.

In extensive systems, where cattle are spread over a wide area, consideration should be given to ensuring that the cattle have had sufficient opportunity to spread virus within each group before sampling.