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

BVD cattle diagnoses: Diagnosis systems for cattle infected with BVDV in the present model were referred to the control measures implemented in Nemuro, including individual tests prior to communal pasture grazing, the herd screening test using bulk milk, and the outbreak investigation of newly infected herds. In Nemuro, all of the cattle kept at the summer communal pasture are mandatorily investigated for BVDV infection by RT-PCR method using serum samples prior to communal pasture grazing [29] . In the present model, the cattle older than three months were assumed to possibly use the summer communal pastures with a certain probability (P c ), which was changed from 10 to 50% in the scenario models. In Nemuro, independent of the use of summer communal pasture, all of the lactating cows are mandatorily screened for BVDV infection by RT-PCR method using bulk tank milk samples. As this screening test is usually performed over six month intervals during every spring and autumn, cows that were not lactating as dry cows at the time of the initial screening test would be investigated during the latter screening test scheme; therefore, theoretically, all of the delivered cattle would be screened no less than once per year. In the present model, PI or TI cattle in the >19 M group were estimated to be screened with a certain test sensitivity (S eb ) using bulk tank milk once a year. In the default setting, bulk milk tests were not performed (S eb was zero), and this was changed to approximately 95% in the scenario models. Under Japanese legislation, when any cattle in a herd are diagnosed with BVD, all of the cattle in the herd are mandatorily tested for BVDV infection either by RT-PCR or virus isolation method using serum samples as an outbreak investigation of a newly infected herd. In the present model, all of the cattle except those in which BVD was detected by any clinical manifestation, by the individual compulsory test, and by the screening test, were assumed to be tested within the outbreak investigation with a probability of 0.3% (P o ), which was calculated by the number of infected herds to that of all the herds in Nemuro. Test sensitivities of BVDV detection from serum and bulk milk samples were as according to the calculations by Kim and Dubovi [20] .