Risk factors associated with within-herd transmission of bovine leukemia virus on dairy farms in Japan

Infected farms were defined as those with more than one ELISA-positive animal and accounted for 110 (79.1%) of the 139 farms in the study. Completed questionnaires obtained from 90 of these 110 farms were used for statistical analysis. Seroprevalence, which was defined as the proportions of animals that tested positive out of all animals tested on the farm, was 17.1%, 48.1%, and 68.5% for the 25th, 50th, and 75th percentiles, respectively. A mixed logistic regression analysis implicated a loose housing system, dehorning, and a large number of horseflies in summer as risk factors (coefficient = 0.71, 1.11, and 0.82; p = 0.03, < 0.01, and 0.01, respectively) and feeding of colostrum to newborn calves from their dams as a protective factor (coefficient = -1.11, p = 0.03) against within-farm transmission of BLV on infected farms.Control of EBL in infected dairy farms in Japan will be improved by focusing particularly on these risk and protective factors.Bovine leukemia virus (BLV), a retrovirus of the family Retroviridae, is the causative agent of enzootic bovine leucosis (EBL). Approximately 30% of cattle infected with BLV have persistent lymphocytosis, and 1-5% develop B-cell lymphosarcoma [1]. With a worldwide distribution, EBL is listed by the World Organization for Animal Health as a disease of importance to international trade [2] and is included in the national eradication program in Australia and some member states of the European Union (EU), several of which have recently eliminated the disease [3,4]. In contrast, 89% of dairy herds in the United States are reported to be infected with BLV [5], and the annual economic loss due to EBL is estimated at $525 million in decreased milk yield [6]. Although EBL causes serious economic damage in the dairy industry [7], thus far only regional voluntary control programs have been implemented in the United States [8].In Japan, EBL is listed as a notifiable disease, but no nationwide control programs have been established.