The sporadic nature of the disease and relatively low losses attributed to MCF in cattle made it a low priority for research funding agencies. A. Reports of an MCF-like disease in cattle in North America appeared as early as the 1920s. The first well-documented case of MCF in bison occurred in South Dakota 1973 and was reported in the scientific literature in 1977. A. Epidemiological studies of field outbreaks strongly suggests that sheep infected with OHV-2 are the principal source of MCF outbreaks in bison and cattle. 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.