Controlling bovine leukemia virus in a large dairy herd by selective culling based on diagnostic testing

Abstract

This intervention field trial aimed to reduce bovine leukemia virus (BLV) from a large dairy herd using a new diagnostic test to identify the most infectious cows for segregation or culling. Bovine leukemia virus infects over 40% of all dairy cattle in the United States. Similar prevalence has been reported in many other countries. Previously hidden economic impacts of BLV on the industry have only recently been recognized. The traditional method of controlling this disease was by culling all antibody-positive cattle. The availability of a new diagnostic test can identify the most infectious cows for removal. A 3,000-cow dairy farm conducted a 4-yr BLV disease control program using blood lymphocyte count (LC), BLV ELISA antibody test, and quantitative-PCR proviral load (PVL) to identify the most infectious animals for segregation or culling. The percentage of cows with a high LC was significantly decreased from 4.22 to 1.04% after the first year of selective culling of cows with high LC. Thereafter, BLV ELISA screening with PVL followup assays were used to target segregation and culling of cows with the highest PVL. After 3 yr, only 0.85% of the tested cows were BLV ELISA positive. The prevalence of animals with detectable PVL decreased from 284 to zero animals. When coupled with the results of an intervention trial in 3 small dairy farms, our findings from a large dairy herd demonstrate that BLV may be eradicated without the simultaneous culling of all ELISA-positive cows.