The fragment amplified in the PCR corresponds to the region that codes for the surface protein gp51, vital in the infectivity cycle of the virus since it is responsible for the recognition of the receptor and the binding of the virus to the host cell. Therefore, it is decisive to know the range of virus hosts (Gao et al. 2020). The primers selected to carry out the nested PCR of the env gene have been used in various diagnostic and genotyping studies of the virus since 1996 due to the low genetic variability of this region concerning other genome regions (Fechner et al. 1996; Polat et al. 2017).

The results obtained in this study for the 575 bovines belonging to the specialized dairy in Antioquia show that the prevalence of bovine leukemia virus in the department is 17.04% (Table 2), i.e., lower than the results obtained in previous studies (Corredor-Figueroa et al. 2020; Ortiz et al. 2016; Úsuga-Monroy et al. 2018c). This result may be due to the fact that this is the first extensive study carried out in the region where cattle of different racial components (Holstein, Jersey, Viking Red, Ayrshire, and their Jerhol crosses and trihybrids, among others), age groups (calves, heifers, and cows in production) and geographical regions of the department were sampled. Furthermore, this research contrasts with previous studies where only the population of one breed (Úsuga-Monroy et al. 2018c), a municipality, or an age group were sampled (Corredor-Figueroa et al. 2020; Ortiz et al. 2016).

Table 2

The low prevalence may also be the result of improved sanitary measures in specialized dairy herds in the department, the implementation of good livestock practices, and biosecurity protocols in the herds that prevent the transmission of the virus, such as the use of one needle per animal, effective as a control strategy for the BLV (Kuczewski et al. 2019). In Colombia, since 2012, the payment of raw milk to producers dependent on compositional and hygienic factors has been implemented, finding that this measure has been effective in stimulating progress in production systems (Carulla and Ortega 2016) and is directly related to the improvement of the sanitary scheme of the production systems and the general management of the animals in it. Nevertheless, the prevalence in herds was relatively high (71.7%) in this study, indicating that in more than half of the productive systems, there is at least one infected animal, existing the risk of an increase in the number of infected animals by contagion. Therefore, it is necessary to evaluate what factors can increase or decrease the risks of spreading the disease in the dairy systems of the department. It is well known that practices such as using shared needles for the application of medicines are one of the main risk factors for the infection of bovines in herds (Kuczewski et al. 2021; Ortiz et al. 2016). Moreover, it is essential to transmit information to producers, creating control strategies to contribute to the progress of this productive sector.

In the department of Antioquia, the main breeds found in specialized dairies are Holstein and Jersey. Crosses between them and with other reference breeds are also widely used when analyzing prevalence data according to racial components (Table 3). It can be inferred that this factor may be influencing the results obtained since the categories Holstein x (Cross) and other crossbreeds are associated with an increased risk of being infected with BLV. There is evidence that genetic factors can make animals susceptible or resistant to diseases. An example is the Colombian Blanco Orejinegro (BON) breed, where some studies have determined the resistance/susceptibility of these individuals to some viral infections such as foot-and-mouth disease and vesicular stomatitis (Lopez-Herrera et al. 2000, 2002).

Table 3

In the case of EBL, it has been found that the molecular prevalence of the virus is lower in BON compared to data from pure breeds (Herrera et al. 2011; Úsuga-Monroy et al. 2018b). The heritability of this susceptibility factor to the bovine leukemia virus infection among Holstein and Jersey cattle populations has also been estimated at 8%, indicating that genetics plays an important role in the incidence of BLV in different cattle breeds (Marawan et al. 2021). In order to analyze the breed factor, several categories were combined to determine whether or not the racial factor has an influence on BLV infection. These results can be a precedent for later studies where this hypothesis was evaluated in depth. It should be noted that the results obtained on the prevalence of BLV in the specialized dairy in Antioquia for the different racial components are not sufficient to make comparative statements between susceptibility/resistance to BLV infection due to the number of individuals for some breeds. However, to establish the association between the breed and the presence of BLV, the following groupings were made: Holstein x Cross, others purebreds and other crossbreeds and compared with the Holstein breed (Table 3). It was identified that the racial component (Holstein x Cross and other crossbreeds) does influence the probability of not having BLV infection (P<0.05) compared to the Holstein breed. Therefore, these results show a trend that generates the hypothesis that there may be breeds with greater resistance to infection by the virus in the Antioquia dairy, however, our data do not allow us to establish individually which breed or breeds are more resistant to BLV infection.

It has been found that in the bovine major histocompatibility complex (BoLA), with a fundamental role in the presentation of antigens and the immune response of the organism against infection, there is a genetic region (BoLA-DRB3) that influences the development of BLV infection (Lo et al. 2020; Notsu et al. 2021). Most cattle with specific mutations in the allele (DRB3*009:02) maintain undetectable proviral loads and are positively related to the development of lymphomas (Lo et al. 2020). Identifying these genetic variations associated with resistance to BLV infection helps develop control programs based on the genetic selection of animals with these genes (Andoh et al. 2021). Studies recommend the evaluation of this factor in BLV positive and negative bovines from dairies in Antioquia to determine if it is the determining factor for the low molecular prevalence in the population studied.

