Fig. 1.

Schematic diagram of the field site and buried TSE infectivity. The completely enclosed site area (14 m × 7 m) held 2 large and 14 small circular lysimeters within secondary containment with a waterproof lining. Cattle heads were interred in numbered and ‘NC’ lysimeters; black circles indicate lysimeters containing boluses. Lysimeters were filled with clay (dark grey) or sand (white). Numbers 1-5 refer to the year in which each 301V-spiked head was exhumed. NC, negative control heads; SC, soil control (lysimeters contained no heads or bolus). Black lines indicate water drainage from each lysimeter to the bunker for filtration, collection and processing. The system also included an overflow tank required to collect water after heavy rain