Abstract
BackgroundAlthough 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.ResultsInfected 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.ConclusionControl of EBL in infected dairy farms in Japan will be improved by focusing particularly on these risk and protective factors.
Highlights
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
Univariate test results showed that cattle housing conditions, availability of own grazing area, the presence of horseflies in summer, dehorning, the use of a plastic sleeve for rectal palpation, not changing needles between animals during herd vaccination, and colostrum feeding were factors possibly associated with seroprevalence (p < 0.15, Table 1), whereas herd size (p = 0.51) and cattle replacement (p = 0.29) were not associated with it
Starting with the seven variables with p values < 0.15 in the univariate analyses, the final model was obtained with four variables with the smallest value of the Akaike information criterion (AIC) (Table 2)
Summary
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. According to animal health statistics, EBL was reported in 159 cattle at 157 farms in 2000 and in 838 cattle at 677 farms in 2007. These data suggest that EBL has been gradually spreading in Japan
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