The mice infected other mice, satisfying one of the Koch postulates on the causation of disease. Independent prion scientist Tom Pringle likened the situation to Typhoid Mary, an English woman who spread the disease even though she didn't have symptoms. Dutch veterinary pathologist Lucien van Keulen used an immune-marking technique to map scrapie infection in sheep. After five months, prions accumulated in lymph tissue and nerve fibers in the small intestine. After 10 months, nerves serving other organs were affected.
Prions can change for the worse over time.
Aiken, for example, passed a mink TSE, identified by the late University of Wisconsin-Madison virologist Richard Marsh, through hamsters, and found that the agent acted faster in successive generations. The first hamsters to receive the mink TSE generally outlived the disease, which took hundreds of days to incubate. When brain material from those hamsters was injected into other hamsters, the disease struck faster. Because, as Marsh had suggested years ago, two strains of prions existed in the first passage, with the long-incubation prion predominating. The prospects for treating TSE seem to be improving.