Although a number of workers have assayed components of the cellular immune
system in the course of research studies, only the humoral response has been used
for diagnostic purposes. Cattle undergoing acute BVD infection seroconvert within
two to three weeks of exposure to the virus; the antibody titre may continue to rise
for some weeks further. Paired acute and convalescent sera, taken three to four weeks
apart, from the same individuals can therefore provide a most useful indication, albeit
retrospective, of acute infection in a group of cattle. Antibody titres persist for years,
probably for life, in recovered cattle (40). The finding of antibody in single serum
samples thus indicates no more than infection at some time in the past and is of limited
diagnostic value. Calves born to seropositive dams acquire antibodies from the
colostrum. The titres decline steadily over the first few months of life (42, 46).
Serology may also be used for herd screening as part of a general health monitoring
programme and, specifically, to identify seronegative cattle which can then be subject
to further testing to determine if they are persistently infected virus carriers.
Nevertheless it should not be assumed that all the persistent carriers are to be found
among the seronegative population. It is known that a small proportion of persistently
infected cattle also have BVD virus-specific antibody in their bloodstream (12).