CHICAGO, May 3 (Reuters) - U.S. health officials on Friday warned that dairy workers remain at risk from the H5N1 bird flu virus that is infecting dairy herds, and urged them to wear protective gear to avoid infection.

The U.S. CDC urged farmers, workers, and emergency responders to wear appropriate protective gear when in direct or close physical contact with sick birds, livestock, feces, raw milk or contaminated surfaces.

The warning coincides with a letter published online in a letter to the New England Journal of Medicine detailing the case of a dairy worker who was infected with the virus in March and developed an eye infection known as conjunctivitis.

Infection of the bird virus is rare in humans, and the dairy worker's case represents only the second known infection in the United States. It follows a concerning spread of the virus in a variety of mammal species, raising concerns that widespread exposure in people could cause the virus to spread more easily in people and spark a global pandemic. (Reporting by Julie Steenhuysen; Editing by Chizu Nomiyama)