PREVALENCE OF CATTLE PERSISTENTLY INFECTED WITH BOVINE VIRAL DIARRHEAVIRUS IN 20 DAIRY HERDS IN 2 COUNTIES IN CENTRAL MICHIGAN AND COMPARISON OF PREVALENCE OF ANTIBODY-POSITIVE CATTLE AMONG HERDS WITH DIFFERENT INFECTION AND VACCINATION STATUS

All cattle in 20 dairy herds randomly selected from herds participatin g in the Dairy Herd Improvement Association program in 2 counties in c entral Michigan were tested for the presence of bovine viral diarrhea virus (BVDV). Virus-positive animals were retested to ascertain persis tent infection with the virus. A total of 5,481 animals were tested fo r presence of BVDV. In 9 of the herds, all animals were also tested fo r virus neutralizing antibody titer. Based on infection and vaccinatio n status, these 9 herds were divided into 3 different herd categories: A, 5 herds with currently no cattle persistently infected (PI) with B VDV and without any vaccination program against BVDV in recent years; B, 2 herds with no current PT cattle but using killed BVDV vaccines; a nd C, 2 herds with PI cattle. PI cattle were detected in 3 out of 20 h erds (15%). A total of 7 of 5,481 animals (0.13%) were PI. The mean pr evalences of antibody carriers in herd categories A, B, and C were 28. 8%, 76.4% and 90.6%, respectively. For one herd in category A, antibod y analyses indicated that mostly young stock was seropositive, suggest ed recent BVDV infection in a previously closed and naive herd. Cattle in category B herds were vaccinated with killed vaccine from the age of 15 months. These herds had several antibody negative animals among the younger cows, suggesting incomplete protection against BVDV infect ion. In the 3 herds in which PI animals were detected, all cattle had been vaccinated with killed vaccine. The antibody-positive animals had antibody titers that were significantly different both among herds an d among herd categories. The antibody titers of animals exposed to PT animals were significantly higher than those of animals vaccinated wit h killed vaccine.