Transgenic animals are produced which animals have (1) their endogenous PrP gene ablated; and (2) have an exogenous PrP gene from a genetically diverse animal. The transgenic animal is preferably a mouse, rat or hamster with mice being particularly preferred. The exogenous PrP gene is preferably from a sheep, cow, or pig with cow PrP genes being particularly preferred. When a mouse of the invention is inoculated with a sample containing prions which generally only infects a genetically diverse species (e.g. a cow) the mouse will become ill within about 250 days or less. Methods of producing the transgenic animals are disclosed including (1) microinjecting a mouse egg (having an ablated endogenous PrP gene) with a bovine PrP gene, or (2) breeding a mouse with an ablated PrP gene with a mouse with a bovine PrP gene. Mice produced are used to test samples for the presence of prions which generally only infect cows.
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