"It's most important right now, till the science tells us otherwise, that we treat this as BSE regardless," the Agriculture Department's chief veterinarian, John Clifford, said in an interview.
The Texas and Alabama cases — confirmed last year and this one, respectively — are drawing international attention.
At a meeting in London last month, experts presented research on the U.S. cases and on similar ones in Europe.
These cows appear to have had an "atypical" strain that scientists are only now starting to identify. Such cases have been described in about a dozen cows in France, Italy and other European countries, as well as in Japan.
In the two U.S. cases, researchers did not detect the telltale spongy lesions caused by prions, the misfolded proteins that deposit plaque on the brain and kill brain cells. In addition, the prions in brain tissue samples from the Texas and Alabama cows seemed to be distributed differently from what would be expected to be found in cows with the classic form.
Laboratory studies on mice in France showed that both the classic and atypical strains could be spread from one animal to another. But scientists theorize the atypical strain might have infected cattle through an unusual way.
Mad cow disease is not transmitted from cow to cow like a cold or the flu. It is believed to spread through feed, when cows eat the contaminated tissue of other cattle. That happens when crushed cattle remains are added to feed as a protein source. This once-common practice ended in the United States in 1997.
Humans can get a related disease, variant Creutzfeldt-Jakob Disease, in similar fashion — by eating meat contaminated with mad cow. Mad cow in humans afflicts younger people; the average age at death is 28 years.
A more common form of CJD — not linked to mad cow — can happen spontaneously and is reported in nearly 300 people in the U.S. each year. This form occurs mostly in older people; the average age at death is 68.
Some scientists are raising the possibility that the atypical strain also might happen spontaneously in cattle. The Texas and Alabama cows were older animals, as were some of the other animals in Europe with seemingly atypical cases.
Linda Detwiler, a former Agriculture Department veterinarian who consults for major food companies, cautioned against making that assumption. "I think it's kind of early to say that would be the case," Detwiler said.
Other theories, she said, suggest the atypical strain might come from a mutation of mad cow disease or even from a related disease in sheep.
Mad cow disease has turned up three times in the United States: in native-born animals in Texas and Alabama and in a Canadian import in Washington state.
In the Texas and Alabama cows, tests found patterns distinct from what turned up in an infected cow in Washington state and a cluster of Canadian cases, researchers say. The Washington and Canadian cases resemble the classic British cases.
No matter what the origins might be of an atypical strain, the government says there is no reason to change federal testing or measures that safeguard animals and people from the disease.
"We still feel confident in the safeguards that we have," Clifford said. "We have to base our assumptions on what is scientifically known and understood."
Meanwhile, mad cow research has been halted at the Agriculture Department's lab in Ames, Iowa, because of employee allegations that the lab improperly was disposing of animal waste.
The department asked a group of international experts to review the lab's disposal practices. The city of Ames also is investigating.