US officials on Wednesday reported the country's fourth human case of bird flu linked to the current outbreak of the virus in dairy cattle.

As with previous cases, the person worked on a farm and was exposed to infected cows, the US Centers for Disease Control and Prevention (CDC) said in a statement, adding that the risk to the general population remained "low."

The infection occurred in Colorado and the affected person only had eye symptoms, was given antiviral medication, and has since recovered, it said.

A first US infection in the southern state of Texas was announced on April 1. Two other cases were subsequently reported in Michigan.

Numerous herds of cows are infected across several American states, an epidemic which was first detected in March.

Experts are concerned about the growing number of mammals infected with the disease, although cases in humans remain rare.

They fear that high circulation could facilitate a mutation of the virus which would allow it to pass from one human to another.

Recent testing has confirmed mice are sickened by exposure to raw milk contaminated with bird flu, but pasteurization destroys the virus.

The disease has additionally been found in farm-raised alpacas in Idaho.