The association between bovine leukemia virus infection and diseases in lactating dairy cows

By Caio Figueiredo, Veterinary Medicine Extension

Enzootic bovine leukosis is a cattle disease induced by the bovine leukemia virus (BLV), a retrovirus that affects the lymphoid tissue. Although only 1-5 % of infected cattle develop malignant lymphoma (USDA-APHIS, 2008), bovine leukosis is one of the most prevalent diseases in dairy cows. Multi-state studies reported that 89% of U.S. dairy herds had cattle seropositive for BLV (Ott et al., 2003) and 84% of dairy herds in the U.S. were positive for BLV when bulk tank milk was analyzed (USDA-APHIS, 2008; Table 1). Similar numbers were observed in Canada, as 91% of dairy herds were positive for BLV (using bulk tank milk; Nekouei et al., 2015). In 2003, BLV was associated with $525 million/year in economic losses to the U.S. dairy industry (mostly due to losses in milk production; Ott et al., 2003), which would represent approximately $900 million/year in economic losses in 2024. In addition to reduction of milk production, BLV has been associated with increased culling (Erskine et al., 2012; Bartlett et al., 2013), and increased risk of mastitis (Nakada et al., 2023) and metritis (Hernandez et al., 2024). For instance, BLV-positive heifers had greater odds of developing metritis in the first lactation compared with BLV-negative heifers (Odds Ratio = 1.31; Hernandez et al., 2024). In addition, greater hazard of culling was observed in BLV-positive heifers and cows compared with BLV-negative animals (Hazard Ratio = 1.33). An association between subclinical mastitis occurrence and proviral load of BLV was recently reported (Nakada et al., 2023). Cows with high-proviral loads of BLV had greater odds of developing subclinical mastitis compared with non-infected cows (Odds Ratio = 2.61), whereas no differences were observed between cows with low-proviral loads and non-infected cows.

Although the mechanisms that underlie disease occurrence in BLV-positive cattle are not fully elucidated, studies have described the detrimental impacts of BLV on immune cell function which may lead to immunological disfunction. Studies have shown that BLV infection was associated with reduced migration capacity and respiratory burst functions in polymorphonuclear cells (PMN; Lv et al., 2021). In addition, an interaction between BLV viral load and the degree of effects in PMN was observed, as PMN exposed to a high-proviral load produced large amounts of neutrophil nets, chemokine CXCL7, adhesion molecule CD18, and pro-inflammatory factors IL-8 and TNF-α, triggering inflammatory responses and tissue damage. Aside from detrimental effects on the innate immune response, BLV was also associated with detrimental effects on the adaptative immune response. Studies showed that infected cells had altered expression of interferon-I signal pathways and genes involved in defense response to virus, highlighting a degree of immune system deregulation (Cuesta et al., 2020). Considering how detrimental BLV infections in dairy herds can be, it is important to be aware of its risk factors. A recent study reported that factors such as milking frequency, frequency of needle change, bedding composition, and herd size were associated with higher prevalence of BLV (Hutchinson et al., 2020).

Percentage of Operations in Which Bulk Tank Milk Tested Positive for BLV via ELISA