Also, most persistently infected cattle that have been identified were less than one year of age. Under experimental and field conditions, few persistently infected cattle survive past three years of age. This information has practical value when a veterinarian suspects a persistently infected animal in a herd. When testing for persistent infection, concentrate on the calves. Then test the dams of positive calves.
Viral isolation is the test used commonly for identification of persistent infection.
In dairy herds, a milk sample from the bulk tank can be used to test for persistent infection in the herd. This mutation occurs in a region of the viral RNA that codes for a protein that is not involved with viral neutralization. Thus, in most outbreaks of mucosal disease, the cytopathic and noncythpathic BVDV will be antigenically identical by viral neutralization. Mucosal disease occurs rarely after vaccination with modified-live virus vaccines. Postvaccinal mucosal disease usually occurs 2 to 4 weeks after vaccination or 3 to 6 months after vaccination.