Research Summary - 2
Productivity, Fertility, and Longevity Outcomes in Virginia High-Prevalence Enzootic Bovine Leukosis Cow-Calf Operations
| Date/Time: |
9/12/2025
17:00
|
| Author: |
Sebastian G Umana Sedo |
| Clinic: |
Virginia Tech |
| City, State, ZIP: |
Blacksburg , VA 24061 |
S.G. Umaña Sedó, DVM, MSc, DVSc
1
;
E.B.S Meira, DVM, PhD
1
;
L. Hungerford, DVM, MPH, PhD, CPH
3
;
K. Lahmers, DVM, PhD, DACVP
2
;
J. Currin, DVM
1
;
1Department of Large Animal Clinical Sciences, Virginia Tech, Blacksburg, Virginia 24061
2Department of Biomedical Sciences and Pathobiology, Blacksburg, Virginia 24061
3Department of Population Health Sciences, Blacksburg, Virginia 24061
Introduction:
Evaluating productivity, fertility, and longevity in high-prevalence Enzootic Bovine Leukosis (EBL) cow-calf operations offers valuable insight into the underexplored impact of this disease within U.S. beef herds. This is a contagious lymphoproliferative disease caused by Bovine leukemia virus (BLV) that is widely spread in North America (LaDronka et al., 2018). While large longitudinal studies in dairy cattle have linked BLV infection to decreased fertility and longevity (LaDronka et al., 2018; Barlett et al., 2013), there is a notable lack of comparable research in beef herds (Benitez et al., 2020). Addressing this gap is essential to understanding the broader implications of BLV infection and evaluating its potential economic burden in cow-calf operations, which may mirror those observed in the dairy sector. Traditionally, BLV is diagnosed using ELISA, but proviral load (PVL)—a measure of integrated viral DNA—has emerged as a more informative marker of infection progression (Panei et al., 2013). To date, no study in the United States has examined the relationship between PVL levels and fertility or productivity in cow-calf operations, nor has explored how PVL values change over time within individual animals. The objectives of this study were 1) to assess the association between BLV infection status and PVL levels with productivity, fertility, and longevity in a commercial beef herd and 2) to evaluate longitudinal changes in PVL for individual cows over a two-year period to understand within-cow dynamics of viral load.
Materials and methods:
A longitudinal cohort study was conducted from Fall 2021 to Spring 2023 across 12 commercial Angus-cross cow-calf operations in Virginia. Cows (n = 1969) were managed under fixed-time artificial insemination protocols with follow-up natural service. Blood samples were collected twice per cow, one year apart, and tested for BLV using ELISA. Proviral load was quantified by qPCR in ELISA-positive cows. Individual records of pregnancy, calf birth and weaning weights were recorded. Information concerning culling was obtained from herd records between Fall 2021 to Fall 2024. Data were analyzed using mixed logistic and linear regression models to evaluate associations between ELISA/PVL status and pregnancy, culling, and weaning weight outcomes. PVL was categorized into four levels (very low, low, moderate and high) to improve interpretability. A Cox proportional hazards model was used to assess time to culling. PVL category changes over time and associated risk factors were assessed using Bowker’s test of symmetry and ordinal logistic regression, respectively.
Results:
The BLV herd level prevalence was 78.1% whereas the BLV cow level prevalence was 75.2%. Elisa status (i.e., positive and negative) was not associated with the risk of pregnancy (P = 0.62) and culling (P = 0.11), and the dam’s calf weaning weight (P = 0.83). The PVL category was not associated with the risk pregnancy (P = 0.06) and culling (P = 0.85), and the dam’s calf weaning weight (P = 0.44). The Elisa status (P = 0.10) and PVL category (P = 0.50) were not associated with an increased or decreased hazard of being culled. Cow PVL categories changed significantly over time (P < 0.01), with 385 cows transitioning between categories. Older cows, regardless of the year, had higher odds of being in higher PVL categories (P < 0.01).
Significance:
In high-prevalence cow-calf herds, BLV ELISA status and PVL category were not linked to pregnancy, culling, or weaning weight. However, PVL levels varied within cows over time, with older cows showing higher PVL, suggesting age-related accumulation. These findings expand the knowledge on the limited information of EBL in beef cattle and support herd-level management and long-term monitoring over culling based on BLV status. Further research is needed to assess cumulative impacts and guide PVL-based control strategies.
