Research Shows Asymptomatic Bird Flu Infections Not Occurring, New Cases Reported 

Last week, the U.S. Centers for Disease Control and Prevention (CDC) confirmed two new bird flu-infected humans (one case CMM reported was presumed positive last week) and linked to cases the CDC confirmed last week in Colorado poultry workers. Since April 2024, 10 human cases of bird flu infection have been reported in the United States: four of these cases were associated with exposure to sick dairy cows and six were associated with exposure to bird flu-infected poultry in Colorado. All cases of illness in humans have been mild. 

On Friday, the CDC also released new research from a Michigan Department of Health and Human Services study of farmworkers around cows sickened by bird flu suggesting asymptomatic infections are not occurring. In blood drawn from 35 people who never had any symptoms, but did have evidence of past infection, none showed antibodies that would indicate bird flu infections.  

The CDC also reported that bird flu diagnosed human cases in Colorado and Michigan both have not shown antiviral resistance. Additionally, the CDC said no changes to the virus have occurred that would suggest the risk to human health has increased, and the human health risk currently remains low. 

Still, U.S. health experts have warned that further spread of the virus among cows could heighten the risk of more human infections. And the number of infected herds continues to grow weekly. As of July 22, the U.S. Department of Agriculture (USDA) has confirmed 168 dairy cow herds in 13 U.S. states. As the state with the most confirmed bird flu-infected humans, Colorado currently is also the state with the most bird flu-infected dairy herds (46 herds infected since the beginning of the outbreak.) Idaho, Michigan, and Texas are other leading states for bird flu outbreaks in dairy herds.  

Additionally, Colorado is a leading state for bird flu infection in poultry. As of July 22, Colorado has reported more than 9.4 million bird flu infected commercial and backyard flocks. All Colorado’s six human cases were associated with poultry workers, who were working in depopulation and disposal activities in sick flocks. USDA reports that since April 2024, 34 commercial flocks and 16 backyard flocks have been affected, for a total of more than 18.3 million infected birds in the U.S. (This number does not reflect wildlife).