As federal and state health officials expand their avian flu surveillance, the San Francisco Department of Public Health has reported that avian flu has been detected in wastewater samples collected in May.

Health officials said Monday that the H5N1 avian flu was detected last month in two asymptomatic chickens at a live bird market in San Francisco, but there is no sign that the public has been exposed.

A third person in the U.S. has tested positive for bird flu, the Centers for Disease Control and Prevention said even as it stressed the risk to the general public remains low.

The chickens were tested for H5N1 as part of routine procedure at a live bird market last month, authorities said. The animals were asymptomatic and no human who was in close contact with them reported any signs of illness.