An important early finding in the U.K. was that feeding ruminant-derived MBM to cattle as a protein supplement was the mechanism by which BSE was transmitted among cattle.2 Early BSE epidemiology studies also noted that a majority of cattle became infected with the BSE agent as young calves.2,3
Other epidemiological studies have found dairy cattle are significantly more likely than beef cattle to be infected with BSE4 because dairy cattle were more likely to be exposed to ruminant-derived protein supplements. Beef cattle in general received far less supplementary feed than did dairy calves or mature dairy cows. BSE statistics from Great Britain show that 81 percent of BSE cases were diagnosed in dairy cows and 12 percent of cases were diagnosed in beef cattle.
The practice of including MBM in the diet of calves for most of the first 12 weeks of their lives began in the mid-1970s in the U.K., but does not appear to have been common in continental Europe and the United States. This practice raises the question of whether there is a sensitive period of susceptibility and young calves are more susceptible than adult cattle to the infective agent.
The U.K. banned the use of ruminant-derived protein supplements in all cattle (and sheep) feed in 1988.
Maternal transmission has been proposed as a possible route of transmission but, if it exists, cannot account for more than 10 percent of offspring of all BSE cases and probably less if transmission occurs only when the dam is in the late stages of incubation1. Recent epidemiological research indicates maternal transmission likely does not exist.5
It should be noted there is no evidence of transmission of BSE through milk. Bovine milk is considered safe (not only for calves but also for humans). A number of studies have not shown any evidence of infectivity in bovine milk or any presence of prions in milk.