Bovine leukemia virus: a perspective insight into the infection and immunity

Abstract

Bovine leukemia virus (BLV) is a member of the Retroviridae family and belongs to the Deltaretrovirus genus. It has a close relationship with human T-cell leukemia virus type I. BLV is responsible for causing enzootic bovine leukosis (EBL), a contagious disease that affects the bovine lymphatic system. This virus poses challenges for the global cattle industry, as it impacts cattle populations all over the world. Despite being widespread and impactful, BLV often goes unnoticed, with many researchers unaware of its presence and the potential consequences it carries. BLV demonstrates varying levels of pathogenicity. The majority of cattle (around 70%) become seropositive asymptomatic carriers, displaying no noticeable clinical symptoms. However, a smaller proportion of infected animals experience persistent lymphocytosis, characterized by an elevated number of lymphocytes in the bloodstream. If not monitored and managed, a subset of these persistently infected cattle may advance to lymphosarcoma. This condition typically presents as tumors in different lymphoid tissues, impacting various organs and overall health and productivity. Furthermore, recent research has highlighted the potential association between the occurrence of breast and lung cancer in humans and the presence of BLV. This review will delve into the recent discoveries concerning BLV, specifically exploring its epidemiology, the economic impact it has on the global cattle industry, its implications for human medicine, and the association between different alleles of the major histocompatibility complex (MHC) and susceptibility or resistance to BLV. Bovine leukemia virus, Enzootic bovine leukosis, Major histocompatibility complex, Retroviridae.

Fig. 1

The structure and genomic composition of BLV. As mentioned in the text, the BLV genome is made up of three main regions: long terminal repeats, the structural and enzyme coding region (including four significant loci known as gag, pro, pol, and env), and the pX region

Fig. 2

The main ways of BLV transmission within farms, and the resulting economic losses and health effects. The main factors that contribute to the transmission of BLV within a herd include cows being exposed to contaminated blood, insect bites, and the consumption of milk and colostrum from infected cows. A major challenge in controlling the spread of BLV is that many infected animals do not show noticeable clinical symptoms, allowing for continuous virus transmission within the herd. The economic losses resulting from the spread of BLV include decreased milk production in the herds, increased mortality of young cows, and additional expenses for treating and replacing culled animals. Furthermore, numerous studies have examined the implications of BLV outbreaks on human health. It has been noted that BLV can be identified as a risk factor for breast and lung cancer. Moreover, since BLV proviral genes have been detected in human blood cells, there might be a potential connection between this virus and other blood-related diseases, which should be investigated seropositive (Heinecke et al., 2017). Many European countries, such as England, Belgium, France, Spain, and Sweden, have succeeded in becoming officially free of BLV by implementing strict control programs. However, there are still reports of this disease in some other countries, such as Italy and Portugal (Bartlett et al., 2014).

Fig. 3

Association between MHC alleles and susceptibility/resistance to BLV. Various studies have revealed that certain MHC alleles, particularly those found in the second exon of the DRB3 gene of MHC class 2, exhibit a noteworthy association with the occurrence of different BLV profiles in cattle. For instance, Nikbakht Brujeni et al. (2016) demonstrated that the BoLA-DRB3.2 ^*0101 allele is associated with susceptibility to the persistent lymphocytosis profile while conferring resistance to the lymphosarcoma form. Other numbers and alleles shown in the figure are also real and drawn based on the mentioned article