This is the first epidemiological study to assess herd management factors related to the seroprevalence of BLV on dairy farms in Japan. Based on our mixed logistic model with a random herd effect, the significant factors were found to be cattle housing conditions, the presence of horseflies in summer, dehorning practices, and colostrum feeding.
With regard to housing conditions, a loose housing system was found to be positively associated with seroprevalence compared with a tied housing system (p = 0.03). A possible explanation may be that direct contact between infected and uninfected cattle is facilitated in loose housing systems such as free stalls and barns. Furthermore, the chances of indirect contact between uninfected and infected cattle may be relatively high in a loose housing system because cattle randomly move about the barn or paddock and are not always handled in the same manner.
Dehorning is a practice in daily herd management that can reduce the risk of injury from horn-butting in farmers and in cattle during herd conflict. This practice was found to be positively associated with seroprevalence compared with farms without it (p < 0.01). A potential explanation for this result is the unhygienic procedure of dehorning, which is a risk factor for BLV transmission. It was previously reported that calves dehorned with contaminated dehorning apparatus were more likely to be infected than those that had not been dehorned [11
]. Farmers often leave the dehorned heifers without hemostasis by cautery (personal communications with clinical veterinarians). Although we did not include a question related to the treatment of dehorned cattle in the questionnaire, the information obtained from clinical veterinarians supported our result.
With regard to the presence of horseflies in summer, a response of "very high" was positively associated with seroprevalence compared with a response of "never to seldom" (p = 0.01); however, there was no significant difference between "sometimes or often" and "never to seldom" (p = 0.49). BLV transmission by hematophagous insects has been reported [14
]. In this study, we did not measure horsefly populations quantitatively on each farm. However, given the effect of the response of "very high" in increasing seroprevalence, our findings support the results of previous studies and suggest more quantitative study in future.
In the present study, colostrum feeding of calves from their dams resulted in a decrease in within-farm seroprevalence (coefficient = -1.11, p = 0.03). Because colostrum contains BLV and its antibodies, ingestion of colostrum from infected dams reduces the risk of BLV infection during the weaning period in calves [20
]. Our result is consistent with previous studies, although there were only seven farms in our study without colostrum feeding, the reference category. In addition, despite marginal significance, pooled colostrum feeding on a farm had a negative impact on seroprevalence (coefficient = -0.90, p = 0.10). Therefore, colostrum feeding could be an effective way to reduce the prevalence of BLV in a herd. The effect of optimal conditions for the treatment of pooled colostrum, such as heating and freezing on the farm, should be evaluated in future studies.
Because this study had a cross-sectional design, it was difficult to elucidate definite relationships between seroprevalence and each risk or protective factor. However, in spite of this constraint, our results were consistent with previous studies and confirmed the important factors that should be controlled.