Contaminated food and meat products. Semen Third party mechanical transfer of virus between two susceptible animals. People, vehicles, tools, equipment moving from a diseased to a clean situation can carry the virus to the clean herd. Sheep are not huge shedders of the virus. Infected pigs excrete huge volumes of the virus from 100,000 to one billion virus particles per hour.
Cattle are in between the two in infectivity.
Spreading infected slurry can lead to airborne spread. Venting on milk tankers in transit. Nostrils of humans for up to five days. One human can spread an infective dose to another. There are very sound reasons as to why the feet and vehicles wheels need special attention.