Use of blood meals from stable flies to evaluate the bovine leukemia virus infection status in cattle herds: a pilot study

The incidence of enzootic bovine leukosis (EBL), a type of B-cell lymphoma, is increasing in Japan. EBL is caused by bovine leukemia virus (BLV; <i>Retroviridae</i>, <i>Deltaretrovirus bovleu</i>) infection and is diagnosed by detecting antibodies against BLV in milk and blood or BLV DNA in blood. We assessed the feasibility of using stable flies (<i>Stomoxys calcitrans</i>) as a sampling tool to assess BLV infection status in cattle herds. First, we collected blood from 3 cattle herds and, based on the measurement of BLV-proviral load (PVL) by quantitative real-time PCR (qPCR), identified 1) a BLV-free herd, 2) a herd with a low prevalence of BLV-infected cattle and low PVL, and 3) a herd wherein half of the cattle were BLV-infected with low-to-high PVLs. Next, we collected stable flies from the 3 herds, extracted DNA from their blood meals, analyzed it for BLV DNA, and measured the BLV PVL. Cattle DNA and BLV DNA, but not other mammalian DNA, were successfully detected by digestion of the flies. Based on fly blood meal qPCR, we identified one herd as BLV-free and the other 2 herds as having <50% prevalence of BLV-infected cattle with low PVLs. Our fly results were not consistent with preliminary BLV-PVL measurements on cattle blood. Our pilot study indicated that, to assess the feasibility of a stable fly blood meal test as an alternative technique for evaluating BLV infection status in dairy and beef cattle, additional investigations involving more cattle herds and stable flies are needed.