Four farm workers, who work at the same poultry farm, were infected with bird flu on Monday, increasing the total number of cases to eight, according to officials Centers for Disease Control and Prevention in the United States.
The cases come, simultaneously, with a worrisome outbreak of the virus among cattle, which has also resulted in human infections.
A fifth case is also suspected but must still be analysed to be confirmed.
The risk for the American population is still "low," the CDC said. The infected workers reported facing conjunctivitis and "flu-like symptoms."
An epidemic of bird flu was detected in the spring in the United States, with an unusual twist: the disease, which first struck mainly poultry and birds, was now being found in cows.
The first human infection via a cow was announced on April 1, in the state of Texas.
Two other cases were then reported in Michigan, and in early July, another case was found in Colorado.
That western US state is now seeing the disease spread among both livestock and poultry.
Experts are concerned about the growing number of mammals infected with the disease, though human cases are still rare.
They fear that high circulation could make it easier for a mutation of the virus to develop that would allow it to pass from one human to another.
The CDC renewed its call for precautions for workers in contact with infected cows or poultry.
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