Natural Association Between Bovine Leukemia Virus and Reproductive Infectious Diseases

Abstract

Background: Enzootic bovine leukosis (EBL) is a widespread infectious disease caused by the bovine leukemia virus (BLV), which results in immune system dysfunction. The resulting immunosuppression may lead to an increased prevalence of other diseases. Dairy cows infected have altered immune function associated with decreased milk production and shortened lifespan and decreased immune response to immunization. BLV infection, however, is often asymptomatic, so any connection between subclinical infection and common reproductive diseases remains unknown. This study aimed to
describe the relationship between naturally occurring subclinical BLV and infectious reproductive diseases seroconversion in the field.
Materials, Methods & Results: The diseases investigated included Bovine viral diarrhoea (BVD), Bovine alfaherpesvirus 1 (BoHV-1), Bovine gammaherpesvirus 4 (BoHV-4), Chlamydiosis, Leptospirosis, Brucellosis and Neosporosis in dairy cattle. Six hundred fifty-five sera samples from the northern and south-central regions of Uruguay, from asymptomatic female Holstein and Holstein crosses without a history of vaccination against reproductive diseases were processed using reference diagnostic methods (Seronautalization, ELISA, MAT, Rose Bengal Plate test). The seroprevalence of BLV was 20.0%. Seroprevalence of reproductive diseases BVD, BoHV-1 and BoHV-4 were 99.3%, 41.2% and 27.3% of the populations, respectively, and the total seroprevalence of Leptospirosis, bovine Neospora caninum and Chlamydiosis
were 19.8%, 29.8% and 33.0% respectively. The results revealed positive associations between naturally contracted BLV and the presence of antibodies against BoHV-1 (P = 0.002), as well as between naturally contracted BLV and presence of
antibodies against Leptospira spp. (P = 0.028).
Discussion: BLV infection can impact innate and adaptive immune system cells and alter the proper functioning of uninfected cells. BLV infection may also induce changes in the complex balance of cytokine expression, cell proliferation,
and programmed cell death in T- and B-lymphocytes, which is critical for immune competence and effective response to
infectious challenges. The progression of BLV infection has a substantial effect on host defense mechanisms. Indeed, lowmagnitude serologic responses to a commercial foot-and-mouth disease vaccine and a J5 Escherichia coli vaccine have been observed. These results are supported by recent trial studies showing a reduced immune response to vaccination against BoHV-1 and Leptospira spp. in asymptomatic animals infected with BLV. These are 2 of the most prevalent infectious reproductive diseases in cattle worldwide, and our results provide evidence that a link between BLV and susceptibility to these diseases may exist. Although there is evidence of the co-occurrence of these diseases, it remains unknown whether there is a direct or indirect effect of BLV on infertility, embryonic loss, or abortion. Another possibility is that natural infection with these reproductive pathogens (BoHV-1, Leptospira, or others) promotes BLV expression, negatively affecting the farms where these pathogens are endemic. Considering the high seroprevalence of BLV in dairy herds in North and South America where the infection is endemic, it was explored BLV’s role as an immunosuppressant by quantifying its co-occurrence with diseases that affect reproductive performance in breeding herds. Future work should clarify the role of BLV and the co-occurring pathogens in causing infertility or abortions.

Keywords: BLV, herpesvirus, Infectious Bovine Rhinotracheitis (IBR), Leptospirosis, reproductive performance.