How do PrPSc Prions Spread between Host Species, and within Hosts?

Prion diseases are sub-acute neurodegenerative diseases that affect humans and some
domestic and free-ranging animals. Infectious prion agents are considered to comprise solely of
abnormally folded isoforms of the cellular prion protein known as PrPSc. Pathology during prion
disease is restricted to the central nervous system where it causes extensive neurodegeneration and
ultimately leads to the death of the host. The first half of this review provides a thorough account
of our understanding of the various ways in which PrPSc prions may spread between individuals
within a population, both horizontally and vertically. Many natural prion diseases are acquired
peripherally, such as by oral exposure, lesions to skin or mucous membranes, and possibly also
via the nasal cavity. Following peripheral exposure, some prions accumulate to high levels within
the secondary lymphoid organs as they make their journey from the site of infection to the brain,
a process termed neuroinvasion. The replication of PrPSc prions within secondary lymphoid organs
is important for their efficient spread to the brain. The second half of this review describes the key
tissues, cells and molecules which are involved in the propagation of PrPSc prions from peripheral
sites of exposure (such as the lumen of the intestine) to the brain. This section also considers how
additional factors such as inflammation and aging might influence prion disease susceptibility.