Bovine leukemia virus (BLV), a member of retroviridae, is an oncovirus that causes a chronic infection in cattle called Enzootic bovine leukosis (EBL) and has a worldwide distribution, but its overall prevalence in Iran is unknown. EBL causes significant economic loss associated with the cost of control and eradication, loss in milk production and difficulties in exports. The bulk tank samples were collected from 92 dairy herds in Mashhad area during summer 2009. The dairy herds were categorized based on the type of herd, herd size and geographical location of herd. A positive ELISA antibody response was detected in 38 (41.3%) out of 92 herds. There was a significant and positive correlation between herd size and PP value (rs=0.345, p<0.01). This showed that larger herds had higher antibody levels in bulk tank milk and probably higher within-herd prevalence. BLV prevalence was significantly higher in herds with more than 100 cattle (p<0.01). Also, there was a significantly higher BLV prevalence in industrial dairy herds compared with semi-industrial dairy herds (p<0.05). BLV prevalence between dairy herds of two regions was not significantly different (p>0.05). This study revealed that BLV infection in dairy herds of Mashhad area is influenced by herd size and type of herd. The test showed that it can be used in an extensive investigation for rapid screening of dairy herds in Iran. We predict a major role for herd management practices in this prevalence but characterization of key risk factors needs more investigations.

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