Cattle persistently infected (PI) with BVD virus are the single biggest means of transmission of infection within and between herds. While most calves tested will be negative on initial testing, a small number (currently 0.65%) will give a positive (or inconclusive) result. This article describes the next steps where this happens.

An initial positive result on a calf means that it is likely that it is PI, but is not definitive, with a minority being transiently infected (TI), typically due to contact with a PI animal after birth. Unlike PI calves, TI animals can mount an immune response and eliminate the virus, typically within a couple of weeks. Following an initial positive result, the dam of the calf is also a potential PI, and is assigned a DAMPI status until further testing has been carried out, consistent with the programme guidelines.

The AHWNI database reports positive results by SMS text message, followed by a letter giving further information, including the registered dam of the calf and a submission form for samples collected as part of follow up testing.

It is recommended that calves with an initial positive or inconclusive result are resampled after a minimum interval of 3 weeks, with a second positive result consistent with the calf being PI, whereas a negative result indicates that it was a TI. The dam of the calf should also be tested at the same time. In both cases this must be done through blood samples collected by your veterinary practitioner.

Once confirmed, it is strongly recommended that PI calves are culled from the herd as quickly as possible, and isolated (with their mother if necessary) until this has been done. Firstly, they are an important source of transient infections in other animals. This can lead to a weakening of the immune system, particularly in other calves, causing illness and death due to scours and pneumonias. Secondly, infection of susceptible pregnant stock can lead to a range of negative reproductive outcomes, the most important of which is the creation of further PI calves that will be born the following calving season. Finally, while they may appear normal at birth, PI animals typically do not thrive, with many dying before they reach slaughter weight. The programme guideline include an undertaking not to sell animals that are suspected or proven to be PI, including DAMPIs.

For further information, including guidance on registration of stillborn and dead calves, see www.bvdni.com or contact 028 8778 9126 (AHWNI) or your own veterinary practice