Evaluation of control measures for bovine viral diarrhea implemented in Nemuro District, Hokkaido, Japan, using a scenario tree model

ISODA, Norikazu; ASANO, Akihiro; ICHIJO, Michiru; WAKAMORI, Shiho; OHNO, Hiroshi; SATO, Kazuhiko; OKAMOTO, Hirokazu; NAKAO, Shigeru; KATO, Hajime; SAITO, Kazuma; ITO, Naoki; USUI, Akira; TAKAYAMA, Hiroaki; SAKODA, Yoshihiro

In Scandinavian countries including Denmark, Sweden, Norway, and Finland, BVD had been eradicated due to implementation of an eradication program without vaccination [24, 30] . Moreover, the Norwegian government banned the use of the vaccination for pestiviruses as the vaccination could prevent the manifestation of severe clinical manifestations, but did not prevent the spread of the virus [24] . In these countries, cattle with antibodies against BVDV as potential risk of BVD spread is easily monitored but it can also cull no-virus-shedding cattle which was transiently infected in past. In Japan, on the other hand, we are monitoring virusshedding cattle and adopt the vaccination so as to prevent the further virus spread. According to the results of the present study, it was revealed that the adoption of mass vaccination decreased the number of missed BVD cattle, although it did not increase the number of detected BVD cattle. This may indicate that, in an area where no interventions for BVD control have been implemented, the adoption of the mass vaccination campaign would be the foremost intervention for BVD control to reduce the number of cattle infected with BVDV. In the present model, herd immunity conferred by mass vaccination did not affect the number of PI cattle, but did affect the number of TI cattle due to a decrease of the sensitive population. Under system of no BVD monitoring, PI cattle would not be detected without showing obvious clinical manifestations, such as reproductive disorder. Given these considerations, the missed BVD cattle under the mass vaccination program would be the TI cattle of any age or PI heifers. As these diseased cattle usually do not show obvious clinical manifestations, it is likely that undetected cattle would be left for a period, thereby allowing the shedding of infectious viruses. Even TI cattle would be a source of BVDV spread, resulting in horizontal and vertical infection over a limited duration.