Objective—To determine the reference interval for WBC counts in Holstein dairy cows from herds with high seroprevalence for anti–bovine leukemia virus (BLV) antibodies, analyze the correlation of total WBC counts and blood proviral load (bPVL) in BLV-infected animals, and determine whether total WBC count can be used a hematologic marker for in vivo infection. Animals—307 lactating cows from 16 dairy herds with high BLV seroprevalence. Procedures—Blood samples were collected for assessment of plasma anti–BLV p24 antibody concentration (all cows), manual determination of WBC count (161 BLV-seronegative cows from 15 herds), and evaluation of bPVL (146 cows from another herd). Results—The WBC count reference interval (ie, mean ± 2 SD) for BLV-seronegative dairy cows was 2,153 to 11,493 cells/μL. Of the 146 cows used to analyze the correlation between WBC count and bPVL, 107 (73%) had WBC counts within the reference interval; of those cows, only 21 (19.6%) had high bPVL. Most cows with high WBC counts (35/39) had high bPVL. Mean WBC count for cows with high bPVL was significantly higher than values for cows with low or undetectable bPVL. White blood cell counts and bPVL were significantly (ρ = 0.71) correlated. Conclusions and Clinical Relevance—These data have provided an updated reference interval for WBC counts in Holstein cows from herds with high BLV seroprevalence. In dairy cattle under natural conditions, WBC count was correlated with bPVL; thus, WBC count determination could be a potential tool for monitoring BLV infection levels in attempts to control transmission. | Instituto de Virología | Fil: Alvarez, Irene. Instituto Nacional de Tecnología Agropecuaria (INTA). Instituto de Virología; Argentina | Fil: Gutierrez, Gerónimo. Instituto Nacional de Tecnología Agropecuaria (INTA). Instituto de Virología; Argentina | Fil: Gammella, Mariela. Instituto Nacional de Tecnología Agropecuaria (INTA). Instituto de Virología; Argentina | Fil: Martinez, Cecilia. Instituto Nacional de Tecnología Agropecuaria (INTA). Instituto de Virología; Argentina | Fil: Politzki, Romina Paula. Instituto Nacional de Tecnología Agropecuaria (INTA). Instituto de Virología; Argentina | Fil: Gonzalez, Cintia. Instituto Nacional de Tecnología Agropecuaria (INTA). Instituto de Virología; Argentina | Fil: Caviglia, Luciana. Instituto Nacional de Tecnología Agropecuaria (INTA). Instituto de Virología; Argentina | Fil: Carignano, Hugo Adrian. Instituto Nacional de Tecnología Agropecuaria (INTA). Instituto de Genética "Ewald A. Favret"; Argentina | Fil: Fondevila, Norberto. Instituto Nacional de Tecnología Agropecuaria (INTA). Instituto de Virología; Argentina | Fil: Poli, Mario Andres. Instituto Nacional de Tecnología Agropecuaria (INTA). Instituto de Genética "Ewald A. Favret"; Argentina | Fil: Trono, Karina Gabriela. Instituto Nacional de Tecnología Agropecuaria (INTA). Instituto de Virología; Argentina