A LONG-TERM EPIDEMIOLOGIC-STUDY OF BOVINE VIRAL DIARRHEA INFECTIONS IN A LARGE HERD OF DAIRY-CATTLE

Epidemiological aspects of bovine viral diarrhoea virus (BVDV) infecti ons were studied longitudinally in a large dairy herd for three years. At the start of the study, practically all the cows more than four ye ars old had BVDV antibody titres, whereas the younger stock were almos t all seronegative. The spread of the virus was monitored in a part of the population that contained only transiently viraemic cattle and in another part that contained persistently viraemic calves. Among the l actating cows the virus circulated for two-and-a-half years, although they had no direct contact with persistently viraemic cattle during th is period. The highest transmission rate occurred when a large number or susceptible heifers was added to the population of cows that contai ned transiently viraemic cattle. The circulation Of BVDV among the lac tating cows ceased while 27 seronegative cows were still present. Both findings are in accordance with predictions from simple epidemic mode ls. The susceptibility of the cows that remained seronegative was conf irmed experimentally. In contrast with the limited circulation Of BVDV caused by transiently viraemic cattle, virtually all susceptible catt le that came into contact with a persistently viraemic calf became ser opositive within three months. Transplacental BVDV infections were not detected in the calves born to cows that had antibodies against the v irus due to an infection that had occurred at least four years earlier . Transplacental transmission Of BVDV did not occur in most of the pre gnant cows that were infected before approximately the 60th day of ges tation, but when cows became infected later in gestation the virus vir tually always invaded the fetus. Clear conclusions on transplacental i nfection were not always possible in fetuses infected in late gestatio n. The precolostral sera or six of 42 prenatally infected calves conta ined both virus and antibodies; the antibody titres were low. After re testing four to five months later, the two calves remaining on the far m were still viraemic, but they had become seronegative.