The US Department of Agriculture has said it is investigating why an animal suspected of having a brain disease was not tested for mad cow disease before it was destroyed.

Last week at a Lone Star Beef Plant in Texas, a federal veterinarian saw the cow "stagger and fall", but instead of being tested for BSE, the carcass was sent straight to rendering, reported Reuters.

"Standard procedures call for animals condemned due to possible CNS (central nervous system) disorder to be kept" until brain tissue has been collected for BSE testing, the USDA said.

The USDA said meat from the animal did not enter the human food chain. Under USDA rules, no crippled or "downer" cattle may be processed for food, but can instead be rendered for use in industrial products. Officials also said that the cow's behaviour could have been due to a broken bone or another illness, not necessarily BSE.

Back in December a single case of BSE was detected in a cow in Washington state, resulting in widespread trade bans on imports of US beef. The US is trying to convince Japan to reopen its border to US beef, but Japan is so far insisting that the US tests 100% of its cattle for the disease. This latest situation will be a setback to the US, which has claimed its testing system is safe and has refused to introduce 100% testing.