Seroprevalence and risk factors associated with bovine Leukemia virus infection in argentine beef cattle.

Bovine leukemia virus (BLV) is the causative agent of enzootic bovine leukosis, an endemic disease in dairy cattle of Argentina. However, little is known about the seroprevalence of BLV in beef cattle. In this study, we conducted a cross-sectional study including farms from thirteen provinces of Argentina. A total of 5827 bovine serum samples were collected from 76 farms and analyzed using an in-house developed enzyme-linked immunosorbent assay. Information about herd management was collected through a questionnaire, and univariate and multivariate analyses were performed to detect risk factors associated with BLV infection. Herd-level seroprevalence was 71.05%, while the mean animal-level seroprevalence was 7.23% (median = 2.69%; min = 0, max = 75). Only two provinces had no positive BLV samples. The other eleven provinces showed more than 50% of their farms infected with BLV. The multivariate model revealed that BLV prevalence was significantly associated with the use of animals raised in the same farm for cattle replacement (P = 0.005), breeding cows by natural mating with a bull (P < 0.001), and weaning calves after 6 months of age (P = 0.011). This extensive study revealed that BLV seroprevalence in Argentine beef farms has increased during the last years and allowed identifying some management practices associated with BLV prevalence. These data deserve special attention because BLV infection in beef cattle seems to lead to a dissemination pattern similar to that observed during the last decades in dairy cattle, especially considering that Argentina is the sixth beef producer in the world, with about 5% of global beef production.

Efficacy of the spray-drying treatment to inactivate the bovine leukemia virus in bovine colostrum.

Previous studies have shown the presence of bovine leukemia virus (BLV) in colostrum and milk of naturally infected cows. The relationship between virus or provirus and specific antibodies in these secretions is particular to each infected cow and will probably determine whether the consumption of colostrum or milk from these naturally infected dams provides an infective or a protective effect in recipient calves. Our recent findings suggest that this issue is a key point in BLV transmission in very young calves. Based on this, the aim of the present study was to determine the effect of the spray-drying treatment of colostrum on BLV infectivity. The treatment was done on scale-down conditions, using fresh colostrum from BLV-negative cows spiked with infective BLV. Residual infectivity was tested in susceptible lambs. Lambs inoculated with colostrum spiked with BLV-infected cells or cell-free BLV showed evidence of infection 60 d after inoculation, whereas none of the lambs inoculated with spray-dried colostrum showed evidence of infection 60 d after inoculation. These results provide direct evidence that the experimental spray-drying process used in this study was effective in inactivating infectious BLV in colostrum. These findings suggest that the risk for BLV transmission could be reduced if milk and colostrum were treated by spray-drying prior to consumption in dairy facilities. The effect of spray-drying on the functional properties and stability of the antibodies present in colostrum under long-term storage should be further investigated.

Infección experimental en ovinos con el virus de leucosis bovina: dosis mínima de células BLV-FLK y virus libre de células, y actividad neutralizante de anticuerpos naturales / Experimental infection of sheep with Bovine leukemia virus (BLV): Minimum dose of BLV-FLK cells and cell-free BLV and neutralization activity of natural antibodies

Abstract Bovine leukemia virus (BLV) is an important cattle pathogen that causes major economic losses worldwide, especially in dairy farms. The use of animal models provides valuable insight into the pathogenesis of viral infections. Experimental infections of sheep have been conducted using blood from BLV-infected cattle, infectious BLV molecular clones or tumor-derived cells. The Fetal Lamb Kidney cell line, persistently infected with BLV (FLK-BLV), is one of the most commonly used long-term culture available for the permanent production of virus. FLK-BLV cells or the viral particles obtained from the cell-free culture supernatant could be used as a source of provirus or virus to experimentally infect sheep. In this report, we aimed to determine the minimum amount of FLK-BLV cells or cell-free supernatant containing BLV needed to produce infection in sheep. We also evaluated the amount of antibodies obtained from a naturally-infected cow required to neutralize this infection. We observed that both sheep experimentally inoculated with 5000 FLK-BLV cells became infected, as well as one of the sheep receiving 500 FLK-BLV cells. None of the animals inoculated with 50 FLK-BLV cells showed evidence of infection. The cell-free FLK-BLV supernatant proved to be infective in sheep up to a 1:1000 dilution. Specific BLV antibodies showed neutralizing activity as none of the sheep became infected. Conversely, the animals receiving a BLV-negative serum showed signs of BLV infection. These results contribute to the optimization of a sheep bioassay which could be useful to further characterize BLV infection.

