The US Food and Drug Administration (FDA) has sent a warning letter to meatpacking giant Tyson Foods over violations in mad cow regulations at one of its rendering plants.

The regulations are designed to prevent BSE spreading throughout the US if it were to reach the country, and the FDA warns Tyson that violations threaten their effectiveness.

Tyson responded by agreeing to fix the violation, but said it will insist to the FDA that the regulations did not apply in its particular case.

The fact that the letter has now been made public (it was dated 12 February) suggests that the FDA is working harder to close the loopholes after a recent General Accounting Office report said its enforcement of the regulations is too lax.

The letter was the first of its kind to be received by a major meat company, although numerous smaller renderers and feed mills have received similar warnings.