Muscle from a sick dairy cow tested positive for the virus. The meat did not enter the commercial food supply, which officials said remained safe.

© Aaron Josefczyk/Reuters

The results bolster evidence that virus-laden raw milk may be unsafe for humans.

© Jonel Aleccia/Associated Press

Tens of millions of farm animals cross state lines every year, traveling in cramped, stressful conditions that can facilitate the spread of disease.

© Rory Doyle for The New York Times

But the scope of the outbreak among cattle remains uncertain, and little human testing has been done.

© Hans Pennink/Associated Press

There is no evidence that the milk is unsafe to drink, scientists say. But the survey result strongly hints that the outbreak may be widespread.

© Robert F. Bukaty/Associated Press