Another inherited human prion disease is so-called Fatal Familial Insomnia. There is some evidence that this results from a deficiency of the normal prion protein, a deficiency that occurs because mutant prions are unable to fulfil their normal functions. In addition to causing disease through inherited genetic mutations, mutant prions are capable of turning into 'rogue' disease agents. Transmitted from an infected animal or human to a new host, they convert any 'normal' prions they encounter into copies of themselves. The transmission of disease depends on the rogue prion being similar enough to the host prion to be able to 'lock in' to its structure and convert it.
Transmission works best between animals of the same species.
For example, a prion disease called 'kuru', found in the Fore tribe of New Guinea, is spread by ritual cannibalism, in which mourners eat the brains of their dead relatives. This practice has since ceased. But there is some evidence that rogue prions can jump the barrier between species, provided that the prions of donor and host are similar enough for the conversion process to occur. Cattle and sheep are extremely close evolutionary relatives. They belong to the family Bovidae, and share a common ancestor that lived probably no more than 20 million years ago.