It has been found that the association between BoLA-DRB3 gene polymorphisms and BLV infection varies between breeds (Hernandez et al. 2018; Lo et al. 2021a), supporting the hypothesis that some breeds are more susceptible than others to infection with BLV. Moreover, environmental factors also relate to the susceptibility/resistance that cattle may have (Andoh et al. 2021). Therefore, more in-depth studies are needed to test this hypothesis as, so far, only animals heterozygous for BoLA-DRB3 have been shown to have advantages against BLV infection regardless of their breed (Hernandez et al. 2018; Lo et al. 2021b).

Another factor that can be decisive in the prevalence results of BLV in this study is the age of the sampled animals. Other studies have found that the number of infected animals increases as age increases (Murakami et al. 2011). Calves are less likely to be infected, as the main transmission route of the virus is iatrogenic in production systems (Ruiz et al. 2018). It is until after the first calving that prevalence rates increase due to increased exposure to the virus. Nevertheless, the presence of the virus has been found in calves because there is a risk of transmission during pregnancy and birth and later with milk consumption (Marawan et al. 2021; Ruiz et al. 2018). It should also be considered that the onset of the persistent lymphocytosis stage in infected cattle generally occurs before 5 years of age, and after that, between 5 and 8 years of age, it passes to the lymphoma stage (Zyrianova and Kovalchuk 2019). Therefore, many adult cattle individuals may be positive for viral infection and show positive serological tests but are negative for the molecular detection test by PCR since BLV-positive cells may be sequestered in lymphomas and not be circulating in the blood.

When detailing the seropositivity of the study samples by the number of calvings (Figure 1), it is observed that as the number of calvings increases, the prevalence rate also increases, reaching the highest percentage in the group of cows from four to six calvings (20.7%). Further, the positivity rate decreases in the interval of more than seven calvings, possibly due to the sequestration of BLV-positive cells in internal tissues. On the other hand, it is known that most of the cattle in specialized dairies throughout the country are discarded before reaching seven calvings due to the reduction in productive traits. It is then possible that the lower prevalence in this range of calvings is because the animals that continue in the system after seven calvings have good health conditions, and their productivity has been efficient throughout their lives. Knowing the effects of BLV infection on productive and reproductive characteristics, the possibility that these animals were not infected with BLV during their productive life and, therefore, have not been discarded should be considered.

Figure 1

Detailing the analysis of seropositivity in the three geographical areas where the study was carried out (Figure 2), the area with the highest prevalence was Valle de Aburrá, with 21.05% positivity to BLV. This area has a lower percentage of livestock production systems dedicated to dairy than the north region of Antioquia, in which the lowest molecular prevalence was found (15.65%).

Figure 2

The analysis of the results obtained in the 16 municipalities evaluated in three regions (Figure 3) shows that there is variability in these, finding values from 7.1% (Carmen de Viboral from the eastern region) to 61.5% (Girardota from the Valle de Aburrá region). Nonetheless, it should be noted that these values cannot be compared because the experimental design was based on the population of each region and not on the one at the municipality level. However, it is a result that must be considered when proposing future studies. In Valle de Aburrá, the municipality with the highest prevalence was Girardota (Figure 3A). Of the three municipalities evaluated in the region, Medellín has the lowest prevalence despite being the one with the largest bovine population included in the study for this region. The second is the east region (Figure 3B), with the municipality of Santuario exhibiting the highest prevalence (50%) with respect to the other four evaluated. Conversely, the municipality with the lowest BLV seropositivity, Carmen de Viboral (7.1%), is also found in this municipality. The north region (Figure 3C) was the one where a higher number of municipalities were evaluated (eight) since it is the one with the most specialized dairy, having most a low prevalence rate (between 7.7 and 33.3%) compared to the municipalities of other regions. On average, seropositivity for this region is 17%, which is the same value that the molecular prevalence of BLV had for the entire department. These results show a variation between the different geographical areas of the department, and future studies would be necessary to evaluate this and the possible reasons for these differences.

Figure 3

The bovine leukemia virus is a pathogen found in most of the specialized dairy systems of the department of Antioquia, generating economic losses to producers. Thus, it is necessary to implement measures to control and prevent this disease in the territory. The results of this study contribute to the creation of BLV control strategies in the specialized dairy of Antioquia since differences in molecular positivity to BLV were detected by the number of calvings (age of the cows), racial component, herd, region, and municipality. It is necessary to identify factors contributing to reducing the presence of the virus in the territory to optimize this agricultural sector of the economy in the department. Thus, future studies are necessary to identify in greater detail the behavior of this pathogen under the different conditions that exist in the dairy production systems of Antioquia, as well as the influence that the different racial components may have.