Infection is usually mild causing a transient rise in temperature, diarrhoea and milk drop in the adult animal. However if the animal is imunocompromised at the time of infection (concurrent disease, mastitis etc), then signs may be more severe. Conversely BVD infection may render an animal more susceptible to other disease e.g. mastitis, pneumonia.

The main problems arise if an animal is infected whilst pregnant.

In the first third of pregnancy the foetus does not have a fully competent immune system. Infection of the cow at this time can cause early embryonic death which manifests as an infertility problem, e.g. irregular oestrus intervals or later on as abortion.

If the foetus survives it will be born as a 'persistently infected' calf. This means it will shed BVD virus throughout its life, being a constant source of infection to other animals.

PI calves may appear 'normal' at birth or they can be born with cataracts or show brain damage in the form of in coordination and tremors. PI calves will often succumb to Mucosal Disease between 6 and 18 months old, when the virus mutates and causes a fatal scour condition. However some will survive into adulthood, become pregnant and calve. These animals will always have a PI calf.

By measuring the level of antibodies in a bulk milk sample will provide an estimate of the level of infection in a herd. Quarterly testing will immediately highlight a rising level indicating a problem. Blood sampling of specific groups of animals particularly maiden heifers will also pick out groups which are completely naïve to the virus and hence are most at risk.