title: Risk factors associated with within-herd transmission of bovine leukemia virus on dairy farms in Japan.
abstract: BACKGROUND
  Although several attempts have been made to control enzootic bovine leukosis (EBL) at the local level, a nationwide control program has not been implemented in Japan, except for passive surveillance. Effective control of EBL requires that the transmission routes of bovine leukemia virus (BLV) infection should be identified and intercepted based on scientific evidence. In this cross-sectional study, we examined the risk factors associated with within-herd transmission of BLV on infected dairy farms in Japan. Blood samples taken from 30 randomly selected adult cows at each of 139 dairy farms were tested by enzyme-linked immunosorbent assay (ELISA). Information on herd management was collected using a structured questionnaire.
  
  
  RESULTS
  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.
  
  
  CONCLUSIONS
  Control of EBL in infected dairy farms in Japan will be improved by focusing particularly on these risk and protective factors.
authors.affiliation: Epidemiological Research Team, National Institute of Animal Health, 3-1-5, Kannondai, Tsukuba-shi, Ibaraki, Japan.
authors.forename: Sota
authors.initials: S
authors.lastname: Kobayashi
authors.forename: Toshiyuki
authors.initials: T
authors.lastname: Tsutsui
authors.forename: Takehisa
authors.initials: T
authors.lastname: Yamamoto
authors.forename: Yoko
authors.initials: Y
authors.lastname: Hayama
authors.forename: Ken-ichiro
authors.initials: K
authors.lastname: Kameyama
authors.forename: Misako
authors.initials: M
authors.lastname: Konishi
authors.forename: Kenji
authors.initials: K
authors.lastname: Murakami
mesh_terms: D000818:Animals; D002417:Cattle; D002418:Cattle Diseases / Q000453:epidemiology* / Q000517:prevention & control / Q000635:transmission*; D003612:Dairying; D016583:Enzootic Bovine Leukosis / Q000453:epidemiology* / Q000517:prevention & control / Q000635:transmission*; D005260:Female; D007564:Japan / Q000453:epidemiology; D001909:Leukemia Virus, Bovine / Q000502:physiology*; D012307:Risk Factors; D016036:Seroepidemiologic Studies