Also like the previous cases, the infected cow was born and probably infected before 1997. This was the year when feeding cattle with the remains of other cattle, a practice that spreads BSE, was banned in North America.

The discovery highlights concerns about surveillance for BSE in the US. The infection probably arrived with cattle imported from the UK in the 1980s. Normal patterns of animal and feed movements should have spread it across the continent since then. Yet while in 2004 Canada tested 22,000 high-risk cattle and found one positive, the US tested more than 120,000 and found none.

Source: New Scientist