Less than 20 per cent of the calves born to EBL positive dams are themselves born infected with the virus. Those calves born infected are a result of the virus crossing the placental barrier and infecting the developing fetus. Research has indicated that neither the egg nor the sperm become infected. Although the disease has not been spread using artificial insemination, there are reports of females becoming infected after natural matings with infected bulls. The spread of EBL virus to susceptible animals requires close contact with infected animals.
Blood is the most common source of infection.
Infected lymphocytes in blood can be transmitted by biting insects such as horse flies, which act as a mechanical vector. Blood contaminated needles, dehorners, and ear tagging and tattoo equipment also transmit the infection. A report in 1983 described a Saskatchewan dairy farm in which 83 per cent of the lactating herd was infected with EBL virus. E. As well, he included an intravenous injection of oxytocin in his milking routine using a tuberculin syringe from which multiple doses had been given. Placing infected blood on plastic sleeves has been shown experimentally to transmit the virus to susceptible animals during rectal palpation.