Diagnostic Measures of Disease Progression in Cattle Following Natural Infection with Bovine Leukemia Virus

Removing bovine leukemia virus infected animals with high proviral load leads to lowering within-herd prevalence and new case reduction

Bovine leukosis is prevalent in the North American dairy industry and its effect on animal health and production is widely documented. However, not all bovine leukemia virus (BLV) infected animals transmit the virus equally. Animals with high BLV proviral loads (HPL) are associated with higher transmission risks and therefore, their removal may reduce transmission and eventually within-herd prevalence. We aimed to evaluate the impact of selectively removing HPL cows on the within-herd BLV prevalence and incidence rate of BLV infection in 10 dairy herds. Annual blood and/or milk samples were collected from adult cows over 3 years. Positivity with BLV were determined by ELISA tests and proviral loads in blood of BLV-positive animals were estimated with BLV SS1 quantitative PCR assays. Herd managers were encouraged to consider the proviral load when making culling decisions and implement BLV control practices. High proviral load cows had the highest relative risk of removal indicating the farmers prioritized HPL cows for culling. The within-herd BLV prevalence decreased significantly in 4 herds whereas BLV incidence rate decreased in 9 herds. Over the 3 years, the proviral load demonstrated a relatively stable level, suggesting a single proviral load test in an adult cow may suffice to make culling decisions.

Effect of heat stress on TNF-α, TNFRI and TNFRII expression in BLV infected dairy cattle

High temperatures for extended periods, which do not allow animals to recover from heat stress, affect in particular those BLV-infected animals that carry a high proviral load. For this study, animals were discriminated between BLV (+) and BLV (-), and those belonging to the first group, were classified based on their proviral load. The expression of the inflammatory cytokine TNF-α and its receptors, which play an important role in disease progression, were quantified by qPCR in two different seasons. During the summer, average temperature was 19.8 °C, maximums higher than 30 °C were frequent. Instead, during the autumn, the average temperature was 12.63 °C, and temperatures never exceeded 27 °C. During this season, almost no periods of temperatures exceeded the comfort limit. Our results revealed that the expression levels of TNF-α and its receptors were downregulated in animals with high proviral load. This fact could affect their antiviral response and predispose to viral dissemination; over time, animals with a poorer immune system are prone to acquiring opportunistic diseases. Conversely, animals with LPL maintained their expression profile, with behavior comparable to non-infected animals. These findings should be considered by producers and researchers, given the problems that global warming is causing lately to the planet.

Relationship of persistent lymphocytosis, antibody titers, and proviral load with expression of interleukin-12, interferon-γ, interleukin-2, interleukin-4, interleukin-10, and transforming growth factor-β in cows infected with bovine leukemia virus from a high-prevalence dairy complex

Bovine leukemia virus (BLV) subclinical infection promotes persistent lymphocytosis (PL), which is related to susceptibility and progression to lymphoma. Moreover, lymphocyte counts directly correlate with BLV antibody titers and proviral load, and cell immune responses are considered atypical due to immune suppression. In order to determine the relationship of PL, antibody titers, and proviral load with interleukin (IL)-12, interferon (IFN)-γ, IL-2, IL-4, IL-10, and transforming growth factor (TGF)-β expression in a 3-month interval, 58 cows were selected (30 BLV⁺ and 28 BLV^-) from a high-prevalence dairy herd to complete 3 monthly blood samplings for the assessment of PL, BLV antibody titers, BLV proviral load, and IL-12, IFN-γ, IL-2, IL-4, IL-10, and TGF-β expression. At sampling conclusion, the BLV-infected cows were grouped according to PL, BLV proviral load, and BLV antibody titers as follows: BLV⁺PL⁺ (n = 16) and BLV⁺PL^- (n = 14); high proviral load (HPL) (n = 18) and low proviral load (LPL) (n = 13); high antibody titers (HAT) (n = 17) and low antibody titers (LAT) (n = 14). The BLV⁺PL⁺ cows showed significantly higher proviral load and antibody titers than the BLV⁺PL^- group; however, the former suggested spread presumably unrelated to lymphoma outcome, because HPL was observed in PL^- cows in the last sampling. Consistent with the data, a higher antibody response strongly indicated BLV susceptibility since it was linked to PL⁺ occurrence and a cytokine profile compatible with immune suppression. Furthermore, a reversion to lower antibody titers was observed in cows with HPL far ahead of time, most likely due to long-term immune suppression. In addition, high expression of IL-10 and TGF-β was associated with reduced IL-12, IFN-γ, IL-2, and IL-4 expression alongside PL, HAT, and HPL in BLV-infected cows, suggesting an IL-10- and TGF-β-induced immune suppression. The IL-10 expression was increasing throughout, implying disease progression, as described. In conclusion, the proliferative expansion of lymphocytes known as PL might enhance a regulatory-rich cell population (Bregs and/or Tregs) that secretes IL-10 and TGF-β, leading to immune suppression. Further studies must be conducted regarding the types of regulatory cells involved in BLV-induced immune suppression.

Bovine Leukaemia Virus: Current Epidemiological Circumstance and Future Prospective

Bovine leukaemia virus (BLV) is a deltaretrovirus that is closely related to human T-cell leukaemia virus types 1 and 2 (HTLV-1 and -2). It causes enzootic bovine leukosis (EBL), which is the most important neoplastic disease in cattle. Most BLV-infected cattle are asymptomatic, which potentiates extremely high shedding rates of the virus in many cattle populations. Approximately 30% of them show persistent lymphocytosis that has various clinical outcomes; only a small proportion of animals (less than 5%) exhibit signs of EBL. BLV causes major economic losses in the cattle industry, especially in dairy farms. Direct costs are due to a decrease in animal productivity and in cow longevity; indirect costs are caused by restrictions that are placed on the import of animals and animal products from infected areas. Most European regions have implemented an efficient eradication programme, yet BLV prevalence remains high worldwide. Control of the disease is not feasible because there is no effective vaccine against it. Therefore, detection and early diagnosis of the disease are essential in order to diminish its spreading and the economic losses it causes. This review comprises an overview of bovine leukosis, which highlights the epidemiology of the disease, diagnostic tests that are used and effective control strategies.