Bovine leukemia virus (BLV) is an exogenous c-type oncovirus in the retroviridae family that causes a chronic infection in cattle, which develops in three possible pathological forms: asymptomatic course, persistent lymphocytosis and lymphosarcoma. Once infected, cattle remain virus carriers for life and start to show a serological reaction within a few weeks after infection. This disease causes significant economic losses associated with the costs of control and eradication, loss in milk production and difficulties in exports. The Agar gel immuno diffusion (AGID) and ELISA tests have been the serological choice assays for routine diagnosis of serum samples. However, due to the higher sensitivity of ELISA, in recent years this method has replaced the AGID in seroepidemiological studies for control and eradication programe. Prevalence studies on BLV infection has not been conducted in the eastern part of Iran. This study was designed to determine the prevalence of BLV in Khorasan Razavi and Khorasan Shomali provinces which are the main provinces located in the north east of Iran. During summer 2007, 429 blood samples were randomly taken from industrial dairy herds of these two provinces and subjected to ELISA test (indirect sandwich ELISA). 25.41% of these samples showed positive reaction. The positive results in khorasane razavi and khorasan shomali provinces were 29.83% and 1.5% , respectively. Moreover, 64.7% of the herds tested, had at least one positive cow. The overall true prevalence was 24.63%. Statistical analysis revealed a direct correlation between age and occurence of BLV. Significant correlation was also found between herd size and BLV infection. In another words, the large herds had the most BLV positive cases. Moreover, number of parity was also shown to have a direct correlation with BLV seropositivity. Results of Odd ratios analysis which is an indication of possible risk factors, confirmed the above findings. Taken together, the results presented in this study along with simillar studies in recent years in Iran confirm an alarming increase in prevalence of BLV infection among dairy herds. Therefore, control strategies based on test and implementation of corrective management should be taken in order to downgrade and possibly eliminate this economically hardship disease.

Keywords