Four new human cases of bird flu were confirmed Sunday by health authorities in the US, bringing to eight the number of people infected with the disease since spring.

The people confirmed to be infected are workers at a poultry facility, the Centers for Disease Control and Prevention (CDC) explained in a statement.

A fifth suspected case has also been identified and further laboratory tests are being conducted to confirm it, the main US federal public health agency said.

The risk to the general population remains “low,” the CDC reiterated. Infected workers have “flu-like symptoms”.

In the US—and internationally—there is an outbreak of bird flu, which is spreading among wild birds, farmed birds and various mammal species. What is unusual is that outbreaks of H5N1 are being recorded on US cow farms.

The first human case was announced as confirmed in Texas, in the southern US, on April 1. It was then the world’s first known case of bird flu transmission to a human; he became infected through contact with a cow-carrier. Two more cases were later identified in Michigan. In early July, a fourth case was identified on a cow farm in Colorado.

Experts are concerned about the increasing number of mammals becoming infected with the disease, although human cases remain rare. The reason is the possible mutation of the virus that would make it easier to transmit it from person to person. The CDC assures that they have not detected any such mutation at this stage.

The agency reiterated its appeal to workers in livestock and poultry units where cases of bird flu are detected to take the maximum possible precautions.

Additional genetic analyzes are expected to be performed on samples from the new cases in Colorado to identify any genome modification that could lead to a “change in risk assessment” for the population by health authorities.