To date, the United States has found a single case of mad cow disease, or bovine spongiform encephalopathy. The discovery of the brain-wasting disease in a dairy cow in Washington state in December 2003 prompted Japan and other nations to halt purchases of American beef.

Former USDA veterinarians Lester Friedlander and Masuo Doi have been quoted separately in recent days by Canadian news outlets saying that the United States may have more cases of mad cow disease.

Friedlander, who retired in 1995 as a food safety inspector in Pennsylvania, alleged that the USDA knew about the additional cases and was hiding the information.

"To suggest USDA has been in engaged in a cover-up in hiding BSE cases is absolutely false," USDA spokesman Ed Loyd said. "We would be completely jeopardizing and risking all of the consumer confidence in the United States."

Agriculture Secretary Mike Johanns told reporters on Tuesday the USDA would not investigate Friedlander's allegation because he provided no proof.

The USDA, which tested about 20,000 cattle for BSE in 2003, significantly expanded its testing program last year to more than 176,000 animals. The government now tests between 9,000 and 10,000 cattle each week.

Lawmakers, meat industry and consumer groups credited the USDA's prompt disclosure of the December 2003 case and its adoption of extra safeguards one week later for maintaining public confidence in the safety of the US meat supply.

More than 130 people, mostly in Britain and Europe, have died from eating meat from infected animals and contracting a human form of the disease.

"When we found our first case of BSE, it was immediately reported," said Jim Rogers, spokesman for USDA's Animal and Plant Health Inspection Service.

Doi, a former veterinarian for the USDA's Food Safety and Inspection Service, alleged the department did not properly analyze two suspected cases of mad cow disease in 1997, according to a report aired on Tuesday by Canada's CBC Television.

The CBC report said the incident occurred in Oriskany Falls, New York, and quoted Doi as saying that important brain samples from one animal went missing.

The USDA denied Doi's allegations and provided Reuters with government documents showing the suspect animals were tested and the results were negative for BSE.