Bovine leukemia virus (BLV) is an exogenous retrovirus and one of the most common infectious viruses of cattle with a worldwide distribution. The bovine target cells are mainly B-lymphocytes having the CD5 marker, however, recently it was found that udder epithelial cells might be infected as well. BLV is the casual agent of enzootic bovine leukemia. BLV infection is slow, life persistent and if lymphomas appear clinically (in less than 1% of the BLV infected cows) they do so late in the animal s life. BLV infection results in persistent lymphocytosis (PL) in about 30% of infected cattle. The immune response of infected animals is impaired, mainly the B-cell response. This impairment is more pronounced in PL animals. The effect of this immune disorder, on the productivity of BLV infected cattle is extensively discussed here. Despite some inconsistencies in some of the studies, the following pathological events can be drawn: most of the BLV infected cows exhibit a reduced economic value that is expressed by higher susceptibility to infectious diseases, decreased productivity, lower reproductivity and higher culling rat. The study was performed by an indirect ELISA technique to determine the presen of antibody against BLV in bulk tank milk of dairy cattle herds of Mashhad suburb.

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