Rise in cases amplifies concerns about consolidation in agriculture.

[In the San Joaquin Valley] the H5N1 virus, commonly known as bird flu, has rippled through the massive herds that provide most of the country’s milk. Farmworkers have piled carcasses into black and white heaps. This week the state reported 19 new confirmed cases in cows and more than 240,000 in chickens. Another 50,000 cases were confirmed at a chicken breeding facility in Oklahoma.

Most worrying, though, is the spillover from livestock to humans. So far, 58 people in the United States have tested positive for bird flu. Fifty-six of them worked either on dairy or poultry farms where millions of birds had to be culled.

The Centers for Disease Control and Prevention confirmed that four of the cases in humans had no known connection to livestock, raising fears that the virus eventually could jump from one human to another, though that hasn’t happened yet. On Thursday, a study published in Science by researchers at The Scripps Research Institute said it would take only a single mutation in the H5N1 virus for it to attach itself to human receptor cells.