Resumen El virus de la leucosis bovina (bovine leukemia virus [BLV]) es un importante agente patógeno del ganado que causa importantes pérdidas económicas en todo el mundo, especialmente en los rodeos lecheros. El uso de modelos animales proporciona información valiosa sobre la patogénesis de las infecciones virales. Se realizaron infecciones experimentales en ovejas usando sangre de bovinos infectados con BLV, clones moleculares de BLV infecciosos o células derivadas de tumores. La línea celular Fetal Lamb Kidney, persistentemente infectada con el BLV (FLK-BLV), es uno de los cultivos a largo plazo más utilizados para la producción permanente de virus. Las células FLK-BLV o las partículas virales obtenidas del sobrenadante del cultivo libre de células podrían usarse como fuente de provirus o de virus para infectar experimentalmente ovejas. En este trabajo, nuestro objetivo fue determinar la cantidad mínima de células FLK-BLV o de sobrenadante libre de células que contiene BLV necesaria para producir infección en ovejas. También evaluamos la cantidad de anticuerpos bovinos anti-BLV necesaria para neutralizar la infección. Observamos que las dos ovejas inoculadas experimentalmente con 5000 células FLK-BLV se infectaron, y que una de las dos ovejas que recibieron 500 células FLK-BLV se infectó. Ninguno de los animales inoculados con 50 células FLK-BLV mostró evidencia de infección. El sobrenadante FLK-BLV libre de células demostró ser infectivo en ovejas hasta la dilución 1:1000. Los anticuerpos BLV específicos mostraron actividad neutralizante, ya que ninguna de las ovejas se infectó. Por el contrario, los animales que recibieron un suero BLV negativo mostraron signos de infección por BLV. Estos resultados contribuyen a la optimización de un bioensayo en ovejas útil para caracterizar la infección por BLV.

Correction: Alvarez, I., et al. Detection of Bovine Leukemia Virus RNA in Blood Samples of Naturally Infected Dairy Cattle. Vet. Sci. 2019, 6, 66.

The authors wish to make the following corrections to this paper [...].

Detection of Bovine Leukemia Virus RNA in Blood Samples of Naturally Infected Dairy Cattle.

The viral expression in vivo, in bovine leukemia virus (BLV)-infected cattle, is considered to be restricted to extremely low levels, and the mitosis of infected B lymphocytes is regarded as the main mode of virus persistence within the infected host. In this study, the presence of BLV RNA in whole blood from seven asymptomatic cows naturally infected with BLV during one year, including a complete milking cycle and two delivery time points, was investigated by nested-PCR using the oligonucleotides complementary to the tax and pol gene. BLV RNA was detected in four cows at different time points, especially in high blood proviral load cows and around delivery time. This study describes for the first time the detection of free BLV RNA in blood from BLV-infected asymptomatic cows. The results obtained suggest the occurrence of persistent low-level expression of the tax and pol genes that could be a result of viral reactivation, within the asymptomatic period. This finding may be important in the pathogenesis of BLV infection, associated with the delivery period.

Experimental infection of sheep with Bovine leukemia virus (BLV): Minimum dose of BLV-FLK cells and cell-free BLV and neutralization activity of natural antibodies.

Bovine leukemia virus (BLV) is an important cattle pathogen that causes major economic losses worldwide, especially in dairy farms. The use of animal models provides valuable insight into the pathogenesis of viral infections. Experimental infections of sheep have been conducted using blood from BLV-infected cattle, infectious BLV molecular clones or tumor-derived cells. The Fetal Lamb Kidney cell line, persistently infected with BLV (FLK-BLV), is one of the most commonly used long-term culture available for the permanent production of virus. FLK-BLV cells or the viral particles obtained from the cell-free culture supernatant could be used as a source of provirus or virus to experimentally infect sheep. In this report, we aimed to determine the minimum amount of FLK-BLV cells or cell-free supernatant containing BLV needed to produce infection in sheep. We also evaluated the amount of antibodies obtained from a naturally-infected cow required to neutralize this infection. We observed that both sheep experimentally inoculated with 5000 FLK-BLV cells became infected, as well as one of the sheep receiving 500 FLK-BLV cells. None of the animals inoculated with 50 FLK-BLV cells showed evidence of infection. The cell-free FLK-BLV supernatant proved to be infective in sheep up to a 1:1000 dilution. Specific BLV antibodies showed neutralizing activity as none of the sheep became infected. Conversely, the animals receiving a BLV-negative serum showed signs of BLV infection. These results contribute to the optimization of a sheep bioassay which could be useful to further characterize BLV infection.

Bovine Leukemia Virus Infection in Neonatal Calves. Risk Factors and Control Measures.

Bovine leukemia virus (BLV) is the causative agent of enzootic bovine leukosis (EBL). Although efficient eradication programs have been successfully implemented in most European countries and Oceania, BLV infection rates are still high worldwide. BLV naturally infects cattle, inducing a persistent infection with diverse clinical outcomes. The virus infects lymphocytes and integrates a DNA intermediate as a provirus into the genome of the cells. Therefore, exposure to biological fluids contaminated with infected lymphocytes potentially spreads the virus. Vertical transmission may occur in utero or during delivery, and about 10% of calves born to BLV-infected dams are already infected at birth. Most frequently, transmission from dams to their offspring occurs through the ingestion of infected colostrum or milk. Therefore, although EBL is not a disease specific to the neonatal period, during this period the calves are at special risk of becoming infected, especially in dairy farms, where they ingest colostrum and/or raw milk either naturally or artificially. Calves infected during the first week of life could play an active role in early propagation of BLV to susceptible animals. This review discusses the main factors that contribute to neonatal BLV infection in dairy herds, as well as different approaches and management practices that could be implemented to reduce the risk of BLV transmission during this period, aiming to decrease BLV infection in dairy herds.