WASHINGTON -- A U.S. senator Tuesday said there still are questions about the adequacy of the federal government's response to the mad cow case in Washington state and urged officials to do more to ensure there are no additional cases of the disease in U.S. herds.

"The fact is, we really do not know whether (mad cow disease) is in U.S. cattle or to what extent if it is," Sen. Tom Harkin, D-Iowa, said during a hearing of the Senate Appropriations Committee, in which he urged the U.S. Department of Agriculture to do more testing for the disease.

He noted the USDA has insisted the Washington cow was an isolated case of mad cow disease, otherwise known as bovine spongiform encephalopathy or BSE.

Without increased testing, however -- especially in seemingly healthy animals -- that cannot be known with any certainty, he said.

Harkin urged the USDA to consider expanding its surveillance program to include testing healthy animals and not just downers, or those unable to stand, in light of three eyewitnesses in Washington state who have insisted the infected animal was able to walk on the day of slaughter.

"If that cow was not a downer, it underscores the fact that BSE may be present in cattle that are apparently healthy and showing no BSE or other disease symptoms," Harkin said. "So why are we not testing more of these seemingly healthy cattle?"

Harkin also slammed the Food and Drug Administration for not expanding its feed ban to prohibit the use of any animal proteins in the feed of ruminant animals. Europe took this step in response to detecting mad cow in its herds and an international panel of experts commissioned by the USDA advised the FDA follow suit in a report it issued earlier this month.

"It is essential that FDA consider these recommendations fully and carefully," Harkin said. "Frankly, the burden should be on FDA to adopt them unless it can show that the tighter restrictions are unnecessary."

By resisting further safeguards, the USDA and FDA risk losing consumer confidence and jeopardizing U.S. beef exports, he said.

"Failing to address these questions promptly and carefully can only damage the credibility of our food safety system, erode consumer confidence and impede reopening of export markets for U.S. beef," he said.

More than 40 countries imposed bans on the import of U.S. beef due to concerns about mad cow and most of them still have the prohibitions in place, including the top three U.S. beef importers, Japan, South Korea and Mexico.

Harkin questioned why the USDA only tests cows above 30 months of age when it is possible that younger animals could carry abnormal prions, the agent thought to cause mad cow disease.

"You don't know that an animal of 15 months that has these abnormal prions is not infectious," Harkin said to Ron DeHaven, USDA's chief veterinarian officer. The concern is that humans can contract a fatal brain disorder known as variant Creutzfeldt-Jakob disease from eating meat infected with the mad cow pathogen.

DeHaven responded that ongoing studies have suggested the brain and spinal cord -- the most infectious parts of a cow with BSE -- do not usually become infectious until after 30 months of age.

Michael Hansen, a microbiologist with the watchdog group Consumers Union in Yonkers, N.Y., disputed that assessment in an interview with United Press International.

Hansen said studies in which sheep have been infected with mad cow disease suggest younger cows could be infectious. After the sheep were infected, they were shown to be able to transmit the disease to other sheep while they were young and before they ever showed any symptoms themselves, he said.

"That evidence should be strong enough" to assume younger cows are infectious, particularly since the disease can have such serious consequences in humans, Hansen said.

In addition, cows younger than 30 months of age have tested positive for the disease in both Europe and Japan, so it is possible for cows this young to be infected, he noted.

DeHaven said the USDA was "actively considering a more robust surveillance program" that would include increased testing and the use of so-called rapid tests, which are in use in Europe and enable authorities there to screen more cattle in one week than the U.S. program does in an entire year.

The FDA expanded its feed ban in January to prohibit mammalian blood, waste from chicken house floors and leftover beef at restaurants from being used in cattle feed. Harkin said that was not enough because poultry and chickens, which could carry infectious prions, still can be incorporated into cattle feed.

"Why don't we just simply ban feeding of all animal protein and bone meal products to ruminant animals?" Harkin asked FDA Deputy Commissioner Lester Crawford.

"It would break the cycle" and keep mad cow disease from spreading among the herds, he said.

Crawford responded that step would not improve the safety very much because pigs, chickens and turkeys are not known to carry the infectious prions.

Hansen countered it is unclear whether poultry and pigs can carry these types of disorders.

A USDA veterinarian detected a possible mad cow-like disease in pigs in 1979, Hansen noted, so it is possible pigs could carry a prion that could infect humans or cows.

Other scientists have suggested chickens also could be silent carriers, he added.

"We can't really say for certain. Therefore you have to assume the worst until you get the appropriate data," he said.

Hansen thinks an appropriate testing regimen would be to screen all cattle over the age of 20 months that go to slaughter because no animals younger than this have ever tested positive for mad cow.

"You do not want this disease getting into the population, because it could be a potentially very serious thing," Hansen said.

A recent incident in the United Kingdom, where a man contracted vCJD from a blood transfusion, highlights the potential risk, he said. The blood donor, a man who also had vCJD, was donating blood for up to three years before he showed any symptoms, Hansen said.

Because donated blood can be diluted and distributed to thousands of people, there is the risk of widespread infection of a deadly disease that cannot currently be detected in blood and for which there is no treatment, he added.

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