By Lisa Richwine
WASHINGTON (Reuters) - U.S. regulators are considering whether feeding chicken litter to cattle poses any risk of transmitting the deadly mad cow disease, a top U.S. health official said Monday.

Dr. Stephen Sundlof, director of the FDA's Center for Veterinary Medicine, told Reuters that practice was one of several regulators were reviewing as they consider whether to tighten the shield against the illness. So far, neither mad cow disease nor its deadly human form have been found in the United States.

Federal rules that took effect in 1997 prohibit feeding cattle or sheep with protein from animals that are potential carriers of mad cow or a related illness. Feeding contaminated animal remains is blamed for spreading the disease to cows throughout Europe. Experts believe people get the brain-wasting illness by eating contaminated meat products.

Now, litter containing waste from chickens legally can be processed and fed to cattle under some circumstances, Sundlof said.

Some have questioned whether chickens that ate material prohibited for cattle could recycle the banned byproducts back to cows that ate their litter. The abnormal proteins believed to cause mad cow disease have proven resilient, and it is unknown whether a chicken's digestive tract could kill them.

``That's another issue we intend to put out there for examination and potentially change our position on that ... Just about everything is open right now,'' Sundlof said in an interview.

In remarks to a public meeting regarding mad cow, also known as bovine spongiform encephalopathy, Sundlof said regulators were re-evaluating other practices now exempt from cattle feed bans.

One exemption under review is the feeding of ``plate waste'' to cattle. Plate waste is food served to people in restaurants that later is discarded. Some companies reprocess the leftovers into animal feed.

``It's another area we are continuing to look at to determine if it is indeed a safe practice,'' Sundlof said.

Dr. Paul Brown, medical director for the National Institute for Neurological Disorders and Strokes, said infected meat from a T-bone steak, if cut from an infected animal, could pose ``a reasonably remote possibility'' of being infectious when recycled as plate waste. The steak might carry spinal cord tissue, which along with brain parts is considered highly infectious.

Some 100 people in Europe have died from or been diagnosed with the human version of mad cow, known as variant Creutzfeldt-Jakob Disease.