frequency of bovine enzootic leukosis in cull reports of selected iranian dairy farms

Bovine enzootic leukosis causes economic losses through early culling, reduced milk production, infertility, increased production prices and other hidden losses in dairy farms. the virus is transmitted through blood lymphocytes and spreads slowly and progressively in herds. due to the suppressive impact of the virus on the immune system, affected animals become susceptible to other diseases and may not respond properly to vaccination. in advanced stages, the disease causes the formation of multiple lymphosarcoma, which is pathognomonic and may lead to culling or death of the cow. post-slaughter examinations lead to the condemnation of the affected carcasses. in a 5-year study on 25,267 cases of culled cows during 2018 to 2022 in 32 iranian dairy herds, 738 cases of leukosis were reported, mainly based on pathognomonic symptoms and tracking of the cows in the slaughterhouse. this equals to 2.9% of culled cows. in addition, 553 cases were culled due to tumors and internal abscesses in uterus, lungs, liver and other organs, which potentially could be due to leukosis. thus, the actual culling rate due to leukosis may be higher (up to 5.1% of all culls). annual culling rates of 2 to 5% of productive cows due to leukosis have been reported. leukosis, along with the other infectious diseases of cattle, may have a cumulative effect on cull rate and increase it above the tolerable levels (different in farms). it seems that the diagnosis of leukosis through clinical symptoms, ultrasonography and tracking of the culled cows has received more attention among veterinarians. considering the economic importance, long-term incubation period, lack of commercial vaccines and creation of a persistent infection, an effective preventive program of the disease should include identifying and culling of animals that have persistent lymphocytosis. in practice, not sharing the tools and equipment that come into direct contact with blood (needles, syringes, surgical instruments, gloves, etc.) or disinfecting them after use are the most applicable critical control points in controlling the spread of the virus.