A strong association between outbreaks in cattle and recent exposure to sheep has been documented repeatedly since 1929. Recent data indicate that a similar pattern occurs in bison. In some outbreaks, however, no sheep were in the vicinity immediately prior to the first case being identified. A. Lambs are infected after birth. Infection occurs between birth and 4 months of age, with most lambs infected by the age of 4 months or earlier.
Placental transmission is possible but rare.
Horizontal transmission through physical contact with flock mates is the major mode of transmission among sheep. The virus does not cause illness in sheep. Cattle, bison, and deer are also infected by horizontal transmission from sheep, apparently through transfer of virus via nasal secretions. There is no evidence that transmission occurs horizontally from one bison to another. There is no direct evidence that bison become infected following contact with birds or insects.