Many human infections with ‘cow flu’ are going undetected

The H5N1 bird flu strain circulating in U.S. dairy cattle is likely infecting far more farm workers than scientists realized, a new study published today by the U.S. Centers for Disease Control and Prevention (CDC) suggests. Tests of 115 dairy workers exposed to the virus in two states found evidence of a recent H5N1 infection in eight of them—an infection rate of 7%—researchers report today in the agency’s Morbidity and Mortality Weekly Report.

The United States has reported 46 H5N1 infections in humans so far, 45 of them linked to infected poultry and cattle. But thousands of dairy workers have likely been exposed to infected herds, and the new study suggests many cases have been missed, says Caitlin Rivers, an epidemiologist at the Johns Hopkins Center for Health Security. “There are dairy infections in dozens of states, and so likely most, or all of those states will have experienced human cases.”

In response to the findings, CDC plans to increase testing of exposed farm workers, and it recommends more people take an antiviral drug to reduce their risk. But the agency stresses there is no evidence the virus has become better at spreading between humans, and it still classifies the risk to the general public as low.