a barrier between species of animals that hinders or prevents the passage of a disease between these species. For TSEs to cross the species barrier it appears that a much larger dose of the infectious agent is required.
There appears to be needed much greater a quantity of infectivity to pass a TSE from one species to another rather to a species that is the same as the one from which the sample has come.
A term referring to the biological divides between different species, for example cows/goats/humans. These biological differences make the transference of viruses between different species difficult and unusual, although not impossible.