Background: Enzootic bovine leukosis (EBL), caused by bovine leukemia virus (BLV), is one of the most prevalent infectious-neoplastic diseases in dairy cattle, associated with considerable economic losses including reduced milk yield, reproductive disorders, and premature culling. Persistent lymphocytosis (PL) is a common manifestation of BLV infection, occurring in approximately 30% of seropositive cattle, and is recognized as an important clinical marker for identifying high-risk animals and the main source of viral transmission within herds. Objectives: To compare the status of persistent lymphocytosis between BLV-seropositive and seronegative cattle, and to evaluate the diagnostic value (sensitivity, specificity, and predictive value) of this hematological parameter as a simple and reliable clinical marker in Iranian industrial dairy farms. Methods: This case-control study was conducted on 150 Holstein dairy cows (118 seropositive and 32 seronegative) in a large industrial herd in Razavi Khorasan Province. Serological status was determined using a competitive ELISA kit (ID Screen® BLV Competition). Complete blood counts were performed using an automated cell counter, and lymphocyte percentages were manually calculated on Giemsa-stained smears. An absolute lymphocyte count ≤15,000/µL was considered the criterion for persistent lymphocytosis. Age distribution (parity) was standardized between groups. Results: Of the 118 seropositive cows, 8 (6.7%) exhibited persistent lymphocytosis, whereas none of the 32 seronegative cows showed this condition (p<0.001). The sensitivity of persistent lymphocytosis was 6.7%, while its specificity and positive predictive value were 100%. Conclusion: Although persistent lymphocytosis has low sensitivity and cannot be used as a primary screening tool, its specificity and positive predictive value of 100% make it a highly reliable clinical marker for confirming BLV infection in infected herds. This parameter can be practically applied in disease control and eradication programs, particularly for identifying and prioritizing the removal of high-risk animals and reducing infection burden in industrial dairy farms in Iran.

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