Bovine Leukemia Virus Control on Japanese Breeding Farms Using the Proviral Load and Its Issues

After bovine leukemia virus (BLV) enters the lymphocytes of the infected host, the viral RNA reverse transcribes into viral DNA and the DNA integrates into the host chromosomal genome to establish infection. The viral DNA integrated into the host genome is called the provirus, and its copy number is called the proviral load. The proviral load of BLV-infected cattle is proportional to the risk of viral transmission to cohabitating cattle. Based on our previous research, we categorize BLV-infected cattle with a proviral load greater than 2000 copies/ 50 ng of genomic DNA as “very high risk”, 500 copies or more as “high risk”, 500 copies or less as “low risk”, and 100 copies or less as “very low risk”. In addition to that, we have proposed the following measures to control BLV on farms: (1) keeping high risk cattle separate from uninfected cattle, (2) not using equipment contaminated with body fluids or blood, and (3) reducing breeding with high risk cattle as much as possible. The most unique feature of our proposed control measures is that it is acceptable for low risk and uninfected cattle to be housed in the same pen. This manuscript introduces our studies that provide the evidence for this control measure and examples of farms where the BLV control have been applied.