It’s still unclear how H5N1 virus jumped into U.S. cattle—and why it keeps doing so. When U.S. Department of Agriculture (USDA) officials reported one year ago today that the H5N1 avian influenza virus had infected dairy cattle in Texas and Kansas, it seemed a freak event—a rare confluence of factors that somehow allowed a bird virus to infect cows.
But 10 months later, on 31 January, USDA said it had detected another jump to cattle, this time in Nevada. And 2 weeks later, another one, in Arizona.
One year into the United States’s cow flu outbreak, many important questions remain unanswered, including how the virus is spreading from one farm to the next. But perhaps the most basic one is how it manages to get into cattle in the first place—and how often that happens. Knowing the answers is “really important,” says Thijs Kuiken, a wildlife pathologist at Erasmus University Medical Center, because “then you can change the system so that it doesn’t happen again.”