Objective: To establish a threshold value of bovine leukemia virus (BLV) proviral load (PVL) to identify increased risk of severe clinical mastitis, and to examine the prognosis and economic loss of clinical mastitis based on the newly established PVL cut-off value.
Animals: 97 lactating Holstein cows with clinical mastitis.
Methods: Blood and milk samples were collected aseptically from each cow. Youden index was used for receiver-operating characteristic curve analysis with the severity rate of clinical mastitis as the dependent variable and PVL as an independent variable. PVL cut-off value was used as a criterion to compare the severity rate of clinical mastitis, percentage of cows with and without systemic treatments, number of treatments, cost of treatment, and prognosis.
Results: PVL cut-off value was 17.8 copies/10 ng DNA for the dependent variable MILD vs SEVERE. The severity rate of clinical mastitis, percentage of cows given systemic treatments, and technical fees for medical treatment were significantly higher in the group above the PVL cut-off value than in the group below the PVL cut-off value and the negative group. Number of treatments was significantly higher in the group above the cut-off value than in the group below the cut-off value. There was no significant difference in prognosis after mastitis among the 3 groups.
Clinical relevance: These results suggested that PVL cut-off value of 17.8 copies/10 ng DNA was a useful threshold for increased economic losses in BLV-infected cows; it may also serve as a new standard value for the detection and culling of BLV-infected cows in Japan.
Keywords: bovine leukemia virus; cut-off value; economic loss; proviral load; severity of clinical mastitis.