Prion diseases arise in three different ways

1. Through horizontal transmission from e.g. a sheep to a cow (BSE).

2. In inherited forms, mutations in the prion gene are transmitted from parent to child.

3. They can arise spontaneously.

Route of infection

When cows are fed with offals prepared from infected sheep, prions are taken up from the gut and transported along nerve fibers to the brain stem. Here prions accumulate and convert normal prion proteins to the disease-causing form, PrPSc. Years later, BSE results when a sufficient number of nerve cells have become damaged, affecting the behaviour of the cows.

The kuru epidemic started a long time ago and has subsided since the 1950s. The mad cow disease epidemic started in 1985 affecting approximately 170,000 cows.

Kuru disease Mad cow disease

The graph shows how kuru disappeared among the Fore-people when they stopped their cannibalistic rituals. In children the disease disappeared more rapidly, while adults continued to be affected. An incubation time of up to 30 years has been reported. The graph shows the course of the BSE epidemic. It reached its maximum in 1992 with 37,000 affected animals. When feedstuff containing sheep offals was banned, the incidence of BSE decreased rapidly. There is now a fear that humans might have been infected by eating products from BSE-affected cows giving rise to a new variant type of CJD.

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